Respiration (Lat. respirare, to breathe), the function by which oxygen is absorbed by the living organism for the maintenance of vitality, and by which carbonic acid is discharged as a product of disintegration or waste of the materials of the tissues. Respiration in some form is common to all living beings. Even in vegetables none of the more active phenomena of life can go on unless the plant be constantly supplied with oxygen; and the intensity with which these phenomena are manifested is in proportion to the rapidity with which oxygen is absorbed by its tissues and carbonic acid exhaled. In animals the process of respiration is still more marked; and it is more active in the warm-blooded birds and mammalia than in the cold-blooded reptiles and fishes. Animals which inhabit the water and breathe by gills absorb through them the oxygen which is in solution in water, and discharge carbonic acid by the same channel. In man and the air-breathing animals, the atmospheric air, which consists of 21 volumes of oxygen mixed with 79 volumes of nitrogen, is drawn by the movement of inspiration into the lungs, and discharged by the movement of expiration. During its stay in the pulmonary cavities it is changed in composition. The first and most important change is a diminution of its oxygen.

As a general rule, the air loses in this way, by the effect of a single respiration, 5 per cent. of its volume in oxygen. As, on the average, 20 cubic inches of air are taken into and discharged from the lungs by each respiratory act, the quantity of oxygen thus removed from the air at each respiration is one cubic inch. The movements of respiration follow each other usually at the rate of 18 or 20 a minute, and are accelerated by any active muscular exertion. The total quantity of air thus used for respiration in 24 hours is not far from 350 cubic feet; and accordingly the daily quantity of oxygen taken from the air and consumed by a healthy adult man is about 17 1/2 cubic feet, or more than four times the volume of the whole body. The amount of carbonic acid given off at each respiration in man is rather less than one cubic inch. The expired air usually contains about 4 per cent. of its volume of carbonic acid; this amounts, under ordinary circumstances, to about 14 cubic feet a day. Although the volume of the carbonic acid exhaled is less than that of the oxygen absorbed, its weight is considerably greater; the whole amount of oxygen consumed during 24 hours being about 10,000 grains, or rather less than 1 1/2 lb. avoirdupois, while that of carbonic acid exhaled during the same time is over 11,000 grains, or rather more than 1 1/2 lb.

A certain amount of watery vapor is discharged with the expired breath. This vapor is invisible at moderately warm temperatures, since it is then in the completely gaseous form; but if it be cooled below a certain point, as by coming in contact with cold air or cold metallic or glass surfaces, it becomes condensed, and is then rendered visible as a cloudy vapor or as a deposit of moisture. The amount of watery vapor thus discharged with the breath during 24 hours is, on the average, rather more than one pound avoirdupois. The oxygen absorbed from the air in the lungs is taken up by the blood, and carried away in the arterial circulation. At the same time the blood loses the dark purple color which it presents before entering the lungs, and assumes a bright scarlet hue. This process is the most immediate effect and the main purpose of respiration, and constitutes the principal distinction between arterial and venous blood. Venous blood is dark because it is deficient in oxygen; arterial blood is bright red because it contains an abundant supply of this necessary ingredient. The brilliant color of arterial blood is therefore an indication that it has absorbed its requisite quantity of oxygen, and is fit to provide for the stimulus and nutrition of the tissues.

As the arterial blood is disseminated throughout the body and comes in contact with the substance of the tissues, it gives up to them its oxygen and resumes a dark purple hue; it is thus reconverted into venous blood. At the same time it absorbs from the tissues a certain proportion of carbon, which has been set free in their substance, and, loaded with this product of disintegration, it returns to the right side of the heart, to be thence distributed to the lungs. There are accordingly two opposite and complementary changes taking place in the blood, during its passage through the lungs and the tissues respectively. - Since the air by respiration is deprived of a portion of its oxygen and loaded with carbonic acid, it will be incapable of supporting respiration continuously, unless renovated as rapidly as it is consumed. This renovation is provided for by the alternate movements of inspiration and expiration, by which the air already in the lungs, which has given up its oxygen to the blood and become mingled with carbonic acid, is discharged externally and replaced by a fresh supply.

The expired air is at once disseminated in the external atmosphere and carried away by the currents which are always in motion; so that, while in the open air, the lungs are constantly supplied with the materials of respiration in a state of purity. But if respiration be carried on in a confined space, the oxygen of the air gradually diminishes in quantity, and carbonic acid accumulates in a corresponding degree. The air is thus at the same time impoverished and vitiated, and after a time its deterioration becomes so marked that it is no longer capable of supporting life. Air is completely unfit for respiration when its natural proportion of oxygen has been reduced one half, and when it has become contaminated with carbonic acid to the extent of one fifth of its volume. But although the most serious results follow when the air has been vitiated to this extent, a much smaller amount of deterioration is unwholesome. This is of the greater importance because, besides its loss in oxygen and its mixture with carbonic acid, the air in respiration is also contaminated by certain organic vapors which may be distinguished by their odor, and which are exhaled in the breath at the same time with the carbonic acid.