PHYSIOLOGY OF TO-DAY 



absorbed and C the carbon dioxide given off by the lungs 

 per minute, we have 



^•^- " Ao-Vo ^ v.- A. 

 Since all of these quantities except the required M.V. can 

 be experimentally determined, the problem is theoretically 

 solved. Technical difficulties, however, in determining 

 the necessary values in the above equation have prevented 

 its wide application even to animals and have been only 

 recently surmounted in the case of the human subject. 

 The Fick principle has been used on animals, where 

 blood samples can be taken directly from the right and 

 left sides of the heart. In the human subject, it is a com- 

 paratively easy matter to determine the quantity of oxygen 

 absorbed at the lungs or the quantity of carbon dioxide 

 given off; such a determination is made in determining 

 the so-called metabolic rate or basal metabolism so much 

 used now in the diagnosis and treatment of thyroid disease. 

 The content of oxygen or carbon dioxide in the arterial 

 blood (that of the left side of the heart) can also be easily 

 obtained by an indirect method of calculation and can 

 be checked directly by obtaining blood for analysis 

 from an artery, a procedure quite feasible in the human 

 subject. The difficulty comes, however, in estimating the 

 gaseous contents of the mixed venous blood of the right 

 side of the heart, and because of this fact the simplicity 

 and directness of the method as used on animals are lost. 

 Blood, of course, can be easily obtained from various 

 veins of the body, as is done every day in hospitals in 

 making various blood tests; but such blood is not the same 

 in its oxygen and carbon dioxide contents as that of the 

 right heart. It has taken a quarter of a century of intensive 

 research to work out satisfactory devices for determining 

 the gas tensions, and hence the gaseous contents, of the 

 mixed venous blood indirectly. All of these schemes 



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