EXCRETION. 25 



the windpipe until the pressure is equalised. 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. The protoplasm of the 

 tissues having a stronger affinity for oxygen than has the 

 haemoglobin, takes as much as it requires. The carbonic 

 acid gas formed as a waste product is absorbed by the serum 

 of the blood, 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 until the pressure is equalised. By the constant 

 repetition of the breathing movements, oxygen is constantly 

 being taken in, and carried to the tissues which are in a 

 marvellous way "hungry" for it, while the waste carbonic 

 acid gas is as constantly being removed. 



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 carbonic acid gas. 



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 in- 

 tricacy of life processes. For the kidneys not only cast out 

 of the blood all the waste products that result from the 

 metabolism of proteids, and contain nitrogen ; but they 

 maintain the composition of the blood at its normal, reject- 

 ing 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, 

 &c., leave the body by the skin, but its chief use is to pro- 



