Lung, either of a pair of elastic, spongy organs used in breathing and respiration. Lungs are present in all mammals, birds, and reptiles. Most amphibians and a few species of fish also have lungs.

 In humans the lungs occupy a large portion of the chest cavity from the collarbone down to the diaphragm, a dome-shaped sheet of muscle that walls off the chest cavity from the abdominal cavity. At birth the lungs are pink, but as a person ages, they become gray and mottled from tiny particles breathed in with the air. Generally, people who live in cities and industrial areas have darker lungs than those who live in the country.

 In humans the lungs occupy a large portion of the chest cavity from the collarbone down to the diaphragm. The right lung is divided into three sections, or lobes. The left lung, with a cleft to accommodate the heart, has only two lobes. The two branches of the trachea, called bronchi, subdivide within the lobes into smaller and smaller air vessels known as bronchioles. Bronchioles terminate in alveoli, tiny air sacs surrounded by capillaries. When the alveoli inflate with inhaled air, oxygen diffuses into the blood in the capillaries to be pumped by the heart to the tissues of the body. At the same time carbon dioxide diffuses out of the blood into the lungs, where it is exhaled.