CHAPTER XXV. 



THE OHEMISTEY OF BESPIKATIOIN 



Nature of the Problems. The study of the respira- 

 tory process from a chemical standpoint has for its object 

 to discover, first, what are, in kind and extent, the inter- 

 changes between the air in the lungs and the blood in the 

 pulmonary capillaries; and, in the second place, the nature 

 and amount of the corresponding gaseous changes between 

 the various Jiving tissues and the blood in the systemic 

 capillaries. These processes are the reverse of one another 

 and in the long run balance, the blood losing as much car- 

 bon dioxide gas in the pulmonary circulation as it gains in 

 the systemic, and gaining as much oxygen in the former 

 as it loses in the latter. To thoroughly comprehend, the 

 matter it is, moreover, 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 -processes occur. So far as the prob- 

 lems 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; for example, 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 exactly how 

 much of the one it gives and of the other it takes is only 

 known approximately; as are also the conditions tinder 

 which this greater interchange during the activity of the 

 muscular tissue is effected: and concerning nearly all the 

 other tissues we know even less than about muscle. In fact, 

 as regards the Body as a whole, it is comparatively easy to 



