Lung

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. 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.

Air travels to the lungs through a series of air tubes and passages. It enters the body through the nostrils or the mouth, passing down the throat to the larynx, or voice box, and then to the trachea, or windpipe. In the chest cavity the trachea divides into two branches, called the right and left bronchi or bronchial tubes, that enter the lungs.