46 PHYSICOCHIvMICAL BASTS OF PHYSIOLOGICAL PROCESSES 



containing C0 2 before measuring the C0 2 content. In the second place, 

 the arterial blood represents the mixed blood of the body, and not that of 

 one locality only, as is the case with blood removed from a peripheral vein. 

 If venous blood is collected with the precaution that the muscles in the 

 corresponding area have been at rest for some time it appears that there is 

 practically no difference between the alkaline reserve of arterial and ve- 

 nous blood; but if there has been any muscular contraction, the venous 

 blood will have a lower reserve than the arterial, because of the lactic acid 

 thrown into it by the active muscles. But even when we take the precau- 

 tion of avoiding muscular action it is probable that there is not a strict 

 parallelism between the buffer action of arterial and venous blood, as in 

 cases in which the demands on the alkaline reserves are such that those of 

 the tissues are being called on as well as those of the blood itself. 



Even when the whole Wood is used, however, we do not necessarily 

 measure the total reserve of the body, a final reserve being afforded by the 

 alkalies and possibly .certain of the proteins of the tissue cells. Now it is 

 clear that there can be no test tube method by which measurement of the 

 magnitude of all of these defensive agencies is possible ; and we are there- 

 fore compelled to supplement them by certain indirect methods. 



Indirect Methods 



The chief criticism against the use of the C0 2 carrying power of blood 

 or blood plasma, is therefore, that it tells little if anything concerning the 

 acid-absorbing powers of the tissues. Is there not, therefore, some test of 

 the acid buffer which can be applied to the intact animal 1 One such is the 

 percentage of C0 2 in alveolar air (see page 361). 



1. Determination of the Tension of C0 2 in Alveolar Air. Since this 

 method is employed more particularly in investigating the hormone con- 

 trol of the respiratory center, we shall defer a description of it until later 

 (page 361). For the present, however, it should be remarked that the alve- 

 olar C0 2 can be a precise gauge of the acid-base equilibrium only provided 

 that the respiratory center is perfectly normal, and that there is no in- 

 terference with the diffusion of C0 2 from the blood into the alveolar air. 

 In order to place an estimate on the "relative value of this method com- 

 parisons have been made between the C0 2 tension of the alveolar air and 

 the C0 2 absorbing power of the blood. This has been done both in nor- 

 mal and pathological subjects. In normal subjects the comparisons have 

 been made under conditions, such as the taking of food and during mus- 

 cular exercise, in which slight alterations in the acid-base equilibrium are 

 known to occur. Van Slyke, Stillman and Cullen 9b found that the ratio 



plasma C 2 varies from 1.27 to 1.80 in different resting individu- 



mm. alveolar C0 2 



als, there being apparently a characteristic ratio for each individual, and 



