BIOLOGY IN HUMAN AFFAIRS 



depend on equilibrating a gas mixture in the lungs with the 

 mixed venous blood, analyzing it, and then finding how 

 much oxygen or carbon dioxide the venous blood of the 

 subject will contain at these gas tensions. 



Because of the difficulty described above in applying 

 the Fick principle to man, other avenues of approach 

 to the problem were sought. Another type of method, 

 based on a determination of the rate of absorption through 

 the lungs of some foreign gas, has been investigated. The 

 idea underlying this type of method is quite simple in 

 principle. If we take some physiologically inert foreign 

 gas which will pass into the blood in simple physical 

 solution, determine its solubility in blood, and then find 

 out how much of this gas is absorbed by the blood passing 

 through the lungs from the right to the left side of the 

 heart in a given short interval of time, we can obviously 

 calculate how much blood must have passed through the 

 lungs in that time to dissolve the observed amount of 

 gas. This amount of blood flowing through the lungs 

 represents the output of either side of the heart. In practice, 

 however, this method bristles with difficulties and ques- 

 tions as to its accuracy. Thus, we must be sure that com- 

 plete equilibrium is attained between the gas mixture in 

 the lungs and the flowing blood, that the time allowed 

 for the experiment is sufficiently short to prevent any 

 return of blood containing the foreign gas; that the heart 

 output is not changed by the procedure used, and so forth. 

 Only quite recently have methods based upon this general 

 principle been put upon a firm basis as to their 

 accuracy. 



Since the above type of method has many advantages as 

 to accuracy and simplicity over that based upon the Fick 

 principle, we may describe briefly just how a determination 

 of the cardiac output is carried out with it. Of the foreign 

 gases used — namely, nitrous oxide, ethylene, and acetylene 



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