RESPIRATION 271 



activity. The chief reason why the oxygen diminishes and CO 2 

 increases so strikingly is that the mixed venous blood contains 

 a much larger proportion of blood from muscles, and that this 

 blood is very poor in oxygen whether the muscles are working or 

 not. During rest the mixed venous blood will contain but little 

 blood from the muscles, and a large proportion from the brain 

 and probably other parts which furnish venous blood relatively 

 rich in oxygen. As indicated by the size of its arteries, the brain 

 has a very rich blood supply, going mainly to the gray matter. 

 Its normal oxygen pressure is evidently very high; and this 

 renders intelligible the fact that it is so sensitive to deficient satu- 

 ration of the arterial blood with oxygen. The rapid circulation 

 explains the promptness of its reaction to changes in quality of 

 the arterial blood. 



The fact that during muscular exertion the mixed venous blood 

 contains much less oxygen and more CO 2 explains why, if the 

 breath is voluntarily held during exertion, the alveolar CO 2 

 percentage shoots up much higher than if it is held for a far 

 longer time during rest. It also explains what would otherwise 

 be a very puzzling fact with regard to congenital heart affections 

 ("morbus coeruleus"). In cases of morbus coeruleus the face 

 becomes intensely blue on muscular exertion. Quite evidently the 

 arterial blood is very imperfectly oxygenated ; and Douglas and I 

 found that the blueness continues even if the patient breathes pure 

 oxygen during the exertion. The blueness is due to part of the 

 venous blood short-circuiting through a congenital direct com- 

 munication between the right and left sides of the heart, so that 

 the mixed arterial blood always contains a certain proportion of 

 unae' rated venous blood. During rest this venous blood contains 

 so much oxygen that the cyanosis is only slight ; but during exer- 

 tion, with much less oxygen in the venous blood, the cyanosis is of 

 course far more marked, and the breathing of oxygen avails very 

 little towards redressing the balance. 



It is evident from the facts just referred to that the increase in 

 blood flow through the lungs during exertion is very much less 

 than the increase in air breathed. At first sight, therefore, it might 

 seem that the regulation of circulation differs fundamentally from 

 the regulation of breathing. A little consideration, however, shows 

 that there are no real grounds for this conclusion. If we take as our 

 measure, not the blood flow through the heart, but the blood flow 

 through individual parts of the body, the facts so far discussed do 

 not point to any other conclusion than that the blood flow, just 



