Diagram to show what the blood loses and gains in one of the air sacs of the lungs.
Changes in the Blood within the Lungs.—Blood, after leaving the lungs, is much brighter red than just before entering them. The change in color is due to a taking up of oxygen by the hæmoglobin of the red corpuscles. Changes taking place in blood are obviously the reverse of those which take place in air in the lungs. Every hundred cubic centimeters of blood going into the lungs contains 8 to 12 c.c. of oxygen, 45 to 50 c.c. of carbon dioxide, and 1 to 2 c.c. of nitrogen. The same amount of blood passing out of the lungs contains 20 c.c. of oxygen, 38 c.c. of carbon dioxide, and 1 to 2 c.c. of nitrogen. The water, of which about half a pint is given off daily, is mostly lost from the blood.
Changes in Air in the Lungs.—Air is much warmer after leaving the lungs than before it enters them. Breathe on the bulb of a thermometer to prove this. Expired air contains a considerable amount of moisture, as may be proved by breathing on a cold polished surface. This it has taken up in the air sacs of the lungs. The presence of carbon dioxide in expired air may easily be detected by the limewater test. Air such as we breathe out of doors contains, by volume:—
| Nitrogen | 76.95 |
| Oxygen | 20.61 |
| Carbon dioxide | .03 |
| Argon | 1.00 |
| Water vapor (average) | 1.40 |
| Air expired from the lungs contains:— | |
| Nitrogen | 76.95 |
| Oxygen | 15.67 |
| Carbon dioxide | 4.38 |
| Water vapor | 2.00 |
| Argon | 1.00 |
In other words, there is a loss between 4 and 5 per cent oxygen, and nearly a corresponding gain in carbon dioxide, in expired air. There are also some other organic substances present.
The respiration of cells.
Cell Respiration.—It has been shown, in the case of very simple animals, such as the amœba, that when oxidation takes place in a cell, work results from this oxidation. The oxygen taken into the lungs is not used there, but is carried by the blood to such parts of the body as need oxygen to oxidize food materials in the cells. Since work is done in the cells of the body, food and oxygen are therefore required. The quantity of oxygen used by the body is nearly dependent on the amount of work performed. Oxygen is constantly taken from the blood by tissues in a state of rest and is used up when the body is at work. This is suggested by the fact that in a given time a man, when working, gives off more oxygen (in carbon dioxide) than he takes in during that time.
While work is being done certain wastes are formed in the cell. Carbon dioxide is given off when carbon is burned. But when proteins are burned, another waste product containing nitrogen is formed. This must be passed off from the cells, as it is a poison. Here again the lymph and blood, the common carriers, take the waste material to points where it may be excreted or passed out of the body.