The lungs have dual arterial supply and venous drainage: pulmonary arteries and veins as well as bronchial arteries and veins: In any organ, the parenchyma refers the functional tissues thus, the strict definition of lung parenchyma only includes the alveoli, alveolar ducts, and respiratory bronchioles. It is composed of respiratory bronchioles, alveolar ducts and alveoli, and is the location of gas transfer within the lung. The respiratory zone is an extension of the tracheobronchial tree at the level of the terminal bronchioles. Their walls contain cilia to remove particulates from the inspired gas and cartilage to ensure that they do not collapse in expiration. Their function is to optimize gas delivery to the functional portion of the lung. The conduction zone is composed of the trachea, bronchi, bronchioles and terminal bronchioles. Gas is warmed and humidified as it is conducted from the oropharynx to the functional portion of the lung where gas exchange occurs.
It can broadly be divided into conduction and respiratory zones. The tracheobronchial tree is a branching structure of tubes of an ever-decreasing diameter that start at the larynx and end in the alveoli. Each lung has an oblique fissure with a horizontal fissure also present in the right lung. The lobes of the lungs are also incompletely separated by fissures. See main article: bronchopulmonary segmental anatomy. They are surrounded by the pleura which separates them from the chest wall. In most individuals, the right lung is composed of three lobes subdivided into 10 segments and the left is composed of two lobes and eight segments. The lungs lie in the thoracic cavity, separated by the heart and mediastinum. While the two lungs are similar, they are not completely symmetrical, having a different number of lobes and a different bronchial and vascular anatomy.