lungs, elastic organs used for breathing in vertebrate animals, excluding most fish, which use gills, and a few amphibian species that respire through the skin. The word is sometimes applied to the respiratory apparatus of lower animals.

The human lungs are paired organs, located on either side of the heart and occupying a large portion of the chest cavity from the collarbone to the diaphragm. Air enters the body through a series of passages, beginning with the nose or mouth. It travels to the chest cavity through the trachea, which divides into two bronchi, each of which enters a lung. The bronchi divide and subdivide into a network of countless tubules. The smallest tubules, or bronchioles, enter cup-shaped air sacs known as alveoli, which number about 700 million in both lungs. Each alveolus is surrounded by a net of capillaries. As blood flows through these vessels, carbon dioxide passes into the alveoli, and oxygen diffuses into the bloodstream. The capillaries are part of a vast network of pulmonary blood vessels that connect the lungs directly to the heart via the large pulmonary arteries and veins. The alveoli are clustered in groups, or lobules, and the lobules are clustered into lobes.

In humans, the left lung has two lobes; the right lung three. The lungs are covered by a thin membrane called the pleura. They are expanded and contracted (thereby inhaling and exhaling air) by the combined movement of the diaphragm and the rib cage, which is alternately raised (expansion) and lowered (contraction) by the chest muscles. In recent years, smoking has been found to cause severe and sometimes fatal diseases of the lung, such as cancer and emphysema. Pneumonia is an inflammation of the lung tissue caused by various agents or organisms such as viruses. Asthma, a hypersensitivity or allergic response to some stimuli, covers a range of severity and is characterized by bronchial spasms and difficult breathing. See respiration.

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