Each lung is a sponge-like cone, pink in children and grey in adults. Its APEX projects into the neck, with the base resting on the DIAPHRAGM. Each lung is enveloped by a closed cavity, the pleural cavity, consisting of two layers of pleural membrane separated by a thin layer of fluid. In healthy states, this allows expansion and retraction as breathing occurs.

The HEART lies in contact with the two lungs, so that changes in lung volume inevitably affect the pumping action of the heart. Furthermore, both lungs are connected by blood vessels to the heart. The pulmonary artery passes from the right ventricle and divides into two branches, one of which runs straight outwards to each lung, entering its substance along with the bronchial tube at the hilum or root of the lung. From this point also emerge the pulmonary veins, which carry the blood oxygenated in the lungs back to the left atrium.

Each main bronchial tube, entering the lung at the root, divides into branches. These subdivide repeatedly until the finest tubes, respiratory bronchioles, have a width of only 0·25 mm (1/100 inch). All these tubes consist of a mucous membrane surrounded by a fibrous sheath. The surface of the mucous membrane comprises columnar cells provided with cilia (hair-like structures) which sweep mucus and unwanted matter such as bacteria to the exterior.

The bronchioles divide into a number of tortuous tubes known as alveolar ducts; these terminate eventually in minute sacs, known as alveoli, of which there are around 300 million.

The branches of the pulmonary artery accompany the bronchial tubes to the furthest recesses of the lung, dividing like the latter into progressively fine branches, and ending in a dense network of capillaries. The air in the air-vesicles is therefore separated from the blood only by two delicate membranes: the wall of the air-vesicle, and the capillary wall, through which exchange of gases (oxygen and carbon dioxide) readily takes place. The essential oxygenated blood from the capillaries is collected by the pulmonary veins, which also accompany the bronchi to the root of the lung.

The lungs also contain an important system of lymph vessels, which originate in spaces situated between the air-vesicles, and eventually leave the lung along with the blood vessels, and are connected with a chain of bronchial glands lying near the end of the TRACHEA.