RESPIRATION 277 



great increase in blood flow through the muscles, with a corre- 

 sponding increase in venous blood pressure, as Henderson and 

 his colleagues found with the apparatus shown in Figure 7O. 17 



The correspondence between blood flow and amount of work 

 done by a muscle seems to appear clearly in data obtained by 

 Markwalder and Starling for the coronary circulation with vary- 

 ing work of the heart in a heart-lung preparation. 17A The amount 

 of blood pumped by the heart, the aortic blood pressure, and the 

 flow through the coronary vessels, were measured simultaneously. 

 The data show that if the work done is estimated by the amount 

 of blood pumped multiplied by the aortic pressure, the coronary 

 blood flow varied within wide limits in proportion to the work 

 done. The variations in coronary blood flow might, of course, be 

 attributed to the variations in aortic blood pressure, but this inter- 

 pretation does not seem to explain more than a small part of the 

 facts. 



At first sight the regulation of the circulation appears to be 

 different from that of respiration, since in the case of the latter 

 the influence of CO 2 predominates. This, however, is simply be- 

 cause when ordinary air is breathed the oxygen pressure in the 

 tissues is not increased when the breathing increases. In reality, 

 there is no fundamental difference. Whenever anoxaemia is pres- 

 ent the respiratory regulation, as already shown in Chapter VII, 

 works just like the local circulatory regulation. The breathing is 

 not then free to increase in such a way as to compensate approxi- 

 mately for increasing anoxaemia, because increased breathing 

 lowers the CO 2 pressure and this tends to diminish the breathing. 

 Similarly the breathing cannot increase freely with increased 

 CO 2 pressure, because the increased breathing would diminish 

 the anoxaemia. Under deep anaesthesia, when the arterial blood 

 becomes dark, CO 2 has very little effect on the breathing. 



There can be little doubt that in the case of circulation, just as 

 in that of respiration, increase in CO 2 pressure stands simply for 

 increase in hydrogen ion concentration. Hence alkalosis due to 

 deficiency of CO 2 in the systemic capillaries, or acidosis due to 

 excess, will tend to be relieved by the slow acclimatization changes 

 described in Chapter VIII. 



When once the fundamental fact is grasped that the general 

 flow of blood throughout the body is correlated with the gas pres- 



17 Yandell Henderson and Haggard, Journ. of Pharmac. and, Exper. Therap. t 

 XI, p. 197, 1918. 



17A Markwalder and Starling, Journ. of Physiol., XLVII, p. 279, 1913. 



