CHAPTER XVIII. 



THE MECHANISM OF RESPIRATION. 



In its course through the circulation the blood undergoes a 

 series of necessary modifications. The condition of the fluid is 

 thus constantly being altered as it passes from one part and 

 organ to another. 



It has already been seen that a quantity of nutrient material 

 is taken up by the blood on its way through the capillaries of 

 the alimentary tract. Further, a stream of lymph and chyle is 

 constantly pouring into the great venous trunks, so that from 

 two sources the blood is steadily increased in quantity. But the 

 most urgently essential addition to the circulatory fluid is that 

 which it receives in the capillaries of the lungs. All the blood 

 passes through these organs, in order that the changes taking 

 place in the general systemic capillaries may be counteracted in 

 the lungs. These gas interchanges will form the subject-matter 

 of the present chapter; and the more special modifications which 

 the blood undergoes in the ductless glands, the spleen, the liver, 

 etc., as well as in the kidneys and other excretory glands, will 

 be considered in subsequent chapters. 



As has already been pointed out (Chapter V), an animal 

 during its life may be said to use the substances supplied to it in 

 food as fuel, and thus to acquire the energy which is bound up 

 in them, for the activities of the various tissues are really com- 

 bustions, being invariably associated with an oxidation of some 

 of the carbon compounds, so as to produce carbon dioxide and 

 water. In order that the structures may undergo this change, 

 they must have a ready supply of oxygen constantly at hand, 

 and, moreover, the carbon dioxide which is formed in the process 

 must be removed, or further combustion would be frustrated. A 

 regular income of oxygen and a regular output of carbon dioxide 

 are, then, essential to life; hence we find in almost all animals 

 special arrangements by means of which these gases can find their 



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