Lungs

Structure

The lungs consist of lung parenchyma and supportive connective tissue. The right lung is shorter due to the position of the liver (2.5 cm difference); however, due to the position of the heart, the left lung is narrower and has a slightly reduced capacity and weight. The right lung weights 625 g and the left 565 g. However, this is proportionate to the stature of the individual, e.g., lungs weigh heavier in males than in females (Standring, 2016).

The lungs are conical in shape, with a superiorly located apex and an inferiorly located base. Each lung has three surfaces:

- medially located mediastinal surface;

- laterally located costal surface;

- inferiorly located diaphragmatic surface (or the base of the lung).

The three lung surfaces are separated by three borders:

- the anterior border separates the anterior end of the mediastinal surface from the costal surface;

- the posterior border separates the posterior end of the mediastinal surface from the costal surface;

- the inferior border separates the inferior ends of both the mediastinal and costal surfaces from the diaphragmatic surface.

Located along the mediastinal surface of each lung is the hilum. This is the site where the arteries, veins, nerves, and bronchi enter the left lung. The collective term for these structures that enter the lung through the hilum is the “root” of the lung.

Anatomical Relations

The apex of each lung is located:

- posterior to the subclavian artery, internal thoracic artery, and scalenus anterior muscle;

- anterior to the neck of the first rib, ventral ramus of first thoracic nerve, first posterior intercostal artery, first posterior intercostal vein, and the sympathetic chain;

- superior to the medial one third of clavicle, by approximately 2.5 cm.

The diaphragmatic surface (or the base) of each lung is located superior to the diaphragm. The right hemidiaphragm separates the right lung from the liver, while the left hemidiaphragm separates the left lung from the spleen and fundus of stomach. The costal surface of each lung lies deep to the ribs and innermost intercostal muscles. The mediastinal surface of each lung lies lateral to the mediastinum. The right lung lies lateral to the right atrium, superior vena cava, right brachiocephalic, azygos vein, trachea, esophagus, and the right phrenic and vagus nerves. The left lung lies lateral to the left ventricle, aorta, left subclavian artery, left brachiocephalic vein, trachea, esophagus, and left phrenic and vagus nerves.

Function

The lungs are involved in respiration, which is the process of gas exchange between the atmospheric air and the blood. The lungs consist of lung parenchyma and supportive connective tissue. The lung parenchyma is the functional tissue of the lungs, where gas exchange occurs with pulmonary capillaries; it is composed of alveolar ducts and alveoli.

Arterial Supply

The right and left bronchial arteries are distal branches of the thoracic aorta. They supply oxygenated blood to the tissues of the tracheobronchial tree and the visceral pleura.

Venous Drainage

The bronchial veins that drain the more proximal regions of the tracheobronchial tree drain into heart via azygos and accessory hemiazygos veins (i.e., via the systemic circulation). The bronchial veins that drain the more distal regions of the tracheobronchial tree form anastomoses with pulmonary veins and drain back to the heart via these veins (i.e., via the pulmonary circulation).

Pulmonary and Bronchial Circulation

The lungs receive their blood supply from two separate sources; the pulmonary circulation and the bronchial circulation.

- The pulmonary circulation is responsible for bringing deoxygenated blood from the body to the lungs for gas exchange, and bringing oxygenated blood back to the heart, which is then pumped into the systemic circulation.

- The bronchial circulation is a part of the systemic circulation. It is responsible for supplying oxygenated blood to the tissues of the tracheobronchial tree and visceral pleura. The anatomy of the bronchial circulation can vary between individuals. However, the branching pattern of both the bronchial arteries and veins generally follows that of the bronchi.

Innervation

The nerve supply to the lungs and visceral pleurae is via the pulmonary plexus. The pulmonary plexus provides autonomic (i.e., visceral) innervation to these structures, and it consists of both parasympathetic and sympathetic fibers.

- Parasympathetic stimulation within the respiratory system promotes bronchoconstriction, vasodilation of pulmonary blood vessels, and secretion from glands within the walls of the tracheobronchial tree.

- Sympathetic stimulation promotes bronchodilation, and vasoconstriction of pulmonary blood vessels.

Lymphatic Drainage

The lymphatic drainage system within the lungs can be divided into two plexuses; the superficial plexus and the deep plexus.

- The superficial lymphatic plexus (or subpleural plexus) is located within the lung parenchyma adjacent to the visceral pleura. This plexus provides lymphatic drainage to the superficial regions of the lungs and the distal ends of the tracheobronchial tree. It drains into the bronchopulmonary lymph nodes.

- The deep lymphatic plexus is located within the walls of the proximal tracheobronchial tree and the lung parenchyma adjacent to it. This plexus drains these areas, and, in turn, drains into the intrapulmonary lymph nodes.

List of Clinical Correlates

- Lung cancer

- Pulmonary collapse (atelectasis)

- Lung resection

References

Standring, S. (2016) Gray's Anatomy: The Anatomical Basis of Clinical Practice. Gray's Anatomy Series 41 edn.: Elsevier Limited.