The lungs are a pair of primary organs of respiration, present in the thoracic cavity beside the mediastinum. They are covered by a thin double-layered serous membrane called the pleura.
The respiratory system consists of two components, the conducting portion, and the respiratory portion. The conducting portion brings the air from outside to the site of the respiration. The respiratory portion helps in the exchange of gases and oxygenation of the blood.
The purpose of the lung is to provide oxygen to the blood. The respiratory system divides into airways and lung parenchyma. The airways consist of the bronchus, which bifurcates off the trachea and divides into bronchioles and then further into alveoli. The parenchyma is responsible for gas exchange and includes the alveoli, alveolar ducts, and bronchioles. Lungs have a spongy texture and have a pinkish-gray hue. Also, they are anatomically described as having an apex, three borders, and three surfaces. Further, they subdivide into lobes and segments. The lung parenchyma also is covered by a pleura.[rx][rx][rx]
Anatomy of The Lungs
Anatomically, the lung has an apex, three borders, and three surfaces. The apex lies above the first rib.
The three borders include the anterior, posterior, and inferior borders. The anterior border of the lung corresponds to the pleural reflection, and it creates a cardiac notch in the left lung. The cardiac notch is a concavity in the lung that formes to accommodate the heart. The inferior border is thin and separates the base of the lung from the costal surface. The posterior border is thick and extends from the C7 to the T10 vertebra, which is also from the apex of the lung to the inferior border.
The three surfaces of the lung include the costal, medial, and diaphragmatic surfaces. The costal surface is covered by the costal pleura and is along the sternum and ribs. It also joins the medial surface at the anterior and posterior borders and diaphragmatic surfaces at the inferior border. The medial surface is divided anteriorly and posteriorly. Anteriorly it is related to the sternum, and posteriorly it is related to the vertebra. The diaphragmatic surface (base) is concave and rests on the dome of the diaphragm; the right dome is also higher than the left dome because of the liver.
The right and left lung anatomy are similar but asymmetrical. The right lung consists of three lobes: the right upper lobe (RUL), the right middle lobe (RML), and the right lower lobe (RLL). The left lung consists of two lobes: the left upper lobe (LUL) and the left lower lobe (LLL). The right lobe is divided by an oblique and horizontal fissure, where the horizontal fissure divides the upper and middle lobe, and the oblique fissure divides the middle and lower lobes. In the left lobe, there is only an oblique fissure that separates the upper and lower lobe.
The lobes further divide into segments that are associated with specific segmental bronchi. Segmental bronchi are the third-order branches off the second-order branches (lobar bronchi) that come off the main bronchus.
The right lung consists of ten segments. There are three segments in the RUL (apical, anterior, and posterior), two in the RML (medial and lateral), and five in the RLL (superior, medial, anterior, lateral, and posterior). The oblique fissure separates the RUL from the RML, and the horizontal fissure separates the RLL from the RML and RUL.
There are eight to nine segments on the left, depending on the division of the lobe. In general, there are four segments in the left upper lobe (anterior, apicoposterior, inferior, and superior lingula) and four or five in the left lower lobe (lateral, anteromedial, superior, and posterior).
The hilum (root) is a depressed surface at the center of the medial surface of the lung and lies anteriorly to fifth through seventh thoracic vertebrae. It is the point at which various structures enter and exit the lung. The hilum is surrounded by pleura, which extends inferiorly and forms a pulmonary ligament. The hilum contains mostly bronchi and pulmonary vasculature, along with the phrenic nerve, lymphatics, nodes, and bronchial vessels. Both left and right hilum contain a pulmonary artery, pulmonary veins (superior and inferior), and bronchial arteries. Also, in the left hilum, there is one bronchus, the principal bronchus, and in the right hilum, there are two bronchi, the eparterial and hyparterial bronchi. From anterior to posterior, the order in the hilum is the vein, artery, and bronchus.
Structure of The Lungs
The function of the lung is to get oxygen from the air to the blood, performed by the alveoli. The alveoli are a single cell membrane that allows for gas exchange to the pulmonary vasculature. There are a couple of muscles that help with inspiration and expiration, such as the diaphragm and intercostal muscles. Sternocleidomastoid and scalene muscles are used for accessory respiration when the patient is in respiratory distress or failure. The muscles help create a negative pressure within the thorax, where the pressure of the lung is less than the atmospheric pressure, to help with inspiration and filling of the lungs. Also, the muscles help with creating a positive pressure within the thorax, where the pressure of the lung is greater than the atmospheric pressure, to help with expiration and emptying of the lung.
Blood Supply of The Lungs
The main distinction is between the pulmonary artery and bronchial arteries. The pulmonary artery takes deoxygenated blood from the heart to be oxygenated by the lung parenchyma. However, the bronchial arteries provide oxygen for survival to the lung parenchyma.
The main pulmonary artery emerges from the right ventricle and bifurcates into the left main and right main pulmonary arteries. The pulmonary artery branches usually trail and expand along the branches of the bronchial tree and eventually become capillaries around the alveoli. The pulmonary veins receive oxygenated blood from the alveoli capillaries and deoxygenated blood from the bronchial arteries and visceral pleura. Four pulmonary veins come together at the right atrium.
Bronchial circulation is part of the systemic circulation. The left bronchial artery arises as two (superior and inferior) from the thoracic aorta. The right brachial artery usually comes from one of the following three: the right posterior intercostal artery, with the left superior bronchial artery off the aorta or directly from the aorta. The bronchial veins collect the deoxygenated blood and empty it into the azygos vein.
The superficial and deep lymphatic plexuses drain the lung. The lymph flow from lung parenchyma first drains into the intraparenchymal nodes and then to the peribronchial nodes. Subsequently, the lymphatics will drain to the tracheobronchial, paratracheal lymph nodes, the bronchomediastinal trunk, and then into the thoracic duct.
Nerves Supply of The Lungs
The phrenic nerve comes from C3,4,5 cervical nerve roots. It innervates the fibrous pericardium, portions of the visceral pleura, and the diaphragm.
The lung receives innervation from two main sources: the pulmonary plexus (a combination of parasympathetic and sympathetic innervation) and the phrenic nerve. The pulmonary plexus is at the root of the lung and consists of efferent and afferent autonomic nerve fibers. It consists of branches of the vagus nerve (parasympathetic) and sympathetic fibers—the plexus branches around the pulmonary vasculature and bronchi. The parasympathetic innervation causes constriction of the bronchi, dilation of the pulmonary vessels, and increase gland secretion. The sympathetic innervation causes dilation of the bronchi and constriction of the pulmonary vessels.
Function of The Lungs
Respiratory functions of lungs
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Respiration – The lungs are the primary organs of respiration, where the exchange of gases takes place. In the alveoli, which are the functional units of lungs, the oxygen is taken up, and carbon dioxide is removed from the bloodstream through the alveolar-capillary bed [rx].
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Air conditioning – The function of conducting part is not only to lead the air but also acts as an efficient air conditioner. This conditioning is done by warming/cooling the inhaled air to bring it to the level of body temperature, humidifying the air, and also the removal of all the foreign particles present in it. The removal of foreign particles like dust, bacteria, virus, etc., is done by mucous secretion, which traps the suspended particles and beating of the cilia, which clears the mucus from the respiratory passage [rx].
Non-respiratory functions of lungs
Even though the lungs are primarily for respiration, studies indicate that they have many non-respiratory functions. Some of the few important ones are mentioned below.
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Converting an inactive chemical precursor into its active form, like turning angiotensin-I into angiotensin-II, which help in raising of blood pressure [rx].
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It is also an essential site for degrading/inactivating important vasoactive chemical mediators like bradykinin, serotonin, and norepinephrine
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Pulmonary epithelium acts as the first line of defense for the inspired air [rx].
The external intercostals are inspiratory muscles used primarily during exercise and respiratory distress. The significant lung volumes/capacities and their definitions are listed below[rx][rx][rx][rx]:
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Inspiratory reserve volume (IRV): Volume that can be breathed after a normal inspiration
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Tidal volume (TV): Volume inspired and expired with each breath
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Expiratory reserve volume (ERV): Volume that can be expired after a normal breath
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Residual volume (RV): Volume remaining in lung after maximal expiration (cannot be measured by spirometry)
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Inspiratory capacity (IC): Volume that can be breathed after normal exhalation
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Functional residual capacity (FRC): Volume remaining in the lungs after normal expiration
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Vital capacity (VC): Maximum volume able to be expired after maximal inspiration
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Total lung capacity (TLC): Volume of air in the lungs after maximal inspiration
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Forced expiratory volume (FEV1): Volume that can be expired in 1 second of maximum forced expiration
Issues of Concern
The lung is a primary location for a large proportion of human disease. Lung disease further classifies into obstructive and restrictive disease.
Obstructive Disease
The definition of obstructive disease is lung disease with impaired expiration. It presents with decreased FVC, decreased FEV1, and most notably, a dramatic decrease in FEV1/FVC. In obstructive disease, the air that should be expired is not, which leads to air trapping and an increased FRC. The two major examples of obstructive disease are listed below:
Asthma: a multifactorial disease characterized by chronic bronchial inflammation leading to eventual air trapping. Several key characteristics are as follows.
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Airway disease is mostly reversible (i.e., with the administration of a beta-agonist).
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Can cause chronic cough, wheeze, tachypnea, and dyspnea.
Chronic obstructive pulmonary disorder (COPD) – a constellation of clinical symptoms that share features of both emphysema and chronic bronchitis leading to expiratory airflow limitation.
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Chronic bronchitis demonstrates long-term airway inflammation causing excessive cough and sputum production.
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Emphysema characteristically shows enlarged airspaces (loss of alveolar elasticity) leading to chronic dyspnea. The overly-distended airspaces prevent the lungs from adequately emptying.
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Smoking is the primary cause of the disease and is directly related to the severity of the disease course.
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Cigarettes induce inflammation in the lungs.
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Airways show small airway disease and parenchymal destruction.
Restrictive Disease – Restrictive lung disease is lung disease in which restricted lung expansion causes decreased lung volumes. Its characteristics include both a decreased FVC and decreased FEV1; however, the FEV1 is more reduced than FVC, causing FEV1/FVC to increase. Several examples of restrictive lung disease are listed below[rx][rx][rx][rx][rx]:
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Idiopathic pulmonary fibrosis
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Pneumoconiosis
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Sarcoidosis
Cellular
Oxygen transport is the primary means by which the circulatory system perfuses tissue. Oxygen gets carried in the body in two major forms: bound to hemoglobin and dissolved. Hemoglobin is the major oxygen carrier in the body. The formula for the oxygen content of blood is as follows:
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CaO2 = 1.34 x [Hgb] x (SaO2 / 100) + 0.003 x PaO2
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CaO2 = oxygen content in blood
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[Hb] = hemoglobin concentration
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SaO2 = percentage of heme groups that are bound to oxygen
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PaO2 = Partial pressure of oxygen
Four subunits comprise hemoglobin, each containing a heme-moiety that binds to iron. One molecule of O2 can bind to each iron atom of the heme group; therefore, each hemoglobin group can bind to four molecules of O2.[rx][rx]
Development
Lung development in-utero occurs in five main stages. The first stage begins with the development of the lung bud from the respiratory diverticulum during week 4 of embryogenesis. The stages are as follows[rx][rx][rx][rx]
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Embryonic: begins from weeks 4 to 7; this is when the formation of the major airways and pleura occur.
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Psuedoglandular: occurs during weeks 5 to 17; this is when the bronchial tree and respiratory parenchyma form.
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Canalicular: occurs during weeks 16 to 26; the distal airway, blood-air barrier, surfactant, and acini form.
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Saccular: occurs during weeks 24 to 38 weeks; the airspaces (alveoli) continue to expand.
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Alveolar: occurs from week 36 of gestation and throughout childhood. The alveoli become septated and more mature – improving airspace and capillary networks.
References
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