THE RESPIRATORY SYSTEM. 383 
once-living protoplasm are carried out with the breath, but 
these other substances are got rid of from the blood by a vital 
process of secretion (excretion), we must believe; which shows 
that the lungs to some degree play the part of glands, and that 
their whole action is not to be explained as if they were merely 
moistened bladders acting in accordance with ordinary physical 
laws. 
An estimation of the amount of atmospheric air required 
may be calculated from data already given. 
Thus, assuming that a man gives up at each breath 4 per 
cent of carbon dioxide to the 500 cc. of tidal air he expires, and 
breathes, say, seventeen times a minute, we get for the amount 
of air thus charged in one hour to the extent of 1 per cent: 
500 X 4 X 17 X60 = 2,040,000 cc., or 2,040 litres. 
But if the air is to be contaminated to the extent of only 
dy per cent of carbonic anhydride, the amount should equal at 
least 2,040 X 10 hourly. 
RESPIRATION IN THE BLOOD. 
It may be noticed that arterial blood kept in a confined 
space grows gradually darker in color, and that the original 
bright scarlet hue may be restored by shaking it up with air. 
When the blood has passed through the capillaries and reached 
the veins, the color has changed to a sort of purple, character- 
istic of venous blood. Putting these two facts together, we are 
led to suspect that the change has been caused in some way by 
oxygen. Exact experiments with an appropriate form of blood- 
pump show that from one hundred volumes of blood, whether 
arterial or venous, about sixty volumes of gas may be obtained ; 
that this gas consists chiefly of oxygen and carbonic anhydride, 
but that the proportions of each present depends upon whether 
the blood is arterial or venous. 
The following table will make this clear: 
Oxygen. Carbonic anhydride. Nitrogen. 
Arterial blood. :...,.....0.5 20 40 1-2 
Venous blood..............- 8-12 46 1-2 
from 100 volumes of blood at 0° C. and 760 millimetre pressure. 
Arterial blood, then, contains 8 to 12 per cent more oxygen 
and about 6 per cent more carbonic dioxide than venous blood. 
It is not, of course, true, as is sometimes supposed, that arterial 
