ETYM Old Eng. lunge, AS. lunge, pl. lungen; akin to Dutch long, German lunge, Icel. and Swed. lunga, Dan. lunge, all prob. from the root of Eng. light. Related to Light not heavy.
Large cavity of the body, used for gas exchange. It is essentially a sheet of thin, moist membrane that is folded so as to occupy less space. Most tetrapod (four-limbed) vertebrates have a pair of lungs occupying the thorax. 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 so that oxygen can pass into the organism and waste carbon dioxide can be passed out. 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.
In humans, the principal diseases of the lungs are tuberculosis, pneumonia, bronchitis, emphysema, and cancer.
Trachea and air movement.
The lung may be regarded as a many-chambered elastic bag placed in the air-tight thorax and having communication with the exterior only by means of the trachea (windpipe). Atmospheric pressure acting down the trachea keeps the lung so far stretched that the two pleural layers are always in apposition, and together with the heart and great blood-vessels they completely fill the thorax. The air passes into and through the bronchi, which somewhat resemble the trachea in structure; the air current then continues through the various subdivisions of bronchi, bronchioles, and bronchial tubes, which, diverging in all directions, never anastomose (join), but terminate separately.
Alveoli.
After a certain stage of subdivision, when the diameter is about 1 mm, the walls of the bronchial tubes develop into blind, cup-shaped pouches termed alveoli, the walls of which consist of a thin membrane of areolar and elastic tissue lined by thin, transparent, flat cells. The cells are about 3.6 mm in diameter, and are said to number upward of 700,000,000 and to present a very large surface area to the air. It is from the air in these cells that the blood obtains a fresh supply of oxygen and gives up its carbon dioxide, for between adjacent alveoli there is a layer of thin-walled capillaries, the vessels twisting first to one side and then to the other of the septa between the alveoli.
Either of two saclike respiratory organs in the chest of vertebrates; serves to remove carbon dioxide and provide oxygen to the blood.