Each lung is enveloped in a double-folded membrane called the pleura. The outer layer of the pleura, nearest the ribs, is the parietal pleura; and the inner layer, closest to the lung, is the visceral pleura.

The right lung, which is the slightly larger of the two, is divided into three lobes and the left lung is divided into two lobes. The uppermost part of the lung is called the apex, and the lower area is the base. The hilum or hilus of the lung is the midline region where blood vessels, nerves, lymphatic tissue, and bronchial tubes enter and exit the organ.

The lungs extend from the collarbone to the diaphragm in the thoracic cavity. The downward movemement of the diaphragm enlarges the area in the thoracic cavity and reduces the internal air pressure, so that air flows into the lungs to equalize the pressure. When the lungs are full, the diaphragm relaxes and elevates, making the area in the thoracic cavity smaller, and thus increasing the pressure; this is called exhalation (expiration).