After ordinary expiration, such as that which expels the breathing air, a certain quantity of air remains in the lungs which may be expelled by a forcible and deeper expiration; this is termed reserve air. But even after the most violent expiratory effort, the lungs are not completely emptied; a certain quantity of air remains in them, over which there is no voluntary control, which may be called residual air. Its amount depends, in great measure, on the absolute size of the chest, and has been variously estimated at from 40 to 200 cubic inches.
| Power of Inspiratory Muscles. | Power of Expiratory Muscles. | ||||||
| 1.5 | inches. | weak | 2.0 | inches. | |||
| 2.0 | inches.,, | ordinary | 2.5 | inches.,, | |||
| 2.5 | inches.,, | strong | 3.5 | inches.,, | |||
| 3.5 | inches.,, | very strong | 4.5 | inches.,, | |||
| 4.5 | inches.,, | remarkable | 5.8 | inches.,, | |||
| 5.5 | inches.,, | very remarkable | 7.0 | inches.,, | |||
| 6.0 | inches.,, | extraordinary | 8.5 | inches.,, | |||
| 7.0 | inches.,, | very extraordinary | 10.0 | inches.,, | |||
The blood as it moves through the respiratory organs is exposed to the air that alternately moves into and out of the air-cells and minute bronchial tubes. The blood is propelled from the right ventricle through the pulmonary capillaries in steady streams, and slowly enough to permit every minute portion of it to be for a few seconds exposed to the air, with only the thin walls of the capillary vessels and air-cells intervening.
The atmosphere we breathe has in every situation in which it has been examined in its natural state a nearly uniform composition. It is a mixture of oxygen and nitrogen, carbonic acid, and watery vapor, with traces of other gases, as ammonia, sulphuretta, hydrogen, etc. Of every 100 volumes of pure atmospheric air, 79 volumes consist of nitrogen and 21 of oxygen, about. The proportion of carbonic acid is extremely small: 10,000 volumes of atmospheric air contains only about 4 or 5 of carbonic acid. The average quantity of watery vapor in the atmosphere in this country is about 1.40 per cent.
The changes produced by respiration on the atmosphere are that: 1. It is warmed; 2. Its carbonic acid is increased; 3. Its oxygen is diminished; 4. Its watery vapor is increased; 5. A minute amount of organic matter and of free ammonia is added to it.
1. The expired air is hotter than the inspired air. The temperature varies from 97° to 99½°.
2. Carbonic acid in respired air is always increased; but the quantity exhaled in a given time is subject to change from various circumstances. From every volume of air inspired about 4½ per cent of oxygen is abstracted; while rather a smaller quantity of carbonic acid is added in its place. Under ordinary circumstances, the quantity of carbonic acid exhaled into the air breathed by a healthy adult man amounts to 1,346 inches, or about 636 grains, per hour. It is estimated that the weight of carbon excreted from the lungs is about 173 grains per hour, or rather more than 8 ounces in 24 hours.
Of course the influence of age, sex, respiratory movements, external temperature, season of the year, purity of the respired air, hygrometric state of the atmosphere, period of day, food and drink, exercise and sleep, have to be taken in consideration.
The oxygen of respired air is always less than in the same air before respiration, and its diminution is generally proportionate to the increase of the carbonic acid. It has been shown that for every volume of carbonic acid exhaled into the air 1.17421 volumes of oxygen are absorbed from it; and that when the average quantity of carbonic acid, i.e., 1,346 cubic inches, or 636 grains, is exhaled in the hour, the quantity of oxygen absorbed in the same time is 1,584 cubic inches, or 542 grains.
The nitrogen in the atmosphere, in relation to the respiratory process is supposed to serve only mechanically, by diluting the oxygen, and moderating the action upon the system.