30 THE FUNCTIONS OF ANIMALS. 



The oxygen of this air combines with a substance called 

 haemoglobin, 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 (b) of the removal of 



