The lungs are a pair of vital organs connected to the trachea via the left and right bronchi. The base of these organs meets the dome-shaped muscle known as the diaphragm. Encased by the pleurae, the lungs contact the mediastinum. The right lung is shorter yet wider, and has a larger volume than the left lung. The left lung has an indentation known as the cardiac notch. The superior region of the lungs is referred to as the apex, whereas the base is the lower region near the diaphragm. The costal surface is the part of the lung in contact with the ribs, and the mediastinal surface contacts the mediastinum.
The structure of each lung is further divided into smaller sections termed lobes, separated by fissures. The right lung has three lobes: the superior middle and inferior lobes. The left lung has only a superior and inferior lobe. Each lobe consists of multiple bronchopulmonary segments, each supplied by a separate tertiary bronchus and artery. As the bronchi branch into bronchioles, they form subdivisions known as pulmonary lobules, each receiving a bronchiole with multiple branches. An interlobular septum, a connective tissue wall, separates one lobule from another.
The lungs' blood supply is pivotal for gas exchange and is a transportation path for gaseous exchange in the lungs. The parasympathetic and sympathetic nervous systems provide critical airway control through constriction and dilation, respectively. The pulmonary circulation transports deoxygenated blood to the lungs, where erythrocytes absorb oxygen. The pulmonary artery, branching out from the pulmonary trunk, carries this deoxygenated blood to the alveoli. The pulmonary artery branches continuously, mirroring the structure of the bronchi, with each branch reducing in diameter. One arteriole and one venule service each pulmonary lobule. Upon nearing the alveoli, the pulmonary arteries become the pulmonary capillary network. These capillaries, devoid of smooth muscle fibers, form the respiratory membrane when the capillary wall meets the alveolar wall. Once oxygenated, blood drains from the alveoli via numerous pulmonary veins, exiting the lungs through the hilum.
The parasympathetic and sympathetic nervous systems govern the dilation and constriction of the airways. The parasympathetic system induces bronchoconstriction, while the sympathetic system induces bronchodilation. Reflex actions such as coughing and the lungs' ability to maintain oxygen and carbon dioxide levels are controlled by these autonomic nervous systems. Sensory nerve fibers originate from the vagus nerve and the second to fifth thoracic ganglia. These nerves enter the lungs at the hilum, forming the pulmonary plexus at the root of the lung. From there, the nerves follow the bronchi within the lungs, branching to innervate muscle fibers, glands, and blood vessels.
