CHAPTER XXVI. 

 THE CHEMISTRY OF RESPIRATION. 



Nature of the Problems. The study of the respiratory 

 process from a chemical standpoint has for its object to dis- 

 cover what are, in kind and extent, the interchanges between 

 the air in the lungs and the blood in the pulmonary capilla- 

 ries; and the nature and amount of the corresponding gaseous 

 changes between the living tissues, and the blood in the sys- 

 temic capillaries. Neglecting some oxygen used up otherwise 

 than in forming carbon dioxide, and some carbon dioxide elim- 

 inated by other organs than the lungs, these processes in the 

 long-run balance, the blood losing as much carbon dioxide gas 

 in the lungs as it gains elsewhere, and gaining as much oxygen 

 in the lungs as it loses in the systemic capillaries. To compre- 

 hend the matter it is necessary to know the physical and chemical 

 conditions of these gases in the lungs, in the blood, and in the 

 tissues generally; for only so can we understand how it is that 

 in different localities of the Body such exactly contrary pro- 

 cesses occur. So far as the problems connected with the 

 external respiration are concerned our knowledge is tolerably 

 complete; but as regards the internal respiration, taking 

 place all through the Body, much has yet to be learnt; 

 we know that a muscle at work gives more carbon dioxide 

 to the blood than one at rest and takes more oxygen from 

 it, but how much of the one it gives and of the other it 

 takes is only known approximately; as are the conditions 

 under which this greater interchange during the activity 

 of the muscular tissue is effected: and concerning nearly 

 all the other issues we know even less than about muscle. 

 In fact, as regards the Body as a whole, it is compara- 

 tively easy to find how great its gaseous interchanges with 

 the air are during work and rest, waking and sleeping, 



