Venous System – Anatomy, Structure, Functions

Venous System – Anatomy, Structure, Functions

The venous system refers to the network of veins that work to deliver deoxygenated blood back to your heart. Veins are a type of blood vessel that returns deoxygenated blood from your organs back to your heart. These are different from your arteries, which deliver oxygenated blood from your heart to the rest of your body.

Deoxygenated blood that flows into your veins is collected within tiny blood vessels called capillaries. Capillaries are the smallest blood vessels in your body. Oxygen passes through the walls of your capillaries to your tissues. Carbon dioxide can also move into your capillaries from the tissue before entering your veins.

Venules

Venules are small blood vessels in the microcirculation that connect capillary beds to veins.

Key Points

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.

High-endothelial venules are specialized post-capillary venous swellings characterized by plump endothelial cells, in contrast with the thinner endothelial cells found in regular venues.

Key Terms

  • high endothelial venule: A specialized post-capillary venous swelling of the lymphatic system that allows lymphocytes (white blood cells) to easily exit the circulatory system.
  • venule: A small blood vessel in the microcirculation that allows deoxygenated blood to return from capillary beds to veins.

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.

This diagram indicates capillaries, arteries, arterioles, venules, tissue cells, and veins.

Venule: Venules form when capillaries come together and converging venules from 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 venues. 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.

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Key Points

The difference between veins and arteries is the direction of blood flow (out of the heart through arteries, returning to the heart through veins).

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.

Veins are also called capacitance vessels because they contain 60% of the body’s blood volume.

The return of blood to the heart is assisted by the action of the skeletal- muscle pump. As muscles move, they squeeze the veins running through them. Veins contain a series of one-way valves, and they are squeezed, blood is pushed through the valves, which then close to prevent backflow.

Key Terms

  • venous pooling: When blood accumulates in the lower extremities, resulting in low venous return to the heart which can result in fainting.
  • skeletal-muscle pump: Rhythmic contraction of limb muscles that occurs during normal locomotory activity (walking, running, swimming), which promotes venous return by the pumping action on veins within muscles.
  • portal vein: A short, wide vein that carries blood to the liver from the organs of the digestive system.

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 the 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.

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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.

image

Venous valve: Venous valves prevent backflow and ensure that blood flows in one direction.

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 into 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.

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.

Which conditions affect the venous system?

Many conditions can affect your venous system. Some of the most common ones include:

  • Deep vein thrombosis (DVT). A blood clot forms in a deep vein, usually in your leg. This clot can potentially travel to your lungs, causing pulmonary embolism.
  • Superficial thrombophlebitis. An inflamed superficial vein, usually in your leg, develops a blood clot. While the clot can occasionally travel to a deep vein, causing DVT, thrombophlebitis is generally less serious than DVT.
  • Varicose veins. Superficial veins near the surface of the skin visibly swell. This happens when one-way valves break down or vein walls weaken, allowing blood to flow backward.
  • Chronic venous insufficiency. Blood collects in the superficial and deep veins of your legs due to the improper functioning of one-way valves. While similar to varicose veins, chronic venous insufficiency usually causes more symptoms, including coarse skin texture and ulcers in some cases.
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What are the symptoms of a venous condition?

While the symptoms of a venous condition can vary widely, some common ones include:

  • inflammation or swelling
  • tenderness or pain
  • veins that feel warm to the touch
  • a burning or itching sensation

These symptoms are especially common in your legs. If you notice any of these and they don’t improve after a few days, make an appointment with your doctor.

They can perform venography. In this procedure, your doctor injects contrast die into your veins to produce an X-ray image of a particular area.

Tips for healthy veins

Follow these tips to keep your vein walls and valves strong and properly functioning:

  • Get regular exercise to keep blood moving through your veins.
  • Try to maintain a healthy weight, which reduces your risk of high blood pressure. High blood pressure can weaken your veins over time due to added pressure.
  • Avoid long periods of standing or sitting. Try to change positions regularly throughout the day.
  • When sitting down, avoid crossing your legs for long periods of time or regularly switch positions so one leg isn’t on top for a long period of time.
  • When flying, drink plenty of water and try to stand up and stretch as often as possible. Even while sitting, you can flex your ankles to encourage blood flow.

Reference

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