126 SCIENCE PRIMERS. [§ viii. 



generally in a particular condition, being combined 

 ivith a little carbonic acid, in the form of what is 

 called urea. If urea is not actually ammonia, it is at 

 least next door to it. 



The three great channels, then, by which the blood 

 purifies itself, by which it gets rid of its waste, are the 

 lungs, the kidneys, and the skin. Through the lungs, 

 carbonic acid and water escape ; through the kidneys, 

 water, ammonia in the shape of urea, and various salts ; 

 through the skin, water and a few salts. As the blood 

 passes through lung, kidney, and skin, it throws off 

 little by little the impurities which clog it, one at 

 one place, another at another, and returns from each 

 purer and fresher. The need to get rid of carbonic 

 acid and' to gain a fresh supply of oxygen is more 

 pressing than the need to get rid of either ammonia 

 or salts. Hence, while all the blood which leaves the 

 left ventricle has to pass through the lungs before it 

 returns to the left ventricle again, only a small part of 

 it passes through the kidneys, just enough to fill at 

 each stroke the small arteries leading to those organs. 

 The blood craves for great draughts of oxygen, and 

 breathes out great mouthfuls of carbonic acid, but is 

 quite content to part with its ammonia an-^ salts in 

 llctle driblets, bit by bit. 



The three channels manage between them to keep 

 the blood pure and fresh, working hard and clearing 

 off much when much food or water is taken or much 

 work is done, and taking their ease and working slow 

 when little food is eaten or when the body is at rist. 



