30 THE FUNCTIONS OF ANIMALS. 
The oxygen of this air combines with a substance called 
hemoglobin, contained in the red corpuscles of the blood, 
and is thus carried to all parts of the body. From the 
blood it passes to the tissues usually through the medium 
of the lymph. It is used in the tissues for oxidation. 
The carbon dioxide formed as a waste product is ab- 
sorbed by the serum of the blood, or enters in part into 
loose chemical combination with its salts, and so in time 
reaches the lungs. But as the partial pressure of the 
carbonic acid in the air is lower than it is in the serum, 
the gas escapes from the latter into the air chambers of 
the lungs. When the size of the chest is decreased, the 
pressure is increased, and the gas escapes by the mouth or 
nose until the pressure is equalised. 
Excretion.—We have seen that the blood carries the 
digested food to the various parts of the body, and that it is 
also the carrier of oxygen and of the waste carbon dioxide. 
But there is much waste resulting from tissue changes, 
which is not gaseous. It is cast into the blood stream by 
the tissues, and has to be got rid of in some way. This is 
effected by the kidneys, which are really filters introduced 
into the blood stream. But they are the most marvellous 
filters imaginable, and give us a good example of the 
intricacy of life processes. For the kidneys not only take 
out of the blood all the waste products that result from 
the metabolism of proteids and contain nitrogen, they also 
maintain the composition of the blood at its normal, 
rejecting any stuffs that vary from that normal, either 
qualitatively or quantitatively, doing this work according to 
laws quite different from the simple ones of diffusion or 
solubility: thus sugar and urea are about equally soluble, 
and yet the sugar is kept in the body, while the urea is cast 
out. Even substances as insoluble as resins are removed 
from the blood by the living cells of the kidneys. 
A considerable quantity of water, and traces of salts, fats, 
etc., leave the body by the skin, but its chief use is to 
protect, and to regulate the temperature by variations in 
the size of its blood vessels. 
This completes our sketch—(a) of the process by which 
the food becomes available for the organism as fuel for the 
maintenance of its life energies, and (4) of the removal of 
