The walls of the air cells are of extreme thinness, consisting of delicate elastic and connective tissue, and lined inside by a single layer of thin epithelial cells. In the connective tissue run capillary vessels belonging to the pulmonary artery and veins. Now these delicate vessels running in the connective tissue are surrounded on all sides by air cells. It is evident, then, that the blood flowing through these capillaries is separated from the air within the cells only by the thin walls of the vessels, and the delicate tissues of the air cells.
This arrangement is perfectly adapted for an interchange between the blood in the capillaries and the air in the air cells. This will be more fully explained in sec. 214.
208. Capacity of the Lungs. In breathing we alternately take into and expel from the lungs a certain quantity of air. With each quiet inspiration about 30 cubic inches of air enter the lungs, and 30 cubic inches pass out with each expiration. The air thus passing into and out of the lungs is called tidal air. After an ordinary inspiration, the lungs contain about 230 cubic inches of air. By taking a deep inspiration, about 100 cubic inches more can be taken in. This extra amount is called complemental air.
After an ordinary expiration, about 200 cubic inches are left in the lungs, but by forced expiration about one-half of this may be driven out. This is known as supplemental air. The lungs can never be entirely emptied of air, about 75 to 100 cubic inches always remaining. This is known as the residual air.
The air that the lungs of an adult man are capable of containing is thus composed:
| Complemental air | 100 | cubic inches. |
| Tidal | 30 | ” ” |
| Supplemental | 100 | ” ” |
| Residual | 100 | ” ” |
| Total capacity of lungs | 330 | ” ” |
If, then, a person proceeds, after taking the deepest possible breath, to breath out as much as he can, he expels:
| Complemental air | 100 | cubic inches. |
| Tidal | 30 | ” ” |
| Supplemental | 100 | ” ” |
| 230 |
This total of 230 cubic inches forms what is called the vital capacity of the chest ([Fig. 90]).
209. The Movements of Breathing. The act of breathing consists of a series of rhythmical movements, succeeding one another in regular order. In the first movement, inspiration, the chest rises, and there is an inrush of fresh air; this is at once followed by expiration, the falling of the chest walls, and the output of air. A pause now occurs, and the same breathing movements are repeated.