The amount of air taken in with each inspiration is about thirty cubic inches. This is called the tidal air, as it is constantly ebbing and flowing from and to the lungs. By means of a very forced inspiration, about 100 cubic inches of additional air can be inspired; and similarly after an ordinary inspiration, one can expire forcibly an additional 100 cubic inches, though there will still be left in the lungs another 100 cubic inches of air. Thus:—
| Tidal air | 30 | cub. in. |
| Complemental air | 100 | „ |
| Supplemental air | 100 | „ |
| Residual air | 100 | „ |
| —— | ||
| Total capacity of lungs | 330 | „ |
Corresponding to the respiratory changes in the lungs, there are changes in the tissues throughout the body. The pure and oxygenated blood leaving the lungs, is carried to all parts of the system. Oxidation and allied processes are actively carried on, the result of which is the formation of urea, carbonic acid, and smaller quantities of other effete matters. These are then carried by the blood to the excretory organs, urea being chiefly eliminated by the kidneys, and carbonic acid by the lungs.
Examination of Expired Air shows that—1. It is heated; in its passage through the nose and deeper respiratory passages it has acquired a temperature approaching that of the blood.
2. Its moisture is increased. By the skin and lungs from 25 to 40 ounces of water pass off in the twenty-four hours; the relative amount varies somewhat.
3. It contains 4 to 5 per cent. less oxygen, and 4 per cent. more carbonic acid than inspired air. The carbonic acid, instead of being 4 parts in 10,000 of air, becomes over 400 in 10,000, while the oxygen is diminished in a somewhat larger proportion. Thus:—
| OXYGEN. | NITROGEN. | CARBONIC ACID. | |
| Inspired air contains | 20·81 | 79·15 | ·04 |
| Expired air contains | 16·033 | 79·557 | 4·38 |
The amount of carbonic acid expired varies under different circumstances. It is increased by active work, by an increase of food, by a diminution of the external temperature; it is greater when the surrounding air is pure, and when it is moist; and it varies with the season, being greatest in spring, and least in autumn.
Children require more oxygen, and expire more carbonic acid than adults, weight for weight. A child six or seven years old requires nearly as much oxygen as one twice that age. Boys usually require more air than girls, as they are more active and exhale a larger amount of carbonic acid and other impurities.
The average amount of carbonic acid eliminated by a healthy adult is at least 0·6 cubic foot per hour, or 14·4 cubic feet per day. This reckoned as carbon is equivalent to 160 grains per hour, or half a pound of carbon in the twenty-four hours. Liebig gives the amount of carbonic acid expired as 0·79 cubic foot per hour, or 19 cubic feet per day.