342 PHYSIOLOGICAL EEGULATIONS 



might be computed relative to that. A large part of the body's total 

 carbon dioxide is fixed in bones; this portion is not measurably 

 touched when increments of ± 75 per cent of the usual alveolar 

 tension of carbon dioxide are experimentally imposed (Irving). 

 If the remainder of the body's carbon dioxide rapidly interchanges 

 with blood, the volume of distribution of increments is roughly 

 70% of Bo; this value is also that found for the distribution of 

 added bicarbonate (Palmer and Van Slyke, '17). In anesthetized 

 cats breathing mixtures of air and 7.5 per cent carbon dioxide, 

 Shaw ('26) found 0.0017 l./kg. of whole body retained for 1 mm. 

 Apco2 in blood, while blood alone had an increment of 0.0033 l./liter 

 (orVD = 52% of Bo). 



Returning to man, I believe that in the range of carbon dioxide 

 contents near balance, the dissociation of carbon dioxide in the body 

 is proportional to that of blood (in other words, that the volume of 

 distribution of increments is constant), and that an increment of 

 1 mm. of mercury tension of blood carbon dioxide (ApcoJ equals 

 an increment of 0.0031 liter/kg. of body (0.7 times an increment of 

 0.0045 l./liter of blood), or about 0.3 per cent of all the carbon di- 

 oxide present. The value 0.0031 resembles the 0.0021 to 0.0028 

 l./kg. for each mm. Apcoa derived by Liljestrand ('16) and from 

 selected data of Adolph et al. ('29) and of Nielsen ('36) ; it is the 

 value used in figure 171. 



Should it turn out that much uncertainty is introduced into fig- 

 ure 171 by Liljestrand 's assumptions (a) that carbon dioxide con- 

 tent of the body is proportional to change in alveolar tension of 

 carbon dioxide, and (b) that production of carbon dioxide in tissues 

 is independent of load, other coordinates could be substituted. 

 Instead of the whole body, the blood alone may have its contents 

 of free or of total carbon dioxide ascertained, and the net rates of 

 exchange may be computed from serial analyses of samples from 

 circulating blood. Instead of carbon dioxide output, rates of total 

 ventilation or of alveolar ventilation may be correlated with carbon 

 dioxide tension in alveoli (as Campbell et al., '14, and others, have 

 already done). It is well known that under chosen conditions the 

 rate of carbon dioxide elimination is precisely proportional to 

 (alveolar) ventilation rate (Douglas and Priestley, '24). In other 

 words, the concentration of carbon dioxide in expired air or in air 

 drawn from alveoli is uniform. But the high correlation between 

 ventilation rate and carbon dioxide tension does not, as always, 

 exclude many other factors from being related to ventilation rate. 



