Category Archive Anatomy A – Z

The Aorta and Its Branches for Circulatory Routes

The aorta is the largest artery in the body and is divided into 3 parts: the ascending aorta, arch of the aorta, and descending aorta.

The aorta is the largest artery in the systemic circulatory system. Blood is pumped from the left ventricle into the aorta and from there branches to all parts of the body. The aorta is an elastic artery, meaning it is able to distend. When the left ventricle contracts to force blood into the aorta, the aorta expands. This stretching generates the potential energy that helps maintain blood pressure during diastole, since during this time the aorta contracts passively.

The aorta is divided into three parts: the ascending aorta, where the aorta initially leaves the heart and points superiorly toward the head; the arch of the aorta where the aorta changes direction; and the descending aorta where the aorta points inferiorly toward the feet.

The ascending aorta has two small branches, the left and right coronary arteries. These arteries provide blood to the heart muscle, and their blockage is the cause myocardial infarctions or heart attacks.

The arch of the aorta has three branches: the brachiocephalic artery, which itself divides into right common carotid artery and the right subclavian artery, the left common carotid artery, and the left subclavian artery. These arteries provide blood to both arms and the head.

The descending aorta is the largest artery in the body; it runs from the heart down the length of the chest and abdomen. It is divided into two portions, the thoracic and abdominal, in correspondence with the two great cavities of the trunk in which it sits. Within the abdomen, the descending aorta branches into the two common iliac arteries that provide blood to the pelvis and, eventually, the legs.

Ascending Aorta

The ascending aorta is the first portion of the aorta; it includes the aortic sinuses, the bulb of the aorta, and the sinotubular junction.

The ascending aorta is a portion of the aorta beginning at the upper part of the base of the left ventricle, on a level with the lower border of the third costal cartilage behind the left half of the sternum; it passes diagonally upward, forward, and to the right, in the direction of the heart’s axis, as high as the upper border of the second right costal cartilage. Its total length is about five centimeters.

The aortic root is the portion of the ascending aorta beginning at the aortic annulus, the fibrous attachment between the heart and the aorta, and extending to the sinotubular junction. Just above the aortic valve are three small dilations called the aortic sinuses. The two anterior sinuses give rise to the coronary arteries, while the third posterior sinus does not usually give rise to any vessels and so is known as the non-coronary sinus.

The sinotubular junction is the point in the ascending aorta where the aortic sinuses end and the aorta becomes a tubular structure.

At the junction of the ascending aorta with the aortic arch, the caliber of the vessel increases with a bulging of its right wall. This dilatation is termed the “bulb of the aorta.” The ascending aorta is contained within the pericardium. It is enclosed in a tube of the serous pericardium, which also encloses the pulmonary artery.

The ascending aorta is covered at its beginning by the trunk of the pulmonary artery and, higher up, is separated from the sternum by the pericardium, the right pleura, the anterior margin of the right lung, some loose areolar tissue, and the remains of the thymus. Posteriorly, it rests upon the left atrium and right pulmonary artery.

Arch of the Aorta

The arch of the aorta follows the ascending aorta and begins at the level of the second sternocostal articulation of the right side.

The arch of the aorta, or the transverse aortic arch, is continuous with the upper border of the ascending aorta and begins at the level of the upper border of the second sternocostal articulation of the right side. The arch of the aorta runs at first upward, backward, and to the left in front of the trachea; it is then directed backward on the left side of the trachea and finally passes downward on the left side of the body of the fourth thoracic vertebra. At the lower border, this vertebra becomes continuous with the descending aorta.

Three vessels come out of the aortic arch: the brachiocephalic artery, the left common carotid artery, and the left subclavian artery. These vessels supply blood to the head, neck, thorax and upper limbs. In approximately 20% of individuals, the left common carotid artery arises from the brachiocephalic artery rather than the aortic arch, and in approximately 7% of individuals the left subclavian artery also arises here.

Thoracic Aorta

The thoracic aorta is the section of the aorta that travels through the thoracic cavity to carry blood to the head, neck, thorax and arms.

The thoracic aorta forms part of the descending aorta and is continuous with the aortic arch at its origin before becoming the abdominal aorta. Contained within the posterior mediastinal cavity, it begins at the lower border of the fourth thoracic vertebra where it is continuous with the aortic arch, and ends in front of the lower border of the twelfth thoracic vertebra at the aortic hiatus in the diaphragm. At its commencement, the thoracic aorta is situated on the left of the vertebral column; it approaches the median line as it descends, and at its termination lies directly in front of the column.

The thoracic aorta’s relation, from above downward, is as follows: anteriorly with the root of the left lung, the pericardium, the esophagus and the diaphragm; posteriorly with the vertebral column; on the right side with the hemiazygos veins and thoracic duct; and on the left side with the left pleura and lung. The esophagus lies on the right side of the aorta for most of its length, but at the lower part of the thorax is placed in front of the aorta and close to the diaphragm, situated on its left side.

As it descends in the thorax, the aorta gives off several paired branches. In descending order these are the bronchial arteries, the mediastinal arteries, the esophageal arteries, the pericardial arteries, and the superior phrenic artery. The posterior intercostal arteries are branches that originate throughout the length of the posterior aspect of the thoracic aorta.

Abdominal Aorta

The abdominal aorta is the largest artery in the abdominal cavity and supplies blood to most of the abdominal organs.

The abdominal aorta is the largest artery in the abdominal cavity. As part of the descending aorta, it is a direct continuation of the thoracic aorta.

It begins at the level of the diaphragm, crossing it via the aortic hiatus. This hole in the diaphragm that allows the passage of the great vessels at the vertebral level of T12. The abdominal aorta travels down the posterior wall of the abdomen, anterior to the vertebral column, following the curvature of the lumbar vertebrae. The abdominal aorta runs parallel to the inferior vena cava, located just to the right of the abdominal aorta.

The abdominal aorta lies slightly to the left of the midline of the body. It is covered anteriorly by the lesser omentum and stomach. Posteriorly, it is separated from the lumbar vertebrae by the anterior longitudinal ligament and left lumbar veins.

The abdominal aorta supplies blood to much of the abdominal cavity through numerous branches that become smaller in diameter as it descends. Terminally it branches into the paired common iliac arteries, which supply the pelvis and lower limbs.

Arteries of the Pelvis and Lower Limbs

The abdominal aorta divides into the major arteries of the leg: the femoral, popliteal, tibial, dorsal foot, plantar, and fibular arteries.

The pelvic cavity is largely supplied by the paired internal iliac arteries, formed when the common iliac artery divides the internal iliac artery at the vertebral level L5 descends inferiorly into the lesser pelvis. The external iliac artery passes into the thigh, becoming the femoral artery.

At the most superior border of the greater sciatic foramen, the large opening to the rear of the pelvis, the internal iliac artery divides into anterior and posterior trunks.

The anterior trunk gives rise to numerous arteries that supply the organs of the pelvis and the gluteal and adductor muscles of the leg. Key branches include the obturator artery, the inferior vesical artery in men and the equivalent vaginal artery in females, and the rectal and gluteal arteries.

The posterior trunk gives rise to arteries that supply the posterior pelvic wall and the gluteal region, including the iliolumbar artery that supplies the psoas major muscle, the lateral sacral arteries, and the superior gluteal artery.

Principal Veins

Veins are blood vessels that carry blood towards the heart, have thin, inelastic walls, and contain numerous valves.

In the circulatory system, veins are blood vessels that carry blood towards the heart. Veins have thin, inelastic walls, and contain numerous valves in order to prevent the backflow of blood. Most veins carry deoxygenated blood from the tissues back to the heart with the exceptions of the pulmonary and umbilical veins, both of which carry oxygenated blood to the heart.

Veins can be broadly classified based on their depth within the body. Superficial veins are located close to the surface of the body and have no corresponding arteries, such as the great saphenous vein which runs the length of the leg. The deep veins lie deeper in the body and often run adjacent to corresponding arteries, such as the femoral vein which sits adjacent to the femoral artery in the thigh. Deep veins are often of larger caliber than superficial veins and carry the majority of the blood within the circulatory system. Communicating veins, or perforator veins if they pass through a large muscle mass, directly connect superficial and direct veins. The above veins form part of the systemic circulatory system. The pulmonary veins and venules that run from the lungs to the heart form part of the pulmonary circulatory system and are distinct from other veins in that they carry oxygenated blood.

Venae Cavae

The venae cavae are the veins with the largest diameter. Both enter the right atrium of the heart with the superior vena cava carrying blood from the arms, head, and thoracic cavity and the inferior vena cava carrying blood from the legs and abdomen. The inferior vena cava runs parallel to the abdominal aorta.

The superior vena cava is formed from the brachiocephalic veins which are in turn formed from the subclavian and internal jugular veins that serve the arm and head respectively. The inferior vena cava is formed from the common iliac veins that serve the legs and abdomen. The renal and hepatic veins from the kidneys and liver respectively also feed into the inferior vena cava.

Other Important Veins

Other important venous systems include the cardiac veins, which return blood from the heart tissue back to the general circulation. The cardiac veins merge into the coronary sinus, which empties directly into the right atrium.

The pulmonary veins are large blood vessels which receive oxygenated blood from the lungs and return it to the left atrium of the heart. There are four pulmonary veins, two from each lung, each of which forms from three to four bronchial veins. In approximately 25% of individuals, the left pulmonary veins may merge into a single vein; the same effect on the right side is only seen in approximately 3% of individuals.

The hepatic portal vein carries blood from the gastrointestinal tract to the liver. The portal vein is often described as a false vein because it conducts blood between capillary networks rather than between a capillary network and the heart. It functions to supply the liver with blood and required metabolites, but also ensures that ingested substances are first processed in the liver before reaching the wider systemic circulation.

Veins of the Head and Neck

In the head and neck, blood circulates from the upper systemic loop, which originates at the aortic arch.

Two main jugular veins are responsible for the venous draining of the head and neck.

The superficial external jugular vein is formed from the retromandibular vein and the posterior auricular vein at a point adjacent to the mandible. The external jugular vein passes down the neck and underneath the clavicle before draining into the subclavian vein.

The deep-lying internal jugular vein receives blood from the dural venous sinuses in the brain as well as the cerebral and cerebellar veins. Dural sinuses are composed of dural mater lined with endothelium, making them distinct from arteries, veins, and capillaries. The dural sinuses receive blood from the veins that drain the brain and skull.

Formed at the base of the brain from the inferior petrosal sinus and the sigmoid sinus, the internal jugular vein runs down the side of the neck adjacent to the internal carotid artery. As well as removing blood from the brain, the anterior retromandibular, facial, and lingual veins also drain into the internal jugular. Upon exiting the neck, the internal jugular vein merges with the subclavian vein to form the brachiocephalic vein.

Additional veins such as the occipital, deep cervical, and thyroid veins drain directly into the brachiocephalic vein.

Veins of the Upper Limbs

The veins of the upper extremity are divided into superficial and deep veins, indicating their relative depths from the skin.

Veins of the arm are either deep or superficial and are responsible for draining the hand and arm.

The major deep veins of the arm are the radial and ulnar veins, which run along the length of their respective bones and merge at the elbow to form the paired brachial vein. The brachial vein runs from the elbow up to the shoulder parallel to the brachial artery.

The major superficial veins of the upper limb are the cephalic, median cubital and basilic veins. The cephalic vein arises from the dorsal venous network of the hand and passes the elbow anteriorly, continuing up the upper arm to the shoulder. The basilic vein follows a similar path but is located medially to the cephalic vein. At the elbow, the basilic and cephalic veins are linked by the median cubital vein, from which blood is often drawn.

At the shoulder, the basilic vein passes deep into the arm and merges with the brachial veins to form the axillary vein, to which the cephaliac vein merges, forming the subclavian vein.

Veins of the Thorax

The veins of the thorax drain deoxygenated blood from the thorax region for return to the heart

Two venae cavae return deoxygenated blood from the systemic circulation to the right atrium of the heart.

The superior vena cava, formed from the left and right brachiocephalic veins, returns deoxygenated blood from the upper half of the body and carries blood from the upper limbs, head, and neck via the thyroid and jugular veins. It is joined just before entering the heart by the azygos vein, which runs up the right side of the thoracic vertebral column and transports blood from the external thoracic cavity.

The internal thoracic vein is a vessel that drains the chest wall and breasts. Bilaterally, it arises from the superior epigastric vein, accompanies the internal thoracic artery along its course, and terminates in the brachiocephalic vein.

The supreme intercostal vein is a paired vein that drains the first intercostal space on its corresponding side. It usually drains into the brachiocephalic vein.

The inferior vena cava returns blood from the abdomen and lower limbs to the right atrium of the heart. The renal veins from the kidney and hepatic veins of the liver drain directly into the inferior vena cava. Additionally, the superior and inferior phrenic veins drain the diaphragm and usually open into the internal mammary vein and inferior vena cava, respectively.

Veins of the Abdomen and Pelvis

The major veins of the abdomen and pelvis return deoxygenated blood from the abdomen and pelvis to the heart.

A number of veins remove deoxygenated blood from the abdomen and pelvis. The external iliac vein, the upward continuation of the femoral vein, passes upward along the pelvis and ends to form the common iliac vein.The tributaries of the external iliac vein are the inferior epigastric, deep iliac circumflex, and pubic veins.

The internal iliac vein begins near the upper part of the greater sciatic foramen, the large opening at the rear of the pelvis, passes upward behind and slightly medial to the internal iliac artery and, at the brim of the pelvis, joins with the external iliac vein to form the common iliac vein.

The left and right common iliac veins come together in the abdomen at the level of the fifth lumbar vertebra, forming the abdominal vena cava. They drain blood from the pelvis and lower limbs.

The superior epigastric vein refers to a blood vessel that carries deoxygenated blood and drains into the internal thoracic vein. It anastomoses with the inferior epigastric vein at the level of the umbilicus and drains the anterior part of the abdominal wall and some of the diaphragm.

The inferior epigastric vein refers to the vein that drains into the external iliac vein and arises from the superior epigastric vein.

The deep circumflex iliac vein is formed by the union of the venae comitantes of the deep iliac circumflex artery and joins the external iliac vein about 2 cm above the inguinal ligament.

Veins of the Lower Limbs

The deep veins of the lower extremity have valves for unidirectional flow and accompany the arteries and their branches.

Running the full length of the leg, making it the longest vein in the body, the great saphenous vein is a superficial vein that returns blood from the foot and superficial muscles of the leg before merging with the femoral vein to form the external iliac vein.

The Aorta and Its Branches for Circulatory Routes

The aorta is the largest artery in the body and is divided into 3 parts: the ascending aorta, arch of the aorta, and descending aorta.

The aorta is the largest artery in the systemic circulatory system. Blood is pumped from the left ventricle into the aorta and from there branches to all parts of the body. The aorta is an elastic artery, meaning it is able to distend. When the left ventricle contracts to force blood into the aorta, the aorta expands. This stretching generates the potential energy that helps maintain blood pressure during diastole, since during this time the aorta contracts passively.

The aorta is divided into three parts: the ascending aorta, where the aorta initially leaves the heart and points superiorly toward the head; the arch of the aorta where the aorta changes direction; and the descending aorta where the aorta points inferiorly toward the feet.

The ascending aorta has two small branches, the left and right coronary arteries. These arteries provide blood to the heart muscle, and their blockage is the cause myocardial infarctions or heart attacks.

The arch of the aorta has three branches: the brachiocephalic artery, which itself divides into right common carotid artery and the right subclavian artery, the left common carotid artery, and the left subclavian artery. These arteries provide blood to both arms and the head.

The descending aorta is the largest artery in the body; it runs from the heart down the length of the chest and abdomen. It is divided into two portions, the thoracic and abdominal, in correspondence with the two great cavities of the trunk in which it sits. Within the abdomen, the descending aorta branches into the two common iliac arteries that provide blood to the pelvis and, eventually, the legs.

Ascending Aorta

The ascending aorta is the first portion of the aorta; it includes the aortic sinuses, the bulb of the aorta, and the sinotubular junction.

The ascending aorta is a portion of the aorta beginning at the upper part of the base of the left ventricle, on a level with the lower border of the third costal cartilage behind the left half of the sternum; it passes diagonally upward, forward, and to the right, in the direction of the heart’s axis, as high as the upper border of the second right costal cartilage. Its total length is about five centimeters.

The aortic root is the portion of the ascending aorta beginning at the aortic annulus, the fibrous attachment between the heart and the aorta, and extending to the sinotubular junction. Just above the aortic valve are three small dilations called the aortic sinuses. The two anterior sinuses give rise to the coronary arteries, while the third posterior sinus does not usually give rise to any vessels and so is known as the non-coronary sinus.

The sinotubular junction is the point in the ascending aorta where the aortic sinuses end and the aorta becomes a tubular structure.

At the junction of the ascending aorta with the aortic arch, the caliber of the vessel increases with a bulging of its right wall. This dilatation is termed the “bulb of the aorta.” The ascending aorta is contained within the pericardium. It is enclosed in a tube of the serous pericardium, which also encloses the pulmonary artery.

The ascending aorta is covered at its beginning by the trunk of the pulmonary artery and, higher up, is separated from the sternum by the pericardium, the right pleura, the anterior margin of the right lung, some loose areolar tissue, and the remains of the thymus. Posteriorly, it rests upon the left atrium and right pulmonary artery.

Arch of the Aorta

The arch of the aorta follows the ascending aorta and begins at the level of the second sternocostal articulation of the right side.

The arch of the aorta, or the transverse aortic arch, is continuous with the upper border of the ascending aorta and begins at the level of the upper border of the second sternocostal articulation of the right side. The arch of the aorta runs at first upward, backward, and to the left in front of the trachea; it is then directed backward on the left side of the trachea and finally passes downward on the left side of the body of the fourth thoracic vertebra. At the lower border, this vertebra becomes continuous with the descending aorta.

Three vessels come out of the aortic arch: the brachiocephalic artery, the left common carotid artery, and the left subclavian artery. These vessels supply blood to the head, neck, thorax and upper limbs. In approximately 20% of individuals, the left common carotid artery arises from the brachiocephalic artery rather than the aortic arch, and in approximately 7% of individuals the left subclavian artery also arises here.

Thoracic Aorta

The thoracic aorta is the section of the aorta that travels through the thoracic cavity to carry blood to the head, neck, thorax and arms.

The thoracic aorta forms part of the descending aorta and is continuous with the aortic arch at its origin before becoming the abdominal aorta. Contained within the posterior mediastinal cavity, it begins at the lower border of the fourth thoracic vertebra where it is continuous with the aortic arch, and ends in front of the lower border of the twelfth thoracic vertebra at the aortic hiatus in the diaphragm. At its commencement, the thoracic aorta is situated on the left of the vertebral column; it approaches the median line as it descends, and at its termination lies directly in front of the column.

The thoracic aorta’s relation, from above downward, is as follows: anteriorly with the root of the left lung, the pericardium, the esophagus and the diaphragm; posteriorly with the vertebral column; on the right side with the hemiazygos veins and thoracic duct; and on the left side with the left pleura and lung. The esophagus lies on the right side of the aorta for most of its length, but at the lower part of the thorax is placed in front of the aorta and close to the diaphragm, situated on its left side.

As it descends in the thorax, the aorta gives off several paired branches. In descending order these are the bronchial arteries, the mediastinal arteries, the esophageal arteries, the pericardial arteries, and the superior phrenic artery. The posterior intercostal arteries are branches that originate throughout the length of the posterior aspect of the thoracic aorta.

Abdominal Aorta

The abdominal aorta is the largest artery in the abdominal cavity and supplies blood to most of the abdominal organs.

The abdominal aorta is the largest artery in the abdominal cavity. As part of the descending aorta, it is a direct continuation of the thoracic aorta.

It begins at the level of the diaphragm, crossing it via the aortic hiatus. This hole in the diaphragm that allows the passage of the great vessels at the vertebral level of T12. The abdominal aorta travels down the posterior wall of the abdomen, anterior to the vertebral column, following the curvature of the lumbar vertebrae. The abdominal aorta runs parallel to the inferior vena cava, located just to the right of the abdominal aorta.

The abdominal aorta lies slightly to the left of the midline of the body. It is covered anteriorly by the lesser omentum and stomach. Posteriorly, it is separated from the lumbar vertebrae by the anterior longitudinal ligament and left lumbar veins.

The abdominal aorta supplies blood to much of the abdominal cavity through numerous branches that become smaller in diameter as it descends. Terminally it branches into the paired common iliac arteries, which supply the pelvis and lower limbs.

Arteries of the Pelvis and Lower Limbs

The abdominal aorta divides into the major arteries of the leg: the femoral, popliteal, tibial, dorsal foot, plantar, and fibular arteries.

The pelvic cavity is largely supplied by the paired internal iliac arteries, formed when the common iliac artery divides the internal iliac artery at the vertebral level L5 descends inferiorly into the lesser pelvis. The external iliac artery passes into the thigh, becoming the femoral artery.

At the most superior border of the greater sciatic foramen, the large opening to the rear of the pelvis, the internal iliac artery divides into anterior and posterior trunks.

The anterior trunk gives rise to numerous arteries that supply the organs of the pelvis and the gluteal and adductor muscles of the leg. Key branches include the obturator artery, the inferior vesical artery in men and the equivalent vaginal artery in females, and the rectal and gluteal arteries.

The posterior trunk gives rise to arteries that supply the posterior pelvic wall and the gluteal region, including the iliolumbar artery that supplies the psoas major muscle, the lateral sacral arteries, and the superior gluteal artery.

Principal Veins

Veins are blood vessels that carry blood towards the heart, have thin, inelastic walls, and contain numerous valves.

In the circulatory system, veins are blood vessels that carry blood towards the heart. Veins have thin, inelastic walls, and contain numerous valves in order to prevent the backflow of blood. Most veins carry deoxygenated blood from the tissues back to the heart with the exceptions of the pulmonary and umbilical veins, both of which carry oxygenated blood to the heart.

Veins can be broadly classified based on their depth within the body. Superficial veins are located close to the surface of the body and have no corresponding arteries, such as the great saphenous vein which runs the length of the leg. The deep veins lie deeper in the body and often run adjacent to corresponding arteries, such as the femoral vein which sits adjacent to the femoral artery in the thigh. Deep veins are often of larger caliber than superficial veins and carry the majority of the blood within the circulatory system. Communicating veins, or perforator veins if they pass through a large muscle mass, directly connect superficial and direct veins. The above veins form part of the systemic circulatory system. The pulmonary veins and venules that run from the lungs to the heart form part of the pulmonary circulatory system and are distinct from other veins in that they carry oxygenated blood.

Venae Cavae

The venae cavae are the veins with the largest diameter. Both enter the right atrium of the heart with the superior vena cava carrying blood from the arms, head, and thoracic cavity and the inferior vena cava carrying blood from the legs and abdomen. The inferior vena cava runs parallel to the abdominal aorta.

The superior vena cava is formed from the brachiocephalic veins which are in turn formed from the subclavian and internal jugular veins that serve the arm and head respectively. The inferior vena cava is formed from the common iliac veins that serve the legs and abdomen. The renal and hepatic veins from the kidneys and liver respectively also feed into the inferior vena cava.

Other Important Veins

Other important venous systems include the cardiac veins, which return blood from the heart tissue back to the general circulation. The cardiac veins merge into the coronary sinus, which empties directly into the right atrium.

The pulmonary veins are large blood vessels which receive oxygenated blood from the lungs and return it to the left atrium of the heart. There are four pulmonary veins, two from each lung, each of which forms from three to four bronchial veins. In approximately 25% of individuals, the left pulmonary veins may merge into a single vein; the same effect on the right side is only seen in approximately 3% of individuals.

The hepatic portal vein carries blood from the gastrointestinal tract to the liver. The portal vein is often described as a false vein because it conducts blood between capillary networks rather than between a capillary network and the heart. It functions to supply the liver with blood and required metabolites, but also ensures that ingested substances are first processed in the liver before reaching the wider systemic circulation.

Veins of the Head and Neck

In the head and neck, blood circulates from the upper systemic loop, which originates at the aortic arch.

Two main jugular veins are responsible for the venous draining of the head and neck.

The superficial external jugular vein is formed from the retromandibular vein and the posterior auricular vein at a point adjacent to the mandible. The external jugular vein passes down the neck and underneath the clavicle before draining into the subclavian vein.

The deep-lying internal jugular vein receives blood from the dural venous sinuses in the brain as well as the cerebral and cerebellar veins. Dural sinuses are composed of dural mater lined with endothelium, making them distinct from arteries, veins, and capillaries. The dural sinuses receive blood from the veins that drain the brain and skull.

Formed at the base of the brain from the inferior petrosal sinus and the sigmoid sinus, the internal jugular vein runs down the side of the neck adjacent to the internal carotid artery. As well as removing blood from the brain, the anterior retromandibular, facial, and lingual veins also drain into the internal jugular. Upon exiting the neck, the internal jugular vein merges with the subclavian vein to form the brachiocephalic vein.

Additional veins such as the occipital, deep cervical, and thyroid veins drain directly into the brachiocephalic vein.

Veins of the Upper Limbs

The veins of the upper extremity are divided into superficial and deep veins, indicating their relative depths from the skin.

Veins of the arm are either deep or superficial and are responsible for draining the hand and arm.

The major deep veins of the arm are the radial and ulnar veins, which run along the length of their respective bones and merge at the elbow to form the paired brachial vein. The brachial vein runs from the elbow up to the shoulder parallel to the brachial artery.

The major superficial veins of the upper limb are the cephalic, median cubital and basilic veins. The cephalic vein arises from the dorsal venous network of the hand and passes the elbow anteriorly, continuing up the upper arm to the shoulder. The basilic vein follows a similar path but is located medially to the cephalic vein. At the elbow, the basilic and cephalic veins are linked by the median cubital vein, from which blood is often drawn.

At the shoulder, the basilic vein passes deep into the arm and merges with the brachial veins to form the axillary vein, to which the cephaliac vein merges, forming the subclavian vein.

Veins of the Thorax

The veins of the thorax drain deoxygenated blood from the thorax region for return to the heart

Two venae cavae return deoxygenated blood from the systemic circulation to the right atrium of the heart.

The superior vena cava, formed from the left and right brachiocephalic veins, returns deoxygenated blood from the upper half of the body and carries blood from the upper limbs, head, and neck via the thyroid and jugular veins. It is joined just before entering the heart by the azygos vein, which runs up the right side of the thoracic vertebral column and transports blood from the external thoracic cavity.

The internal thoracic vein is a vessel that drains the chest wall and breasts. Bilaterally, it arises from the superior epigastric vein, accompanies the internal thoracic artery along its course, and terminates in the brachiocephalic vein.

The supreme intercostal vein is a paired vein that drains the first intercostal space on its corresponding side. It usually drains into the brachiocephalic vein.

The inferior vena cava returns blood from the abdomen and lower limbs to the right atrium of the heart. The renal veins from the kidney and hepatic veins of the liver drain directly into the inferior vena cava. Additionally, the superior and inferior phrenic veins drain the diaphragm and usually open into the internal mammary vein and inferior vena cava, respectively.

Veins of the Abdomen and Pelvis

The major veins of the abdomen and pelvis return deoxygenated blood from the abdomen and pelvis to the heart.

A number of veins remove deoxygenated blood from the abdomen and pelvis. The external iliac vein, the upward continuation of the femoral vein, passes upward along the pelvis and ends to form the common iliac vein.The tributaries of the external iliac vein are the inferior epigastric, deep iliac circumflex, and pubic veins.

The internal iliac vein begins near the upper part of the greater sciatic foramen, the large opening at the rear of the pelvis, passes upward behind and slightly medial to the internal iliac artery and, at the brim of the pelvis, joins with the external iliac vein to form the common iliac vein.

The left and right common iliac veins come together in the abdomen at the level of the fifth lumbar vertebra, forming the abdominal vena cava. They drain blood from the pelvis and lower limbs.

The superior epigastric vein refers to a blood vessel that carries deoxygenated blood and drains into the internal thoracic vein. It anastomoses with the inferior epigastric vein at the level of the umbilicus and drains the anterior part of the abdominal wall and some of the diaphragm.

The inferior epigastric vein refers to the vein that drains into the external iliac vein and arises from the superior epigastric vein.

The deep circumflex iliac vein is formed by the union of the venae comitantes of the deep iliac circumflex artery and joins the external iliac vein about 2 cm above the inguinal ligament.

Veins of the Lower Limbs

The deep veins of the lower extremity have valves for unidirectional flow and accompany the arteries and their branches.

Running the full length of the leg, making it the longest vein in the body, the great saphenous vein is a superficial vein that returns blood from the foot and superficial muscles of the leg before merging with the femoral vein to form the external iliac vein.

Types of Shock

Circulatory shock is a life-threatening medical condition that occurs due to inadequate substrate for aerobic cellular respiration.

Circulatory shock, commonly known simply as shock, is a life-threatening medical condition that occurs due to inadequate substrates for aerobic cellular respiration. In the early stages, this is generally caused by an inadequate tissue level of oxygen. The typical signs of shock are low blood pressure, a rapid heartbeat, and signs of poor end-organ perfusion or decompensation (such as low urine output, confusion, or loss of consciousness). In some people with circulatory shock, blood pressure remains stable.

The presentation of shock is variable with some people having only minimal symptoms such as confusion and weakness. While the general signs for all types of shock are low blood pressure, decreased urine output, and confusion, these may not always be present. Specific subtypes of shock may have additional symptoms.

Hypovolemic Shock

Hypovolemic shock, the most common type, is caused by insufficient circulating volume, typically from hemorrhage although severe vomiting and diarrhea are also potential causes.

Hypovolemic shock is graded on a four-point scale depending on the severity of symptoms and level of blood loss. Typical symptoms include a rapid, weak pulse due to decreased blood flow combined with tachycardia, cool, clammy skin, and rapid and shallow breathing.

Cardiogenic Shock

Cardiogenic shock is caused by a failure of the heart to pump correctly, either due to damage to the heart muscle through myocardial infarction or through cardiac valve problems, congestive heart failure, or dysrhythmia.

Obstructive Shock

Obstructive shock is caused by an obstruction of blood flow outside of the heart. This typically occurs due to a reduction in venous return, but may also be caused by blockage of the aorta.

Distributive Shock

Distributive shock is caused by an abnormal distribution of blood to tissues and organs and includes septic, anaphylactic, and neurogenic causes.

Septic

Septic shock is the most common cause of distributive shock and is caused by an overwhelming systemic infection that cannot be cleared by the immune system, resulting in vasodilation and hypotension.

Anaphylactic

Anaphylactic shock is caused by a severe reaction to an allergen, leading to the release of histamine that causes widespread vasodilation and hypotension.

Neurogenic

Neurogenic shock arises due to damage to the central nervous system, which impairs cardiac function by reducing heart rate and loosening the blood vessel tone, resulting in severe hypotension.

or

1. Distributive Shock

Characterized by peripheral vasodilatation.

Types of distributive shock include:

Septic Shock

Sepsis is defined as life-threatening organ dysfunction resulting from dysregulated host response to infection.[2] Septic shock is a subset of sepsis with severe circulatory, cellular, and metabolic abnormalities resulting in tissue hypoperfusion manifested as hypotension which requires vasopressor therapy and elevated lactate levels (more than 2 mmol/L)

The most common pathogens associated with sepsis and septic shock in the United States are gram-positive bacteria, including streptococcal pneumonia and Enterococcus.

Systemic Inflammatory Response Syndrome

Systemic inflammatory response syndrome (SIRS) is a clinical syndrome of the vigorous inflammatory response caused by either infectious or noninfectious causes. Infectious causes include pathogens such as gram-positive (most common) and gram-negative bacteria, fungi, viral infections (e.g., respiratory viruses), parasitic (e.g., malaria), rickettsial infections. Noninfectious causes of SIRS include but are not limited to pancreatitis, burns, fat embolism, air embolism, and amniotic fluid embolism

Anaphylactic Shock

Anaphylactic shock is a clinical syndrome of severe hypersensitivity reaction mediated by immunoglobulin E (Ig-E), resulting in cardiovascular collapse and respiratory distress due to bronchospasm. The immediate hypersensitivity reactions can occur within seconds to minutes after the presentation of the inciting antigen. Common allergens include drugs (e.g., antibiotics, NSAIDs), food, insect stings, and latex.

Neurogenic Shock

Neurogenic shock can occur in the setting of trauma to the spinal cord or the brain. The underlying mechanism is the disruption of the autonomic pathway resulting in decreased vascular resistance and changes in vagal tone.

Endocrine Shock

Due to underlying endocrine etiologies such as adrenal failure (Addisonian crisis) and myxedema.

2. Hypovolemic Shock

Hypovolemic shock is characterized by decreased intravascular volume and increased systemic venous assistance (compensatory the mechanism to maintain perfusion in the early stages of shock). In the later stages of shock due to progressive volume depletion, cardiac output also decreases and manifest as hypotension. Hypovolemic shock divides into two broad subtypes: hemorrhagic and non-hemorrhagic.

Common causes of hemorrhagic hypovolemic shock include

• Gastrointestinal bleed (both upper and lower gastrointestinal bleed (e.g., variceal bleed, portal hypertensive gastropathy bleed, peptic ulcer, diverticulosis) trauma
• Vascular etiologies (e.g., aortoenteric fistula, ruptured abdominal aortic aneurysm, tumor eroding into a major blood vessel)
• Spontaneous bleeding in the setting of anticoagulant use (in the setting of supratherapeutic INR from drug interactions)

Common causes of non-hemorrhagic hypovolemic shock include:

• GI losses – the setting of vomiting, diarrhea, NG suction, or drains.
• Renal losses – medication-induced diuresis, endocrine disorders such as hypoaldosteronism.
• Skin losses/insensible losses – burns, Stevens-Johnson syndrome, Toxic epidermal necrolysis, heatstroke, pyrexia.
• Third-space loss – in the setting of pancreatitis, cirrhosis, intestinal obstruction, trauma.

3. Cardiogenic Shock

Due to intracardiac causes leading to decreased cardiac output and systemic hypoperfusion. Different subtypes of etiologies contributing to cardiogenic shock include:

• Cardiomyopathies – include acute myocardial infarction affecting more than 40% of the left ventricle, acute myocardial infarction in the setting of multi-vessel coronary artery disease, right ventricular myocardial infarction, fulminant dilated cardiomyopathy, cardiac arrest (due to myocardial stunning), myocarditis.
• Arrhythmias – both tachy- and bradyarrhythmias
• Mechanical – severe aortic insufficiency, severe mitral insufficiency, rupture of papillary muscles, or chordae tendinae trauma rupture of ventricular free wall aneurysm.

4. Obstructive Shock

Mostly due to extracardiac causes leading to a decrease in the left ventricular cardiac output

• Pulmonary vascular – due to impaired blood flow from the right heart to the left heart. Examples include hemodynamically significant pulmonary embolism, severe pulmonary hypertension.[3]
• Mechanical – impaired filling of right heart or due to decreased venous return to the right heart due to extrinsic compression. Examples include tension pneumothorax, pericardial tamponade, restrictive cardiomyopathy, constrictive pericarditis.

Homeostatic Responses to Shock

An organism responds with numerous reactions during each of the four stages of shock in an attempt to maintain cellular homeostasis.

Circulatory shock, commonly known simply as shock, is a life-threatening medical condition that occurs due to inadequate substrate for aerobic cellular respiration. In the early stages this is generally an inadequate level of oxygen in the tissues.

There are four stages of shock. As it is a complex and continuous condition there is no sudden transition from one stage to the next.

Initial Stage

During the initial stage, the state of hypoperfusion causes hypoxia. Due to the lack of oxygen, the cells perform anaerobic respiration. Since oxygen is not abundant, the Kreb’s cycle is slowed, resulting in lactic acidosis (the accumulation of lactate).

Compensatory Stage

The compensatory stage is characterized by the employment of neural, hormonal, and biochemical mechanisms in the body’s attempt to reverse the lactic acidosis. The increase in acidity will initiate the Cushing reflex, generating the classic symptoms of shock. The individual will begin to hyperventilate to rid the body of carbon dioxide to raise the blood pH (lower the acidity). As a result, the baroreceptors in the arteries detect the hypotension and initiate the release of epinephrine and norepinephrine to increase heart rate and blood pressure.

Progressive Stage

Should the cause of the crisis not be successfully treated, the shock will proceed to the progressive stage, in which the compensatory mechanisms begin to fail. As anaerobic metabolism continues, increasing the body’s metabolic acidosis, the arteriolar smooth muscle and precapillary sphincters relax. Blood remains in the capillaries, leading to leakage of fluid and protein into the surrounding tissues. As fluid is lost, blood concentration and viscosity increase, causing blockage of the microcirculation. The prolonged vasoconstriction will also cause the vital organs to be compromised due to reduced perfusion. If the bowel becomes sufficiently ischemic, bacteria may enter the blood stream, resulting in the additional complication of endotoxic shock.

Refractory Stage

At the refractory stage, the vital organs have failed and shock can no longer be reversed. Brain damage and cell death are occurring, and death is imminent. Shock is irreversible at this point since a large amount of cellular ATP has been degraded into adenosine in the absence of oxygen as an electron receptor in the mitochondrial matrix. Adenosine easily perfuses out of cellular membranes into extracellular fluid, furthering capillary vasodilation, and then is transformed into uric acid. Because cells can only produce adenosine at a rate of about 2% of the cell’s total need per hour, even restoring oxygen is futile at this point because there is no adenosine to phosphorylate into ATP.

Signs and Symptoms of Shock

The clinical manifestation of shock varies depending on the type of shock and the individual, but there are some general symptoms.

The presentation of shock varies. Some people presenting only minimal symptoms, such as confusion and weakness. Typical symptoms of shock include elevated but weak heart rate, low blood pressure, and poor organ function, typically observed as low urine output, confusion or loss of consciousness.

While a fast heart rate is common, those on beta blockers and those who are athletic may have a normal or slow heart rate. This also occurs in 30% of cases of shock caused by abdominal bleeding. Specific subtypes of shock may have additional symptoms.

Hypovolemic shock results from the direct loss of effective circulating blood volume. This leads to a rapid, weak pulse due to decreased blood flow combined with tachycardia, stimulation of vasoconstriction, and cool, clammy skin. It also presents with acidosis as well as rapid, shallow breathing due to sympathetic nervous system stimulation. Hypothermia due to decreased perfusion and evaporation of sweat, and thirst and dry mouth due to fluid depletion, may also be present.

Gas Exchange

A vital example of gas exchange occurs between the terminal portions of the lungs and pulmonary capillaries. Therefore, pulmonary capillaries possess characteristics that allow for rapid and efficient diffusion. The capillaries optimize the diffusion rate by receiving a constant blood supply. They also have an average membrane thickness of only 0.6 micrometers and form a network of capillaries over the alveoli. Furthermore, the alveoli themselves have an extremely large surface area of seventy square meters to further increase the surface area available for diffusion.

However, common diseases can interfere with this optimization. A useful way of thinking about these diseases is to frame them with respect to the variables of Fick’s law. For example, some pulmonary diseases cause fibrosis or edema. This increases the diffusion distance that the molecule has to travel, thus decreasing the diffusion rate. Other diseases, such as emphysema, result in damage to the walls of the alveoli causing them to rupture. This consequently forms one larger air space and decreases the surface area available for gas exchange.

Finally, if the lungs are unable to ventilate correctly, such as in restrictive lung diseases, a shallower concentration gradient is established, and the diffusion rate is impaired.

Capillary Dynamics

Hydrostatic and osmotic pressure are opposing factors that drive capillary dynamics.

Capillary exchange refers to the exchange of material between the blood and tissues in the capillaries. There are three mechanisms that facilitate capillary exchange: diffusion, transcytosis, and bulk flow.

Capillary Exchange Mechanisms

Diffusion, the most widely-used mechanism, allows the flow of small molecules across capillaries such as glucose and oxygen from the blood into the tissues and carbon dioxide from the tissue into the blood. The process depends on the difference of gradients between the interstitium and blood, with molecules moving to low-concentrated spaces from high-concentrated ones.

Transcytosis is the mechanism whereby large, lipid-insoluble substances cross the capillary membranes. The substance to be transported is endocytosed by the endothelial cell into a lipid vesicle which moves through the cell and is then exocytosed to the other side.

Bulk flow is used by small, lipid-insoluble solutes in water to cross the capillary wall. The movement of materials across the wall is dependent on pressure and is bi-directional depending on the net filtration pressure derived from the four Starling forces that modulate capillary dynamics.

Capillary Dynamics

The four Starling forces modulate capillary dynamics.

• Oncotic or colloid osmotic pressure is a form of osmotic pressure exerted by proteins in the blood plasma or interstitial fluid.
• Hydrostatic pressure is the force generated by the pressure of fluid within or outside of the capillary on the capillary wall.

The net filtration pressure derived from the sum of the four forces described above determines the fluid flow into or out of the capillary. Movement from the bloodstream into the interstitium is favored by blood hydrostatic pressure and interstitial fluid oncotic pressure. Alternatively, movement from the interstitium into the bloodstream is favored by blood oncotic pressure and interstitial fluid hydrostatic pressure.

Due to the pressure of the blood in the capillaries, blood hydrostatic pressure is greater than interstitial fluid hydrostatic pressure, promoting a net flow of fluid from the blood vessels into the interstitium. However, because large plasma proteins, especially albumin, cannot easily cross through the capillary walls, their effect on the osmotic pressure of the capillary interiors will to some extent balance the tendency for fluid to leak from the capillaries.In conditions where plasma proteins are reduced (e.g. from being lost in the urine or from malnutrition), or blood pressure is significantly increased, a change in net filtration pressure and an increase in fluid movement across the capillary result in excess fluid build-up in the tissues (edema).

Transcytosis

Transcytosis is a process by which molecules are transported into the capillaries.

Transcytosis, or vesicle transport, is one of three mechanisms that facilitate capillary exchange, along with diffusion and bulk flow.

Substances are transported through the endothelial cells themselves within vesicles. This mechanism is mainly used by large molecules, typically lipid-insoluble preventing the use of other transport mechanisms. The substance to be transported is endocytosed by the endothelial cell into a lipid vesicle which moves through the cell and is then exocytosed to the other side. Vesicles are capable of merging, allowing for their contents to mix, and can be transported directly to specific organs or tissues.

Pathology

Due to the function of transcytosis, it can be a convenient mechanism by which pathogens can invade a tissue. Transcytosis has been shown to be critical to the entry of Cronobacter sakazakii across the intestinal epithelium and the blood-brain barrier.

Listeria monocytogenes has been shown to enter the intestinal lumen via transcytosis across goblet cells. Shiga toxin secreted by entero-hemorrhagic E. coli has been shown to be transcytosed into the intestinal lumen. These examples illustrate that transcytosis is vital to the process of pathogenesis for a variety of infectious agents.

Transcytosis in Pharmaceuticals

Pharmaceutical companies are currently exploring the use of transcytosis as a mechanism for transporting therapeutic drugs across the human blood-brain barrier. Exploiting the body’s own transport mechanism can help to overcome the high selectivity of this barrier, which blocks the uptake of most therapeutic antibodies into the brain and central nervous system.

Bulk Flow: Filtration and Reabsorption

Capillary fluid movement occurs as a result of diffusion (colloid osmotic pressure), transcytosis, and filtration.

Bulk flow is one of three mechanisms that facilitate capillary exchange, along with diffusion and transcytosis.

Bulk Flow Process

Bulk flow is used by small, lipid-insoluble solutes in water to cross the the capillary wall and is dependent on the physical characteristics of the capillary. Continuous capillaries have a tight structure reducing bulk flow. Fenestrated capillaries permit a larger amount of flow and discontinuous capillaries allow the largest amount of flow.

The movement of materials across the capillary wall is dependent on pressure and is bidirectional depending on the net filtration pressure derived from the four Starling forces.

When moving from the bloodstream into the interstitium, bulk flow is termed filtration, which is favored by blood hydrostatic pressure and interstitial fluid oncotic pressure. When moving from the interstitium into the bloodstream, the process is termed reabsorption and is favored by blood oncotic pressure and interstitial fluid hydrostatic pressure.

Modern evidence shows that in most cases, venular blood pressure exceeds the opposing pressure, thus maintaining a positive outward force. This indicates that capillaries are normally in a state of filtration along their entire length.

The Kidneys and Bulk Flow

The kidney is a major site for bulk flow transport. Blood that enters the kidneys is filtered by nephrons, the functional unit of the kidney. Each nephron begins in a renal corpuscle composed of a glomerulus containing numerous capillaries enclosed in a Bowman’s capsule. Proteins and other large molecules are filtered out of the oxygenated blood in the glomerulus and pass into Bowman’s capsule and the tubular fluid contained within. Blood continues to flow around the nephron until it reaches another capillary-rich region the peritubular capillaries, where the previously filtered molecules are reabsorbed from the tubule of the nephron.

Tubular reabsorption is the process by which solutes and water are removed from the tubular fluid and transported into the blood. Reabsorption is a two-step process beginning with the active or passive extraction of substances from the tubule fluid into the renal interstitium, and then the transport of these substances from the interstitium into the bloodstream

Introduction to Blood Pressure

Blood pressure is a vital sign reflecting the pressure exerted on blood vessels when blood is forced out of the heart during contraction.

Blood pressure is the pressure that blood exerts on the wall of the blood vessels. This pressure originates in the contraction of the heart, which forces blood out of the heart and into the blood vessels.

Two mechanisms take place in the heart: diastole and systole. Diastole is the relaxation of the chambers of the heart and systole is the contraction of the heart chambers. Systolic pressure is thus the pressure that your heart emits when blood is forced out of the heart and diastolic pressure is the pressure exerted when the heart is relaxed. This is the main mechanism by which blood pressure operates.

Blood pressure is one of the principal vital signs. During each heartbeat, blood pressure varies between a maximum (systolic) and a minimum (diastolic) pressure. Normal blood pressure should be around 120/80, with the systolic pressure expressed first.

Differences in mean blood pressure are responsible for blood flow from one location to another in circulation. The rate of mean blood flow depends on the resistance to flow presented by the blood vessels. Mean blood pressure decreases as circulating blood moves away from the heart through arteries, capillaries, and veins due to viscous loss of energy. Mean blood pressure decreases during circulation, although most of this decrease occurs along the small arteries and arterioles. Gravity affects blood pressure via hydrostatic forces (for example, during standing) Valves in veins, breathing, and pumping from contraction of skeletal muscles also influence venous blood pressure.

Arterial Blood Pressure

The measurement of blood pressure without further specification usually refers to systemic arterial pressure measured at the upper arm.

The measurement of blood pressure without further specification usually refers to the systemic arterial pressure, defined as the pressure exerted by circulating blood upon the walls of blood vessels. Pressure is typically measured with a blood pressure cuff ( sphygmomanometer ) wrapped around a person’s upper arm, which measures the pressure in the brachial artery. A person’s blood pressure is usually expressed in terms of the systolic pressure over diastolic pressure and is measured in millimeters of mercury (mmHg), for example, 140/90.

Blood pressure in the arteries is much higher than in the veins, in part due to receiving blood from the heart after contraction, but also due to their contractile capacity. The tunica media of arteries is thickened compared to veins, with smoother muscle fibers and elastic tissue. Together, these generate of elastic recoil and blood vessel contraction, allowing for the maintenance of higher pressure.

Blood Pressure and Cardiovascular Disease

While average values for arterial pressure could be computed for any given population, there is extensive variation from person to person and even from minute to minute for an individual. Additionally, the average arterial pressure of a given population has only a questionable correlation with its general health. However, in a study of 100 human subjects with no known history of hypertension, the average blood pressure of 112/64 mmHg, currently classified as a desirable or “normal” value. Normal values fluctuate through the 24-hour cycle, with the highest readings in the afternoons and lowest readings at night

The risk of cardiovascular disease increases progressively above 115/75 mmHg. In the past, hypertension was only diagnosed if secondary signs of high arterial pressure were present along with a prolonged high systolic pressure reading over several visits. Hypotension is typically diagnosed only if noticeable symptoms are present. Clinical trials demonstrate that people who maintain arterial pressures at the low end of these ranges have much better long-term cardiovascular health. The principal medical debate concerns the aggressiveness and relative value of methods used to lower pressures into this range for those with high blood pressure. Elevations more commonly seen in older people, though often considered normal, are associated with increased morbidity and mortality.

Arterial Hypertension

Arterial hypertension can be an indicator of other problems and may have long-term adverse effects. Sometimes it can be an acute problem, such as a hypertensive emergency. All levels of arterial pressure put mechanical stress on the arterial walls. Higher pressures increase heart workload and progression of unhealthy tissue growth (atheroma) that develops within the walls of arteries. The higher the pressure, the more stress that is present, the more the atheroma tends to progress, and the more heart muscle may thicken, enlarge, and weaken over time.

Persistent hypertension is one of the risk factors for strokes, heart attacks, heart failure, and arterial aneurysms, and is the leading cause of chronic renal failure. Even moderate elevation of arterial pressure leads to shortened life expectancy. At mean arterial pressures 50% or more above average, a person can expect to live no more than a few years unless appropriately treated.

In the past, most attention was paid to diastolic pressure, but now we know that both high systolic pressure and high pulse pressure (the numerical difference between systolic and diastolic pressures) are also risk factors for disease. In some cases, a decrease in excessive diastolic pressure can actually increase risk, probably due to the increased difference between systolic and diastolic pressures. If systolic blood pressure is elevated (>140) with a normal diastolic blood pressure (<90), it is called “isolated systolic hypertension” and may present a health concern.

Venous Blood Pressure

Venous pressure is the vascular pressure in a vein or the atria of the heart and is much lower than arterial pressure.

Blood pressure generally refers to the arterial pressure in the systemic circulation. However, measurement of pressures in the human venous system and the pulmonary vessels play an important role in intensive care medicine and are physiologically important in ensuring the proper return of blood to the heart, maintaining flow in the closed circulatory system.

Systemic Venous Pressure

Venous pressure is the vascular pressure in a vein or the atria of the heart. It is much lower than arterial pressure, with common values of 5 mmHg in the right atrium and 8 mmHg in the left atrium. Variants of venous pressure include:

• Central venous pressure, a good approximation of right atrial pressure, which is a major determinant of right ventricular end-diastolic volume.
• Jugular venous pressure (JVP), the indirectly observed pressure over the venous system. It can be useful in differentiating different forms of heart and lung disease.
• Portal venous pressure or the blood pressure in the portal vein. It is normally 5–10 mmHg.

Vein Structure and Function

In general, veins function to return deoxygenated blood to the heart and are essentially tubes that collapse when their lumens are not filled with blood. Compared with arteries, the tunica media of veins, which contains smooth muscle or elastic fibers allowing for contraction, is much thinner, resulting in a compromised ability to deliver pressure. The actions of the skeletal-muscle pump and the thoracic pump of breathing during respiration aid in the generation of venous pressure and the return of blood to the heart.

The pressure within the circulatory circuit as a whole is mean arterial pressure (MAP). This value is a function of the cardiac output (total blood pumped) and total peripheral resistance (TPR). TPR is primarily a function of the resistance of the systemic circulation. The resistance to flow generated by veins, due to their minimal ability to contract and reduce their diameter, means that regulation of blood pressure by veins is minimal in contrast to that of muscular vessels, primarily arterioles. The latter can actively contract, reduce the diameter, and increase resistance and pressure. In addition, veins can easily distend or stretch. A vein’s ability to increase in diameter in response to a given blood volume also contributes to the very low pressures within this segment of the circulatory system.

Pooling and Fainting

Standing or sitting for a prolonged period of time can cause low venous return in the absence of the muscle pump, resulting in venous pooling (vascular) and shock. Fainting can occur, but usually, baroreceptors within the aortic sinuses initiate a baroreflex, triggering angiotensin II and norepinephrine release and consequent vasoconstriction and heart rate increases to augment blood flow return.

Neurogenic and hypovolemic shock can also cause fainting. The smooth muscles surrounding the veins become slack and the veins fill with the majority of the blood in the body, keeping blood away from the brain and causing unconsciousness. Jet pilots wear pressurized suits to help maintain their venous return and blood pressure since high-speed maneuvers increase venous pooling in the legs. Pressure suits specifically squeeze the lower extremities, increasing venous return to the heart. This ensures that end-diastolic volumes are maintained and that the brain will receive adequate blood, preventing loss of consciousness.

Lifestyle

• Quit smoking. Your doctor can recommend programs and products to help.
• Follow a healthy diet. Eat a variety of fruits, vegetables, and whole grains, plus lean meat, poultry, fish, and low-fat/fat-free milk. Your diet should be low in fat, cholesterol, sodium, and sugar.
• Watch your weight. A daily record of your weight can help you be aware of rapid weight gain, which may be a sign that your pulmonary hypertension is worsening.
• Stay active. Incorporate physical activity such as walking into your lifestyle. Discuss the level of activity with your doctor. Avoid straining or lifting heavyweights. Rest when you need to.
• Avoid sitting in a hot tub or sauna, or taking long baths, which will lower your blood pressure.
• Be cautious about air travel or high-altitude locales. You may need to travel with extra oxygen.
• Get support for the anxiety and stress of living with pulmonary hypertension. Talk with your healthcare team, or ask for a referral to a counselor. A support group for people living with pulmonary hypertension can be invaluable in learning how to cope with the illness.

References

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Role of the Cardiovascular Center

The cardiovascular system plays a role in body maintenance by transporting hormones and nutrients and removing waste products.

The cardiovascular center forms part of the autonomic nervous system and is responsible for the regulation of cardiac output. Located in the medulla oblongata, the cardiovascular center contains three distinct components: the cardio accelerator center, the cardioinhibitory center, and the vasomotor center.

The cardio accelerator center stimulates cardiac function by regulating heart rate and stroke volume via sympathetic stimulation from the cardiac accelerator nerve. The cardioinhibitory center slows cardiac function by decreasing heart rate and stroke volume via parasympathetic stimulation from the vagus nerve. The vasomotor center controls vessel tone or contraction of the smooth muscle in the tunica media. Changes in diameter affect peripheral resistance, pressure, and flow, which in turn affect cardiac output. The majority of these neurons act via the release of the neurotransmitter norepinephrine from sympathetic neurons. Although each center functions independently, they are not anatomically distinct.

The cardiovascular center can respond to numerous stimuli. Hormones such as epinephrine and norepinephrine or changes in pH such as acidification due to carbon dioxide accumulation in tissue during exercise are detected by chemoreceptors. Baroreceptors that detect stretch can also signal to the cardiovascular center to alter heart rate.

Short-Term Neural Control

Neural regulation of blood pressure is achieved through the role of cardiovascular centers and baroreceptor stimulation.

The autonomic nervous system plays a critical role in the regulation of vascular homeostasis. The primary regulatory sites include the cardiovascular centers in the brain that control both cardiac and vascular functions.

Neurological regulation of blood pressure and flow depends on the cardiovascular centers located in the medulla oblongata. This cluster of neurons responds to changes in blood pressure as well as blood concentrations of oxygen, carbon dioxide, and other factors such as pH.

Baroreceptor Function

Baroreceptors are specialized stretch receptors located within thin areas of blood vessels and heart chambers that respond to the degree of stretch caused by the presence of blood. They send impulses to the cardiovascular center to regulate blood pressure. Vascular baroreceptors are found primarily in sinuses (small cavities) within the aorta and carotid arteries. The aortic sinuses are found in the walls of the ascending aorta just superior to the aortic valve, whereas the carotid sinuses are located in the base of the internal carotid arteries. There are also low-pressure baroreceptors located in the walls of the venae cavae and right atrium.

When blood pressure increases, the baroreceptors are stretched more tightly and initiate action potentials at a higher rate. At lower blood pressures, the degree of stretch is lower and the rate of firing is slower. When the cardiovascular center in the medulla oblongata receives this input, it triggers a reflex that maintains homeostasis.

Baroreceptor Reflexes

When blood pressure rises too high, baroreceptors fire at a higher rate and trigger parasympathetic stimulation of the heart. As a result, cardiac output falls. Sympathetic stimulation of the peripheral arterioles will also decrease, resulting in vasodilation. Combined, these activities cause blood pressure to fall.

When blood pressure drops too low, the rate of baroreceptor firing decreases. This triggers an increase in sympathetic stimulation of the heart, causing the cardiac output to increase. It also triggers sympathetic stimulation of the peripheral vessels, resulting in vasoconstriction. Combined, these activities cause blood pressure to rise.

The baroreceptors in the venae cavae and right atrium monitor blood pressure as the blood returns to the heart from the systemic circulation. Normally, blood flow into the aorta is the same as blood flow back into the right atrium. If blood is returning to the right atrium more rapidly than it is being ejected from the left ventricle, the atrial receptors will stimulate the cardiovascular centers to increase sympathetic firing and cardiac output until homeostasis is achieved. The opposite is also true. This mechanism is referred to as the atrial reflex.

Other neural mechanisms can also have a significant impact on cardiovascular function. These include the limbic system, which links physiological responses to psychological stimuli, chemoreceptor reflexes, generalized sympathetic stimulation, and parasympathetic stimulation.

Short-Term Chemical Control

Blood pressure is controlled chemically through dilation or constriction of the blood vessels by vasodilators and vasoconstrictors.

Many physical factors influence arterial pressure. Each may in turn be influenced by physiological factors such as diet, exercise, disease, drugs or alcohol, stress, and obesity. In practice, each individual’s autonomic nervous system responds to and regulates all of these interacting factors so that the actual arterial pressure response varies widely because of both split-second and slow-moving responses of the nervous system and end organs. These responses are very effective in changing the variables and resulting blood pressure from moment to moment.

Chemical Vasoconstriction

Vasoconstriction is the narrowing of blood vessels resulting from the contraction of the muscular wall of the vessels, particularly the large arteries and small arterioles. Generalized vasoconstriction usually results in an increase in systemic blood pressure, but may also occur in specific tissues, causing a localized reduction in blood flow.

The mechanism that leads to vasoconstriction results from the increased concentration of calcium (Ca2+ ions) and phosphorylated myosin within vascular smooth muscle cells. When stimulated, a signal transduction cascade leads to increased intracellular calcium from the sarcoplasmic reticulum through IP3 mediated calcium release, as well as enhanced calcium entry across the sarcolemma through calcium channels.

The rise in intracellular calcium interacts with calmodulin, which in turn activates myosin light chain kinase. This enzyme is responsible for phosphorylating the light chain of myosin to stimulate cross-bridge cycling. Once elevated, the intracellular calcium concentration is returned to its basal level through a variety of protein pumps and calcium exchanges located on the plasma membrane and sarcoplasmic reticulum. This reduction in calcium removes the stimulus necessary for contraction allowing for a return to baseline.

Endogenous vasoconstrictors include ATP, epinephrine, and angiotensin II.

Chemical Vasodilation

Vasodilation is the widening of blood vessels resulting from the relaxation of smooth muscle cells within the vessel walls, particularly in the large veins, large arteries, and smaller arterioles. Generalized vasodilation usually results in a decrease in systemic blood pressure, but may also occur in specific tissues causing a localized increase in blood flow.

The primary function of vasodilation is to increase blood flow in the body to tissues that need it most. This is often in response to a localized need for oxygen but can occur when the tissue in question is not receiving enough glucose, lipids, or other nutrients. Localized tissues increase blood flow by several methods, including the release of vasodilators, primarily adenosine, into the local interstitial fluid, which diffuses to capillary beds provoking local vasodilation. Some physiologists have suggested the lack of oxygen itself causes capillary beds to vasodilate by the smooth muscle hypoxia of the vessels in the region.

As with vasoconstriction, vasodilation is modulated by calcium ion concentration and myosin phosphorylation within vascular smooth muscle cells. Dephosphorylation by myosin light-chain phosphatase and induction of calcium symporters and antiporters that pump calcium ions out of the intracellular compartment both contribute to smooth muscle cell relaxation and therefore vasodilation. This is accomplished through the reuptake of ions into the sarcoplasmic reticulum via exchangers and expulsion across the plasma membrane. Endogenous vasodilators include arginine and lactic acid.

Long-Term Renal Regulation

Consistent and long-term control of blood pressure is determined by the renin-angiotensin system.

Along with vessel morphology, blood viscosity is one of the key factors influencing resistance and hence blood pressure. A key modulator of blood viscosity is the renin-angiotensin system (RAS) or the renin-angiotensin-aldosterone system (RAAS), a hormone system that regulates blood pressure and water balance.

When blood volume is low, juxtaglomerular cells in the kidneys secrete renin directly into circulation. Plasma renin then carries out the conversion of angiotensinogen released by the liver to angiotensin I. Angiotensin I is subsequently converted to angiotensin II by the enzyme angiotensin-converting enzyme found in the lungs. Angiotensin II is a potent vasoactive peptide that causes blood vessels to constrict, resulting in increased blood pressure. Angiotensin II also stimulates the secretion of the hormone aldosterone from the adrenal cortex.

Aldosterone causes the tubules of the kidneys to increase the reabsorption of sodium and water into the blood. This increases the volume of fluid in the body, which also increases blood pressure. If the renin-angiotensin-aldosterone system is too active, blood pressure will be too high. Many drugs interrupt different steps in this system to lower blood pressure. These drugs are one of the main ways to control high blood pressure (hypertension), heart failure, kidney failure, and the harmful effects of diabetes.

It is believed that angiotensin I may have some minor activity, but angiotensin II is the major bioactive product. Angiotensin II has a variety of effects on the body: throughout the body, it is a potent vasoconstrictor of arterioles.

Checking Circulation

Checking circulation involves the measurement of blood pressure and pulse through a variety of invasive and noninvasive methods.

Circulatory health can be measured in a variety of ways as follows.

Pulse

While a simple pulse rate measurement can be achieved by anyone, trained medical staff are capable of much more accurate measurements. The radial pulse is commonly measured using three fingers: the finger closest to the heart is used to occlude the pulse pressure, the middle finger is used to get a crude estimate of blood pressure, and the finger most distal to the heart is used to nullify the effect of the ulnar pulse as the two arteries are connected via the palmar arches.

Where more accurate or long-term measurements are required, pulse rate, pulse deficits, and much more physiologic data are readily visualized by the use of one or more arterial catheters connected to a transducer and oscilloscope. This invasive technique has been commonly used in intensive care since the 1970s. The rate of the pulse is observed and measured by tactile or visual means on the outside of an artery and recorded as beats per minute (BPM). The pulse may be further indirectly observed under light absorbencies of varying wavelengths with assigned and inexpensively reproduced mathematical ratios. Applied capture of variances of the light signal from the blood component hemoglobin under oxygenated vs. deoxygenated conditions allows the technology of pulse oximetry.

Heart Rate

Heart rate can be measured by listening to the heart directly through the chest, traditionally using a stethoscope. For more accurate or long-term measurements, electrocardiography may be used.

During each heartbeat, a healthy heart has an orderly progression of depolarization that starts with pacemaker cells in the sinoatrial (SA) node, spreads out through the atrium, passes through the atrioventricular node down into the bundle of His and into the Purkinje fibers, and down and to the left throughout the ventricles. This organized pattern of depolarization can be detected through electrodes placed on the skin and recorded as the commonly seen ECG tracing. ECG provides a very accurate means to measure heart rate, rhythm, and other factors such as chamber sizing, as well as identifying possible regions of damage.

Blood Pressure

Arterial pressure is most commonly measured via a sphygmomanometer, which historically used the height of a column of mercury to reflect the circulating pressure. Blood pressure values are generally reported in millimeters of mercury (mmHg), though aneroid and electronic devices do not use mercury. For each heartbeat, blood pressure varies between systolic and diastolic pressures. Systolic pressure is peak pressure in the arteries, which occurs near the end of the cardiac cycle when the ventricles are contracting. Diastolic pressure is minimum pressure in the arteries, which occurs near the beginning of the cardiac cycle when the ventricles are filled with blood. An example of normal measured values for a resting, healthy adult human is 120 mmHg systolic and 80 mmHg diastolic.

Hypertension refers to abnormally high arterial pressure, as opposed to hypotension, when it is abnormally low. Along with body temperature, respiratory rate, and pulse rate, blood pressure is one of the four main vital signs routinely monitored by medical professionals and healthcare providers.

Measuring pressure invasively by penetrating the arterial wall to take the measurement is much less common and usually restricted to a hospital setting. The noninvasive auscultatory and oscillometric measurements are simpler and faster than invasive measurements, require less expertise, have virtually no complications, are less unpleasant and painful for the patient. However, noninvasive methods may yield somewhat lower accuracy and small systematic differences in numerical results. Noninvasive measurement methods are more commonly used for routine examinations and monitoring.

The Auscultatory Method

The auscultatory method uses a stethoscope and a sphygmomanometer. This comprises an inflatable cuff placed around the upper arm at roughly the same vertical height as the heart, attached to a mercury or aneroid manometer. The mercury manometer, considered the gold standard, measures the height of a column of mercury, giving an absolute result without need for calibration.

A cuff of appropriate size is fitted smoothly and snugly, then inflated manually by repeatedly squeezing a rubber bulb until the artery is completely occluded. Listening with the stethoscope to the brachial artery at the elbow, the examiner slowly releases the pressure in the cuff. When blood just starts to flow in the artery, the turbulent flow creates a “whooshing” or pounding (first Korotkoff sound). The pressure at which this sound is first heard is the systolic blood pressure. The cuff pressure is further released until no sound can be heard (fifth Korotkoff sound), at the diastolic arterial pressure. The auscultatory method is the predominant method of clinical measurement.

Pulse

Pulse is a measurement of heart rate by touching and counting beats at several body locations, typically at the wrist radial artery.

The pulse is the physical expansion of an artery generated by the increase in pressure associated with systole of the heart. Pulse is often used as an equivalent of heart rate due to the relative ease of measurement; heart rate can be measured by listening to the heart directly through the chest, traditionally using a stethoscope.

Pulse rate or velocity is usually measured either at the wrist from the radial artery and is recorded as beats per minute (bpm). Other common measurement locations include the carotid artery in the neck and popliteal artery behind the knee

Pulse varies with age; a newborn or infant can have a heart rate of about 130-150 bpm. A toddler’s heart will beat about 100-120 times per minute, an older child’s heartbeat is around 60-100 bpm, adolescents around 80-100 bpm, and a healthy adults pulse rate is anywhere between 50 and 80 bpm.

The heart rate may be greater or less than the pulse rate depending upon physiologic demand. In this case, the heart rate is determined by auscultation or audible sounds at the heart apex, not the pulse. The pulse deficit (difference between heartbeats and pulsations at the periphery) is determined by simultaneous palpation at the radial artery and auscultation at the heart apex.

Measurement Techniques

While a simple measurement of pulse rate is achievable by anyone, trained medical staff are capable of much more accurate measurements. Radial pulse is commonly measured using three fingers: the finger closest to the heart used to occlude the pulse pressure, the middle finger used get a crude estimate of blood pressure, and the finger most distal to the heart used to nullify the effect of the ulnar pulse as the two arteries are connected via the palmar arches.

Where more accurate or long-term measurements are required, pulse rate, pulse deficits, and more physiologic data are readily visualized by the use of one or more arterial catheters connected to a transducer and oscilloscope. This invasive technique has been commonly used in intensive care since the 1970’s. The rate of the pulse is observed and measured by tactile or visual means on the outside of an artery and is recorded as beats per minute. The pulse may be further indirectly observed under light absorbencies of varying wavelengths with assigned and inexpensively reproduced mathematical ratios. Applied capture of variances of light signal from the blood component hemoglobin under oxygenated vs. deoxygenated conditions allows the technology of pulse oximetry.

Measuring Blood Pressure

Measurement of blood pressure includes systolic pressure during cardiac contraction and diastolic pressure during cardiac relaxation.

Blood pressure is the pressure blood exerts on the arterial walls. It is recorded as two readings: the systolic blood pressure (the top number) occurs during cardiac contraction, and the diastolic blood pressure or resting pressure (the bottom number), occurs between heartbeats when the heart is not actively contracting.

Normal blood pressure is about 120 mmHg systolic over 80 mmHg diastolic. Usually, the blood pressure is read from the left arm, although blood pressures are also taken at other locations along the extremities. These pressures, called segmental blood pressures, are used to evaluate blockage or arterial occlusion in a limb (for example, the ankle-brachial pressure index). The difference between systolic and diastolic pressure is called pulse pressure.

The measurement of these pressures is usually performed with an aneroid or electronic sphygmomanometer. The classic measurement device is a mercury sphygmomanometer, using a column of mercury measured off in millimeters. In the United States and the UK, the common form is millimeters of mercury (mm Hg), while elsewhere SI units of pressure are used. There is no natural or normal value for blood pressure, but rather a range of values that are associated with increased risks for disease and health:

• Hypotension: under 90 mmHg systolic and under 60 mmHg diastolic.
• Normal: 90–119 mmHg systolic and 60–79 mmHg diastolic.
• Prehypertensive: 120–139 mmHg systolic and 80–89 mmHg diastolic.
• Hypertensive: 140 mmHg and above systolic and 90 mmHg and above diastolic.

The guidelines for acceptable readings also take into account other cofactors for disease, such as pre-existing health factors. Therefore, hypertension is indicated when the systolic number is persistently over 140–160 mmHg. Low blood pressure, or hypotension, is indicated when the systolic number is persistently below 90 mmHg.

Extremes in Blood Pressure

Chronically elevated blood pressure is called hypertension, while chronically low blood pressure is called hypotension.

In healthy adults, physiological blood pressure should fall between the range of 100-140 mmHg systolic and 60-90 mmHg diastolic. Blood pressures above this are classed as hypertension and those below are hypotension, both considered medical conditions.

Hypertension

Hypertension or high blood pressure, sometimes called arterial hypertension, is a chronic medical condition in which the blood pressure in the arteries is elevated above 140/90 mmHg.

Hypertension is classified as either primary (essential) hypertension or secondary hypertension; about 90–95% of cases are categorized as “primary hypertension” which means high blood pressure with no obvious underlying medical cause. The remaining 5–10% of cases (secondary hypertension) are caused by other conditions that affect the kidneys, arteries, heart, or endocrine system.

Hypertension is a major risk factor for stroke, myocardial infarction (heart attacks), heart failure, aneurysms of the arteries (e.g. aortic aneurysm), peripheral arterial disease and a cause of chronic kidney disease. Even moderate elevation of arterial blood pressure is associated with a shortened life expectancy. Dietary and lifestyle changes can improve blood pressure control and decrease the risk of associated health complications, although drug treatment is often necessary in people for whom lifestyle changes prove ineffective or insufficient.

Hypotension

Hypotension is a medical condition in which the blood pressure in the arteries is reduced below 100/60 mmHg. Hypotension is best understood as a physiological state rather than a disease and is often associated with shock, though not necessarily indicative of it. However, blood pressure is considered too low only if noticeable symptoms are present.

For some people who exercise and are in top physical condition, hypotension is a sign of good health and fitness. For many people, low blood pressure can cause dizziness and fainting or indicate serious heart, endocrine, or neurological disorders. Severely low blood pressure can deprive the brain and other vital organs of oxygen and nutrients, leading to a life-threatening condition called shock.

References

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Introduction to Blood Flow, Pressure, Mechanisms

Blood flow

The heart is the driver of the circulatory system, pumping blood through rhythmic contraction and relaxation. The rate of blood flow out of the heart (often expressed in L/min) is known as cardiac output (CO).

Blood is pumped out of the heart first enters the aorta, the largest artery of the body. It then proceeds to divide into smaller and smaller arteries, then into arterioles, and eventually capillaries, where oxygen transfer occurs. The capillaries connect to venules, and the blood then travels back through the network of veins to the right heart. The micro-circulation — the arterioles, capillaries, and venules —constitutes most of the area of the vascular system and is the site of the transfer of O₂, glucose, and enzyme substrates into the cells. The venous system returns the deoxygenated blood to the right heart where it is pumped into the lungs to become oxygenated and CO₂ and other gaseous wastes exchanged and expelled during breathing. Blood then returns to the left side of the heart where it begins the process again.

In a normal circulatory system, the volume of blood returning to the heart each minute is approximately equal to the volume that is pumped out each minute (the cardiac output).^[rx] Because of this, the velocity of blood flow across each level of the circulatory system is primarily determined by the total cross-sectional area of that level. This is mathematically expressed by the following equation:

v = Q/A

where

• v = velocity (cm/s)
• Q = blood flow (ml/s)
• A = cross-sectional area (cm²)

Anatomical features

The circulatory system of species subjected to orthostatic blood pressure (such as arboreal snakes) has evolved with physiological and morphological features to overcome circulatory disturbance. For instance, in arboreal snakes, the heart is closer to the head, in comparison with aquatic snakes. This facilitates blood perfusion to the brain.^[rx][rx]

Turbulence

Blood flow is also affected by the smoothness of the vessels, resulting in either turbulent (chaotic) or laminar (smooth) flow. Smoothness is reduced by the buildup of fatty deposits on the arterial walls.

The Reynolds number (denoted NR or Re) is a relationship that helps determine the behavior of a fluid in a tube, in this case, blood in the vessel.

The equation for this dimensionless relationship is written as:^[rx]

{\displaystyle NR={\frac {\rho vL}{\mu }}}

• ρ: density of the blood
• v: mean velocity of the blood
• L: the characteristic dimension of the vessel, in this case, diameter
• μ: viscosity of blood

The Reynolds number is directly proportional to the velocity and diameter of the tube. Note that NR is directly proportional to the mean velocity as well as the diameter. A Reynolds number of less than 2300 is laminar fluid flow, which is characterized by constant flow motion, whereas a value of over 4000, is represented as turbulent flow.^[11] Due to its smaller radius and lowest velocity compared to other vessels, the Reynolds number at the capillaries is very low, resulting in laminar instead of turbulent flow.^[rx]

Velocity

Often expressed in cm/s. This value is inversely related to the total cross-sectional area of the blood vessel and also differs per cross-section, because in normal conditions the blood flow has laminar characteristics. For this reason, the blood flow velocity is the fastest in the middle of the vessel and slowest at the vessel wall. In most cases, the mean velocity is used.^[rx] There are many ways to measure blood flow velocities, like video capillary micro scoping with frame-to-frame analysis, or laser Doppler anemometry.^[rx] Blood velocities in arteries are higher during systole than during diastole. One parameter to quantify this difference is the pulsatility index (PI), which is equal to the difference between the peak systolic velocity and the minimum diastolic velocity divided by the mean velocity during the cardiac cycle. This value decreases with distance from the heart.^[rx]

{\displaystyle PI={\frac {v_{systole}-v_{diastole}}{v_{mean}}}}

Relation between blood flow velocity and total cross-section area in human

Type of blood vessels	Total cross-section area	Blood velocity in cm/s
Aorta	3–5 cm2	40 cm/s
Capillaries	4500–6000 cm2	0.03 cm/s[rx]
Vena cavae inferior and superior	14 cm2	15 cm/s

The circulatory system is the continuous system of tubes that pumps blood to tissues and organs throughout the body.

The circulatory system is the continuous system of tubes through which the blood is pumped around the body. It supplies the tissues with their nutritional requirements and removes waste products. The pulmonary circulatory system circulates deoxygenated blood from the heart to the lungs via the pulmonary artery and returns it to the heart via the pulmonary vein. The systemic circulatory system circulates oxygenated blood from the heart around the body into the tissues before returning deoxygenated blood to the heart.

Resistance, Pressure, and Flow

Three key factors influence blood circulation.

Resistance

Resistance to flow must be overcome to push blood through the circulatory system. If resistance increases, either pressure must increase to maintain flow, or flow rate must reduce to maintain pressure. Numerous factors can alter resistance, but the three most important are vessel length, vessel radius, and blood viscosity. With increasing length, increasing viscosity, and decreasing radius, resistance is increased. The arterioles and capillary networks are the main regions of the circulatory system that generate resistance, due to the small caliber of their lumen. Arterioles in particular are able to rapidly alter resistance by altering their radius through vasodilation or vasoconstriction.

The resistance offered by peripheral circulation is known as systemic vascular resistance (SVR), while the resistance offered by the vasculature of the lungs is known as pulmonary vascular resistance (PVR).

Blood Pressure

Blood pressure is the pressure that blood exerts on the wall of the blood vessels. The pressure originates in the contraction of the heart, which forces blood out of the heart and into the blood vessels. If the flow is impaired through increased resistance then blood pressure must increase, so blood pressure is often used as a test for circulatory health. Blood pressure can be modulated through altering cardiac activity, vasoconstriction, or vasodilation.

Blood Flow

Flow is the movement of the blood around the circulatory system. A relatively constant flow is required by the body’s tissues, so pressure and resistance are altered to maintain this consistency. A too-high flow can damage blood vessels and tissue, while flow that’s too low means tissues served by the blood vessel may not receive sufficient oxygen to function.

Distribution of Blood Flow

Humans have a closed cardiovascular system, meaning that blood never leaves the network of arteries, veins, and capillaries.

Humans have a closed cardiovascular system, meaning that the blood never leaves the network of arteries, veins, and capillaries. Blood is circulated through blood vessels by the pumping action of the heart, pumped from the left ventricle through arteries to peripheral tissues and returning to the right atrium through veins. It then enters the right ventricle and is pumped through the pulmonary artery to the lungs and returns to the left atrium through the pulmonary veins. Blood then enters the left ventricle to be circulated again.

Vasoconstriction is the narrowing of the blood vessels resulting from the contraction of the muscular wall of the vessels, particularly the large arteries and small arterioles. The process is the opposite of vasodilation, the widening of blood vessels. The process is particularly important in staunching hemorrhage and acute blood loss. When blood vessels constrict, the flow of blood is restricted or decreased, thus retaining body heat or increasing vascular resistance. This makes the skin turn paler because less blood reaches the surface, reducing the radiation of heat.

On a larger level, vasoconstriction is one mechanism by which the body regulates and maintains mean arterial pressure. Substances causing vasoconstriction are called vasoconstrictors or vasopressors. Generalized vasoconstriction usually results in an increase in systemic blood pressure, but it may also occur in specific tissues, causing a localized reduction in blood flow. The extent of vasoconstriction may be slight or severe depending on the substance or circumstance.

Vasodilation

Vasodilation refers to the widening of blood vessels resulting from the relaxation of smooth muscle cells within the vessel walls, particularly in the large veins, large arteries, and smaller arterioles. The process is essentially the opposite of vasoconstriction. When blood vessels dilate, the flow of blood is increased due to a decrease in vascular resistance. Therefore, dilation of arterial blood vessels (mainly the arterioles) causes a decrease in blood pressure. The response may be intrinsic (due to local processes in the surrounding tissue) or extrinsic (due to hormones or the nervous system). Additionally, the response may be localized to a specific organ (depending on the metabolic needs of a particular tissue, as during strenuous exercise), or it may be systemic (seen throughout the entire systemic circulation). Substances that cause vasodilation are termed vasodilators.

Introduction to Blood Flow, Pressure, and Resistance

Blood flow

The heart is the driver of the circulatory system, pumping blood through rhythmic contraction and relaxation. The rate of blood flow out of the heart (often expressed in L/min) is known as cardiac output (CO).

Blood being pumped out of the heart first enters the aorta, the largest artery of the body. It then proceeds to divide into smaller and smaller arteries, then into arterioles, and eventually capillaries, where oxygen transfer occurs. The capillaries connect to venules, and the blood then travels back through the network of veins to the right heart. The micro-circulation — the arterioles, capillaries, and venules —constitutes most of the area of the vascular system and is the site of the transfer of O₂, glucose, and enzyme substrates into the cells. The venous system returns the deoxygenated blood to the right heart where it is pumped into the lungs to become oxygenated and CO₂ and other gaseous wastes exchanged and expelled during breathing. Blood then returns to the left side of the heart where it begins the process again.

In a normal circulatory system, the volume of blood returning to the heart each minute is approximately equal to the volume that is pumped out each minute (the cardiac output).^[rx] Because of this, the velocity of blood flow across each level of the circulatory system is primarily determined by the total cross-sectional area of that level. This is mathematically expressed by the following equation:

v = Q/A

where

• v = velocity (cm/s)
• Q = blood flow (ml/s)
• A = cross-sectional area (cm²)

Anatomical features

The circulatory system of species subjected to orthostatic blood pressure (such as arboreal snakes) has evolved with physiological and morphological features to overcome circulatory disturbance. For instance, in arboreal snakes, the heart is closer to the head, in comparison with aquatic snakes. This facilitates blood perfusion to the brain.^[rx][rx]

Turbulence

Blood flow is also affected by the smoothness of the vessels, resulting in either turbulent (chaotic) or laminar (smooth) flow. Smoothness is reduced by the buildup of fatty deposits on the arterial walls.

The Reynolds number (denoted NR or Re) is a relationship that helps determine the behavior of a fluid in a tube, in this case, blood in the vessel.

The equation for this dimensionless relationship is written as:^[rx]

{\displaystyle NR={\frac {\rho vL}{\mu }}}

• ρ: density of the blood
• v: mean velocity of the blood
• L: the characteristic dimension of the vessel, in this case, diameter
• μ: viscosity of blood

The Reynolds number is directly proportional to the velocity and diameter of the tube. Note that NR is directly proportional to the mean velocity as well as the diameter. A Reynolds number of less than 2300 is laminar fluid flow, which is characterized by constant flow motion, whereas a value of over 4000, is represented as turbulent flow.^[11] Due to its smaller radius and lowest velocity compared to other vessels, the Reynolds number at the capillaries is very low, resulting in laminar instead of turbulent flow.^[rx]

Velocity

Often expressed in cm/s. This value is inversely related to the total cross-sectional area of the blood vessel and also differs per cross-section, because in normal conditions the blood flow has laminar characteristics. For this reason, the blood flow velocity is the fastest in the middle of the vessel and slowest at the vessel wall. In most cases, the mean velocity is used.^[rx] There are many ways to measure blood flow velocities, like video capillary micro scoping with frame-to-frame analysis, or laser Doppler anemometry.^[rx] Blood velocities in arteries are higher during systole than during diastole. One parameter to quantify this difference is the pulsatility index (PI), which is equal to the difference between the peak systolic velocity and the minimum diastolic velocity divided by the mean velocity during the cardiac cycle. This value decreases with distance from the heart.^[rx]

{\displaystyle PI={\frac {v_{systole}-v_{diastole}}{v_{mean}}}}

Relation between blood flow velocity and total cross-section area in human

Type of blood vessels	Total cross-section area	Blood velocity in cm/s
Aorta	3–5 cm2	40 cm/s
Capillaries	4500–6000 cm2	0.03 cm/s[rx]
Vena cavae inferior and superior	14 cm2	15 cm/s

The circulatory system is the continuous system of tubes that pumps blood to tissues and organs throughout the body.

The circulatory system is the continuous system of tubes through which the blood is pumped around the body. It supplies the tissues with their nutritional requirements and removes waste products. The pulmonary circulatory system circulates deoxygenated blood from the heart to the lungs via the pulmonary artery and returns it to the heart via the pulmonary vein. The systemic circulatory system circulates oxygenated blood from the heart around the body into the tissues before returning deoxygenated blood to the heart.

Resistance, Pressure and Flow

Three key factors influence blood circulation.

Resistance

Resistance to flow must be overcome to push blood through the circulatory system. If resistance increases, either pressure must increase to maintain flow, or flow rate must reduce to maintain pressure. Numerous factors can alter resistance, but the three most important are vessel length, vessel radius, and blood viscosity. With increasing length, increasing viscosity, and decreasing radius, resistance is increased. The arterioles and capillary networks are the main regions of the circulatory system that generate resistance, due to the small caliber of their lumen. Arterioles in particular are able to rapidly alter resistance by altering their radius through vasodilation or vasoconstriction.

The resistance offered by peripheral circulation is known as systemic vascular resistance (SVR), while the resistance offered by the vasculature of the lungs is known as pulmonary vascular resistance (PVR).

Blood Pressure

Blood pressure is the pressure that blood exerts on the wall of the blood vessels. The pressure originates in the contraction of the heart, which forces blood out of the heart and into the blood vessels. If the flow is impaired through increased resistance then blood pressure must increase, so blood pressure is often used as a test for circulatory health. Blood pressure can be modulated through altering cardiac activity, vasoconstriction, or vasodilation.

Blood Flow

Flow is the movement of the blood around the circulatory system. A relatively constant flow is required by the body’s tissues, so pressure and resistance are altered to maintain this consistency. A too-high flow can damage blood vessels and tissue, while flow that’s too low means tissues served by the blood vessel may not receive sufficient oxygen to function.

Distribution of Blood Flow

Humans have a closed cardiovascular system, meaning that blood never leaves the network of arteries, veins, and capillaries.

Humans have a closed cardiovascular system, meaning that the blood never leaves the network of arteries, veins, and capillaries. Blood is circulated through blood vessels by the pumping action of the heart, pumped from the left ventricle through arteries to peripheral tissues and returning to the right atrium through veins. It then enters the right ventricle and is pumped through the pulmonary artery to the lungs and returns to the left atrium through the pulmonary veins. Blood then enters the left ventricle to be circulated again.

Vasoconstriction is the narrowing of the blood vessels resulting from the contraction of the muscular wall of the vessels, particularly the large arteries and small arterioles. The process is the opposite of vasodilation, the widening of blood vessels. The process is particularly important in staunching hemorrhage and acute blood loss. When blood vessels constrict, the flow of blood is restricted or decreased, thus retaining body heat or increasing vascular resistance. This makes the skin turn paler because less blood reaches the surface, reducing the radiation of heat.

On a larger level, vasoconstriction is one mechanism by which the body regulates and maintains mean arterial pressure. Substances causing vasoconstriction are called vasoconstrictors or vasopressors. Generalized vasoconstriction usually results in an increase in systemic blood pressure, but it may also occur in specific tissues, causing a localized reduction in blood flow. The extent of vasoconstriction may be slight or severe depending on the substance or circumstance.

Vasodilation

Vasodilation refers to the widening of blood vessels resulting from the relaxation of smooth muscle cells within the vessel walls, particularly in the large veins, large arteries, and smaller arterioles. The process is essentially the opposite of vasoconstriction. When blood vessels dilate, the flow of blood is increased due to a decrease in vascular resistance. Therefore, dilation of arterial blood vessels (mainly the arterioles) causes a decrease in blood pressure. The response may be intrinsic (due to local processes in the surrounding tissue) or extrinsic (due to hormones or the nervous system). Additionally, the response may be localized to a specific organ (depending on the metabolic needs of a particular tissue, as during strenuous exercise), or it may be systemic (seen throughout the entire systemic circulation). Substances that cause vasodilation are termed vasodilators.

Introduction to Blood Flow, Pressure, and Resistance

Blood flow

The heart is the driver of the circulatory system, pumping blood through rhythmic contraction and relaxation. The rate of blood flow out of the heart (often expressed in L/min) is known as cardiac output (CO).

Blood being pumped out of the heart first enters the aorta, the largest artery of the body. It then proceeds to divide into smaller and smaller arteries, then into arterioles, and eventually capillaries, where oxygen transfer occurs. The capillaries connect to venules, and the blood then travels back through the network of veins to the right heart. The micro-circulation — the arterioles, capillaries, and venules —constitutes most of the area of the vascular system and is the site of the transfer of O₂, glucose, and enzyme substrates into the cells. The venous system returns the deoxygenated blood to the right heart where it is pumped into the lungs to become oxygenated and CO₂ and other gaseous wastes exchanged and expelled during breathing. Blood then returns to the left side of the heart where it begins the process again.

In a normal circulatory system, the volume of blood returning to the heart each minute is approximately equal to the volume that is pumped out each minute (the cardiac output).^[rx] Because of this, the velocity of blood flow across each level of the circulatory system is primarily determined by the total cross-sectional area of that level. This is mathematically expressed by the following equation:

v = Q/A

where

• v = velocity (cm/s)
• Q = blood flow (ml/s)
• A = cross-sectional area (cm²)

Anatomical features

The circulatory system of species subjected to orthostatic blood pressure (such as arboreal snakes) has evolved with physiological and morphological features to overcome circulatory disturbance. For instance, in arboreal snakes, the heart is closer to the head, in comparison with aquatic snakes. This facilitates blood perfusion to the brain.^[rx][rx]

Turbulence

Blood flow is also affected by the smoothness of the vessels, resulting in either turbulent (chaotic) or laminar (smooth) flow. Smoothness is reduced by the buildup of fatty deposits on the arterial walls.

The Reynolds number (denoted NR or Re) is a relationship that helps determine the behavior of a fluid in a tube, in this case, blood in the vessel.

The equation for this dimensionless relationship is written as:^[rx]

{\displaystyle NR={\frac {\rho vL}{\mu }}}

• ρ: density of the blood
• v: mean velocity of the blood
• L: the characteristic dimension of the vessel, in this case, diameter
• μ: viscosity of blood

The Reynolds number is directly proportional to the velocity and diameter of the tube. Note that NR is directly proportional to the mean velocity as well as the diameter. A Reynolds number of less than 2300 is laminar fluid flow, which is characterized by constant flow motion, whereas a value of over 4000, is represented as turbulent flow.^[11] Due to its smaller radius and lowest velocity compared to other vessels, the Reynolds number at the capillaries is very low, resulting in laminar instead of turbulent flow.^[rx]

Velocity

Often expressed in cm/s. This value is inversely related to the total cross-sectional area of the blood vessel and also differs per cross-section, because in normal conditions the blood flow has laminar characteristics. For this reason, the blood flow velocity is the fastest in the middle of the vessel and slowest at the vessel wall. In most cases, the mean velocity is used.^[rx] There are many ways to measure blood flow velocities, like video capillary micro scoping with frame-to-frame analysis, or laser Doppler anemometry.^[rx] Blood velocities in arteries are higher during systole than during diastole. One parameter to quantify this difference is the pulsatility index (PI), which is equal to the difference between the peak systolic velocity and the minimum diastolic velocity divided by the mean velocity during the cardiac cycle. This value decreases with distance from the heart.^[rx]

{\displaystyle PI={\frac {v_{systole}-v_{diastole}}{v_{mean}}}}

Relation between blood flow velocity and total cross-section area in human

Type of blood vessels	Total cross-section area	Blood velocity in cm/s
Aorta	3–5 cm2	40 cm/s
Capillaries	4500–6000 cm2	0.03 cm/s[rx]
Vena cavae inferior and superior	14 cm2	15 cm/s

The circulatory system is the continuous system of tubes that pumps blood to tissues and organs throughout the body.

The circulatory system is the continuous system of tubes through which the blood is pumped around the body. It supplies the tissues with their nutritional requirements and removes waste products. The pulmonary circulatory system circulates deoxygenated blood from the heart to the lungs via the pulmonary artery and returns it to the heart via the pulmonary vein. The systemic circulatory system circulates oxygenated blood from the heart around the body into the tissues before returning deoxygenated blood to the heart.

Resistance, Pressure and Flow

Three key factors influence blood circulation.

Resistance

Resistance to flow must be overcome to push blood through the circulatory system. If resistance increases, either pressure must increase to maintain flow, or flow rate must reduce to maintain pressure. Numerous factors can alter resistance, but the three most important are vessel length, vessel radius, and blood viscosity. With increasing length, increasing viscosity, and decreasing radius, resistance is increased. The arterioles and capillary networks are the main regions of the circulatory system that generate resistance, due to the small caliber of their lumen. Arterioles in particular are able to rapidly alter resistance by altering their radius through vasodilation or vasoconstriction.

The resistance offered by peripheral circulation is known as systemic vascular resistance (SVR), while the resistance offered by the vasculature of the lungs is known as pulmonary vascular resistance (PVR).

Blood Pressure

Blood pressure is the pressure that blood exerts on the wall of the blood vessels. The pressure originates in the contraction of the heart, which forces blood out of the heart and into the blood vessels. If the flow is impaired through increased resistance then blood pressure must increase, so blood pressure is often used as a test for circulatory health. Blood pressure can be modulated through altering cardiac activity, vasoconstriction, or vasodilation.

Blood Flow

Flow is the movement of the blood around the circulatory system. A relatively constant flow is required by the body’s tissues, so pressure and resistance are altered to maintain this consistency. A too-high flow can damage blood vessels and tissue, while flow that’s too low means tissues served by the blood vessel may not receive sufficient oxygen to function.

Distribution of Blood Flow

Humans have a closed cardiovascular system, meaning that blood never leaves the network of arteries, veins, and capillaries.

Humans have a closed cardiovascular system, meaning that the blood never leaves the network of arteries, veins, and capillaries. Blood is circulated through blood vessels by the pumping action of the heart, pumped from the left ventricle through arteries to peripheral tissues and returning to the right atrium through veins. It then enters the right ventricle and is pumped through the pulmonary artery to the lungs and returns to the left atrium through the pulmonary veins. Blood then enters the left ventricle to be circulated again.

Vasoconstriction is the narrowing of the blood vessels resulting from the contraction of the muscular wall of the vessels, particularly the large arteries and small arterioles. The process is the opposite of vasodilation, the widening of blood vessels. The process is particularly important in staunching hemorrhage and acute blood loss. When blood vessels constrict, the flow of blood is restricted or decreased, thus retaining body heat or increasing vascular resistance. This makes the skin turn paler because less blood reaches the surface, reducing the radiation of heat.

On a larger level, vasoconstriction is one mechanism by which the body regulates and maintains mean arterial pressure. Substances causing vasoconstriction are called vasoconstrictors or vasopressors. Generalized vasoconstriction usually results in an increase in systemic blood pressure, but it may also occur in specific tissues, causing a localized reduction in blood flow. The extent of vasoconstriction may be slight or severe depending on the substance or circumstance.

Vasodilation

Vasodilation refers to the widening of blood vessels resulting from the relaxation of smooth muscle cells within the vessel walls, particularly in the large veins, large arteries, and smaller arterioles. The process is essentially the opposite of vasoconstriction. When blood vessels dilate, the flow of blood is increased due to a decrease in vascular resistance. Therefore, dilation of arterial blood vessels (mainly the arterioles) causes a decrease in blood pressure. The response may be intrinsic (due to local processes in the surrounding tissue) or extrinsic (due to hormones or the nervous system). Additionally, the response may be localized to a specific organ (depending on the metabolic needs of a particular tissue, as during strenuous exercise), or it may be systemic (seen throughout the entire systemic circulation). Substances that cause vasodilation are termed vasodilators.

Introduction to Blood Flow, Pressure, and Resistance

Blood flow

The heart is the driver of the circulatory system, pumping blood through rhythmic contraction and relaxation. The rate of blood flow out of the heart (often expressed in L/min) is known as cardiac output (CO).

Blood being pumped out of the heart first enters the aorta, the largest artery of the body. It then proceeds to divide into smaller and smaller arteries, then into arterioles, and eventually capillaries, where oxygen transfer occurs. The capillaries connect to venules, and the blood then travels back through the network of veins to the right heart. The micro-circulation — the arterioles, capillaries, and venules —constitutes most of the area of the vascular system and is the site of the transfer of O₂, glucose, and enzyme substrates into the cells. The venous system returns the deoxygenated blood to the right heart where it is pumped into the lungs to become oxygenated and CO₂ and other gaseous wastes exchanged and expelled during breathing. Blood then returns to the left side of the heart where it begins the process again.

In a normal circulatory system, the volume of blood returning to the heart each minute is approximately equal to the volume that is pumped out each minute (the cardiac output).^[rx] Because of this, the velocity of blood flow across each level of the circulatory system is primarily determined by the total cross-sectional area of that level. This is mathematically expressed by the following equation:

v = Q/A

where

• v = velocity (cm/s)
• Q = blood flow (ml/s)
• A = cross-sectional area (cm²)

Anatomical features

The circulatory system of species subjected to orthostatic blood pressure (such as arboreal snakes) has evolved with physiological and morphological features to overcome circulatory disturbance. For instance, in arboreal snakes, the heart is closer to the head, in comparison with aquatic snakes. This facilitates blood perfusion to the brain.^[rx][rx]

Turbulence

Blood flow is also affected by the smoothness of the vessels, resulting in either turbulent (chaotic) or laminar (smooth) flow. Smoothness is reduced by the buildup of fatty deposits on the arterial walls.

The Reynolds number (denoted NR or Re) is a relationship that helps determine the behavior of a fluid in a tube, in this case, blood in the vessel.

The equation for this dimensionless relationship is written as:^[rx]

{\displaystyle NR={\frac {\rho vL}{\mu }}}

• ρ: density of the blood
• v: mean velocity of the blood
• L: the characteristic dimension of the vessel, in this case, diameter
• μ: viscosity of blood

The Reynolds number is directly proportional to the velocity and diameter of the tube. Note that NR is directly proportional to the mean velocity as well as the diameter. A Reynolds number of less than 2300 is laminar fluid flow, which is characterized by constant flow motion, whereas a value of over 4000, is represented as turbulent flow.^[11] Due to its smaller radius and lowest velocity compared to other vessels, the Reynolds number at the capillaries is very low, resulting in laminar instead of turbulent flow.^[rx]

Velocity

Often expressed in cm/s. This value is inversely related to the total cross-sectional area of the blood vessel and also differs per cross-section, because in normal conditions the blood flow has laminar characteristics. For this reason, the blood flow velocity is the fastest in the middle of the vessel and slowest at the vessel wall. In most cases, the mean velocity is used.^[rx] There are many ways to measure blood flow velocities, like video capillary micro scoping with frame-to-frame analysis, or laser Doppler anemometry.^[rx] Blood velocities in arteries are higher during systole than during diastole. One parameter to quantify this difference is the pulsatility index (PI), which is equal to the difference between the peak systolic velocity and the minimum diastolic velocity divided by the mean velocity during the cardiac cycle. This value decreases with distance from the heart.^[rx]

{\displaystyle PI={\frac {v_{systole}-v_{diastole}}{v_{mean}}}}

Relation between blood flow velocity and total cross-section area in human

Type of blood vessels	Total cross-section area	Blood velocity in cm/s
Aorta	3–5 cm2	40 cm/s
Capillaries	4500–6000 cm2	0.03 cm/s[rx]
Vena cavae inferior and superior	14 cm2	15 cm/s

The circulatory system is the continuous system of tubes that pumps blood to tissues and organs throughout the body.

The circulatory system is the continuous system of tubes through which the blood is pumped around the body. It supplies the tissues with their nutritional requirements and removes waste products. The pulmonary circulatory system circulates deoxygenated blood from the heart to the lungs via the pulmonary artery and returns it to the heart via the pulmonary vein. The systemic circulatory system circulates oxygenated blood from the heart around the body into the tissues before returning deoxygenated blood to the heart.

Resistance, Pressure and Flow

Three key factors influence blood circulation.

Resistance

Resistance to flow must be overcome to push blood through the circulatory system. If resistance increases, either pressure must increase to maintain flow, or flow rate must reduce to maintain pressure. Numerous factors can alter resistance, but the three most important are vessel length, vessel radius, and blood viscosity. With increasing length, increasing viscosity, and decreasing radius, resistance is increased. The arterioles and capillary networks are the main regions of the circulatory system that generate resistance, due to the small caliber of their lumen. Arterioles in particular are able to rapidly alter resistance by altering their radius through vasodilation or vasoconstriction.

The resistance offered by peripheral circulation is known as systemic vascular resistance (SVR), while the resistance offered by the vasculature of the lungs is known as pulmonary vascular resistance (PVR).

Blood Pressure

Blood pressure is the pressure that blood exerts on the wall of the blood vessels. The pressure originates in the contraction of the heart, which forces blood out of the heart and into the blood vessels. If the flow is impaired through increased resistance then blood pressure must increase, so blood pressure is often used as a test for circulatory health. Blood pressure can be modulated through altering cardiac activity, vasoconstriction, or vasodilation.

Blood Flow

Flow is the movement of the blood around the circulatory system. A relatively constant flow is required by the body’s tissues, so pressure and resistance are altered to maintain this consistency. A too-high flow can damage blood vessels and tissue, while flow that’s too low means tissues served by the blood vessel may not receive sufficient oxygen to function.

Distribution of Blood Flow

Humans have a closed cardiovascular system, meaning that blood never leaves the network of arteries, veins, and capillaries.

Humans have a closed cardiovascular system, meaning that the blood never leaves the network of arteries, veins, and capillaries. Blood is circulated through blood vessels by the pumping action of the heart, pumped from the left ventricle through arteries to peripheral tissues and returning to the right atrium through veins. It then enters the right ventricle and is pumped through the pulmonary artery to the lungs and returns to the left atrium through the pulmonary veins. Blood then enters the left ventricle to be circulated again.

Vasoconstriction is the narrowing of the blood vessels resulting from the contraction of the muscular wall of the vessels, particularly the large arteries and small arterioles. The process is the opposite of vasodilation, the widening of blood vessels. The process is particularly important in staunching hemorrhage and acute blood loss. When blood vessels constrict, the flow of blood is restricted or decreased, thus retaining body heat or increasing vascular resistance. This makes the skin turn paler because less blood reaches the surface, reducing the radiation of heat.

On a larger level, vasoconstriction is one mechanism by which the body regulates and maintains mean arterial pressure. Substances causing vasoconstriction are called vasoconstrictors or vasopressors. Generalized vasoconstriction usually results in an increase in systemic blood pressure, but it may also occur in specific tissues, causing a localized reduction in blood flow. The extent of vasoconstriction may be slight or severe depending on the substance or circumstance.

Vasodilation

Vasodilation refers to the widening of blood vessels resulting from the relaxation of smooth muscle cells within the vessel walls, particularly in the large veins, large arteries, and smaller arterioles. The process is essentially the opposite of vasoconstriction. When blood vessels dilate, the flow of blood is increased due to a decrease in vascular resistance. Therefore, dilation of arterial blood vessels (mainly the arterioles) causes a decrease in blood pressure. The response may be intrinsic (due to local processes in the surrounding tissue) or extrinsic (due to hormones or the nervous system). Additionally, the response may be localized to a specific organ (depending on the metabolic needs of a particular tissue, as during strenuous exercise), or it may be systemic (seen throughout the entire systemic circulation). Substances that cause vasodilation are termed vasodilators.

Venules Structure

Venule walls have three layers: An inner endothelium composed of squamous endothelial cells that act as a membrane, a middle layer of muscle and elastic tissue, and an outer layer of fibrous connective tissue. The middle layer is poorly developed so that venules have thinner walls than arterioles. They are porous so that fluid and blood cells can move easily from the bloodstream through their walls.

Short portal venules between the neural and anterior pituitary lobes provide an avenue for rapid hormonal exchange via the blood.^[rx] Specifically within and between the pituitary lobes is anatomical evidence for confluent interlope venules providing blood from the anterior to the neural lobe that would facilitate moment-to-moment sharing of information between lobes of the pituitary gland.^[rx]

In contrast to regular venules, high endothelial venules are a special type of venue where the endothelium is made up of simple cuboidal cells. Lymphocytes exit the bloodstream and enter the lymph nodes via these specialized venules when an infection is detected. Compared with arterioles, the venules are larger with a much weaker muscular coat. They are the smallest united common branch in the human body. Venules are small blood vessels in the microcirculation that connect capillary beds to veins.

Vein Classification

Veins are classified in a number of ways, including superficial vs. deep, pulmonary vs. systemic, and large vs. small:

• Superficial veins: Superficial veins are close to the surface of the body and have no corresponding arteries.
• Deep veins: Deep veins are deeper in the body and have corresponding arteries.
• Communicating veins: Communicating veins (or perforator veins) directly connect superficial veins to deep veins.
• Pulmonary veins: The pulmonary veins deliver oxygenated blood from the lungs to the heart.
• Systemic veins: Systemic veins drain the tissues of the body and deliver deoxygenated blood to the heart.

A venule is a small blood vessel in the microcirculation that allows deoxygenated blood to return from capillary beds to larger blood vessels called veins. Venules range from 8 to 100μm in diameter and are formed when capillaries come together. Many venules unite to form a vein.

Venule walls have three layers: an inner endothelium composed of squamous endothelial cells that act as a membrane, a middle layer of muscle and elastic tissue, and an outer layer of fibrous connective tissue. The middle layer is poorly developed so that venules have thinner walls than arterioles. Venules are extremely porous so that fluid and blood cells can move easily from the bloodstream through their walls.

In contrast to regular venules, high-endothelial venules (HEV) are specialized post-capillary venous swellings. They are characterized by plump endothelial cells as opposed to the usual thinner endothelial cells found in regular venules. HEVs enable lymphocytes (white blood cells) circulating in the blood to directly enter a lymph node by crossing through the HEV.

Veins

Veins are blood vessels that carry blood from tissues and organs back to the heart; they have thin walls and one-way valves.

Veins are blood vessels that carry blood towards the heart. Most carry deoxygenated blood from the tissues back to the heart, but the pulmonary and umbilical veins both carry oxygenated blood to the heart. The difference between veins and arteries is the direction of blood flow (out of the heart through arteries, back to the heart through veins), not their oxygen content. Veins differ from arteries in structure and function. For example, arteries are more muscular than veins, veins are often closer to the skin, and veins contain valves to help keep blood flowing toward the heart, while arteries do not have valves and carry blood away from the heart. The precise location of veins is much more variable than that of arteries, since veins often display anatomical variation from person to person.

Veins are also called capacitance vessels because they contain 60% of the body’s blood volume. In systemic circulation, oxygenated blood is pumped by the left ventricle through the arteries to the muscles and organs of the body, where its nutrients and gases are exchanged at capillaries. The blood then enters venules, then veins filled with cellular waste and carbon dioxide. The deoxygenated blood is taken by veins to the right atrium of the heart, which transfers the blood to the right ventricle, where it is then pumped through the pulmonary arteries to the lungs. In pulmonary circulation the veins return oxygenated blood from the lungs to the left atrium, which empties into the left ventricle, completing the cycle of blood circulation.

Mechanisms to Return Blood

The return of blood to the heart is assisted by the action of the skeletal-muscle pump and by the thoracic pump action of breathing during respiration. As muscles move, they squeeze the veins that run through them. Veins contain a series of one-way valves. As the vein is squeezed, it pushes blood through the valves, which then close to prevent backflow. Standing or sitting for prolonged periods can cause low venous return from venous pooling. In venous pooling, the smooth muscles surrounding the veins become slack and the veins fill with the majority of the blood in the body, keeping blood away from the brain, which can cause unconsciousness.

Although most veins take the blood back to the heart, portal veins carry blood between capillary beds. For example, the hepatic portal vein takes blood from the capillary beds in the digestive tract and transports it to the capillary beds in the liver. The blood is then drained in the gastrointestinal tract and spleen, where it is taken up by the hepatic veins and blood is taken back into the heart. Since this is an important function in mammals, damage to the hepatic portal vein can be dangerous. Blood clotting in the hepatic portal vein can cause portal hypertension, which results in a decrease of blood fluid to the liver.