These are fourfold—changes in its temperature, in its moisture, in its chemical composition, and in its volume.

Temperature Changes

The air taken into the lungs is nearly always cooler than that expired, which has a temperature of about 36° C. (97° F.). The temperature of a room is usually about 21° C. (70° F.). The warmer the inspired air, the less the heat which is lost to the body in the breathing process.

Changes In Moisture

Inspired air always contains more or less water vapor, but is rarely saturated—that is, rarely contains so much but it can take up more without showing it as mist; the warmer air is, the more water vapor it requires to saturate it. The expired air is nearly saturated for the temperature at which it leaves the body, as is readily shown by the vapor deposited when it is slightly cooled, as when a mirror is breathed upon; or by the clouds seen issuing from the nostrils on a frosty day, these being due to the fact that the air as soon as it is cooled cannot hold all the water vapor which it took up when warmed in the body. We therefore conclude that air when breathed gains water vapor and carries it off from the lungs. The quantity of water thus removed from the body is about nine ounces each twenty-four hours.

Chemical Changes

The most important changes brought about in the breathed air are those in its chemical composition. Pure air when completely dried consists in 100 parts of—

What changes are produced in the air on its being once breathed? How does expired air differ in temperature from inspired? How does expired air differ from inspired in moisture? How much water is evaporated from the lungs daily?

By Volume.

By Weight.







When breathed once, such air gains rather more than 4 volumes in 100 of carbon dioxide, and loses rather more than 5 of oxygen. More accurately, 100 volumes of expired air, when dried, consist of—

Oxygen............................................ _____



.....-........... 7.2

Carbon dioxide...................

.................. 4.3

The expired air also contains volatile organic substances in quantities too minute for chemical analysis, but readily detected by the nose upon coming into a close room in which a number of persons have been collected.

The Quantity Of Oxygen Taken Up By The Lungs In A Day

We have already seen that the quantity of air breathed in a day is 648,000 cubic inches. This loses 5.4 per cent, of oxygen or 35,000 cubic inches, weighing 12,818 grains (1 4/5 lbs.): the body therefore gains this amount of that gas through the lungs daily.

The Amount Of Carbon Dioxide Passed Out From The Lungs In A Day

This being 4.3 per cent, of the total bulk of the air breathed, is 27,864 cubic inches; it weighs 14,105 grains or about 2 lbs.

We thus find that though each breath seems in itself a very little thing, on calculation it is obvious that the total amount of matter received into the body from the lungs, and that passed out of it by these organs, every day of our lives is considerable. In a year each adult breathes about 10,000 lbs. of air; from it he takes 657 lbs. of oxygen, and to it he gives off 730 lbs. of carbon dioxide.

What is the chemical composition of pure air by volume ? By weight ? What substances does air gain and lose when once breathed ? What is the composition by volume of dried expired air? What does dried expired air contain besides oxygen, nitrogen, and carbon dioxide?

What bulk of oxygen does the air breathed in a day lose ? What weight ? How much oxygen does the body take up daily by means of the lungs ?

What bulk of carbon dioxide is carried off by breathing in each day ? What does it weigh ?

Changes Of Volume In Air Once Breathed

If the expired air be measured as it leaves the body its bulk will be found greater than that of the inspired air, since it not only has water vapor added to it, but is expanded in consequence of its higher temperature. If, however, it be dried and reduced to the same temperature as the inspired air, its volume will be found diminished, since it has lost 5.4 volumes of oxygen for every 4 3 volumes of carbon dioxide which it has gained.