lung

The tiny air sacs, called alveoli, in the lungs are covered with a network of blood capillaries, allowing oxygen to enter the blood. The alveoli of an adult have a total surface area of 70 sq m/750 sq ft.

The human lungs contain 300,000 million tiny blood vessels which would stretch for 2,400 km/1,500 mi if laid end to end. A healthy adult at rest breathes 12 times a minute; a baby breathes at twice this rate. Each breath brings 350 millilitres of fresh air into the lungs, and expels 150 millilitres of stale air from the nose and throat.

The two phases of the process of respiration. Gas exchange occurs in the alveoli, tiny air tubes in the lungs.

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.

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, flattened cells. The cells are about 3.6 mm in diameter, and are said to number upwards 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. The alveoli are kept moist by a liquid that acts like a detergent, stopping the alveoli collapsing and sticking together. Babies born prematurely may not make the detergent and this results in these babies having difficulty inflating their lungs. They may have to be put on a ventilator as a consequence.