204 RESPIRATION 



produced without any measureable cause. In reality the acidosis 

 is not completely compensated, and the incompleteness is only 

 hidden by the extreme roughness of the method of measurement 

 in comparison with the fineness of the physiological reaction. 



The table seems to indicate that the normal PH is not quite the 

 same, though very nearly the same, in different individuals. For 

 the present, however, this conclusion is rather doubtful, in view 

 of the fact that the measurements were for imperfectly reduced 

 blood. We have seen already that in spite of the accuracy of regu- 

 lation there are individual differences in the normal alveolar CO 2 

 pressure, the normal composition of haemoglobin, and the normal 

 dissociation curve of blood for CO 2 . As regards every detail of 

 structure and function we may be certain of rinding similar differ- 

 ences when the measurements are made with sufficient accuracy; 

 and this doubtless applies also to even the PH of the blood. 



We have already considered one cause which alters the PH to 

 which the respiratory center regulates. This cause is anoxaemia. 

 At high altitudes the body is in the long run projected to a large 

 extent from the effects of the alkalosis thus produced, because the 

 kidneys and liver still react almost true to the normal PH. There 

 can be no doubt that other causes, such as the action of anaes- 

 thetics or poisons, or of other small changes in the composition of 

 the blood, would have a similar effect in altering the standard 

 to which the PH regulation of the arterial blood is set. This 

 question, and the question how the PH is regulated, not merely in 

 the arterial blood, but in the systemic capillaries, will be deferred 

 to Chapters X and XIV. 



We can now see much more clearly why it is that the resting 

 alveolar CO 2 pressure is not quite steady in spite of the extreme 

 sensitiveness of the respiratory center to the minutest variation 

 in alveolar CO 2 pressure. There are various causes tending to 

 disturb the constancy of the reaction of the blood ; and the respira- 

 tory center, and not merely the kidneys and liver, must do its share 

 in compensating for these disturbances. Hence the alveolar CO 2 

 pressure cannot remain completely steady during rest. One of 

 these causes is the secretion of acid or alkaline digestive juices. 

 On account of the secretion of acid gastric juices the alveolar 

 CO 2 pressure rises distinctly very soon after a meal. The effects of 

 a meal on alveolar CO 2 pressure have been investigated recently 

 by Dodds. 52 He found that there is normally a sharp rise varying 

 in different individuals, but usually amounting to about 4 mm. half 



82 Dodds, Journ. of Physiol., LIV, p. 342, 1921. 



