lung

In mammals, large cavity of the body, used for gas exchange. Most tetrapod (four-limbed) vertebrates have a pair of lungs occupying the thorax. The lungs are essentially an infolding of the body surface – a sheet of thin, moist membrane made of a single layer of cells, which is folded so as to occupy less space while having a large surface area for the uptake of oxygen and loss of carbon dioxide. The folding creates tiny sacs called alveoli. Outside the walls of the alveoli there are lots of blood capillaries for transporting the products of gas exchange. The lung tissue, consisting of multitudes of air sacs and blood vessels, is very light and spongy, and functions by bringing inhaled air into close contact with the blood for efficient gas exchange. The efficiency of lungs is enhanced by breathing movements, by the thinness and moistness of their surfaces, and by a constant supply of circulating blood.

The lungs inflate and deflate as a result of breathing movements (ventilation). Breathing movements are caused by movements of muscles between the ribs and the muscles of the diaphragm. Air flows into the mouth and then along ever narrower tubes, trachea, bronchi, and tiny broncheoles. However, the last part of the journey to the alveoli is by diffusion only, as is the exchange with the blood.

Dust in the air is usually trapped by the mucus lining the tubes leading to the lungs. Cells lining the tubes are specialized cells (see epithelium) and have hair-like structures – cilia – that sweep the trapped dust up to the mouth where it is swallowed. Some dust may reach the lungs where white blood cells may destroy it. However, the lungs can be damaged if dust is not removed. Many miners suffer from lungs damaged by the effects of coal dust, and many other forms of industrial dusts are equally dangerous.

In humans, the principal diseases of the lungs are tuberculosis, pneumonia, bronchitis, emphysema, and cancer. Bronchitis is an irritation of the airways resulting in them becoming narrower than normal so that a person cannot breathe fully. Emphysema is permanent damage to the alveolar walls resulting in too little surface for gas exchange. This too results in difficulties in breathing. The commonest cause of both bronchitis and emphysema is smoking.