CHANGES ACCOMPANYING MUSCULAR EXERCISE 433 



ever, that sufficient change occurs in the heart to cause dilatation of 

 the coronary arteries, and thus improve the bloodflow (page 267). It 

 should be pointed out here, however, that a much more important factor 

 determining the coronary bloodflow is the pressure in the aorta. When 

 this is lower than 90 mm. adequate circulation in the coronary arteries is 

 difficult to maintain even though these vessels be dilated to the full. 

 Above 90 mm. slight further elevations in aortic blood pressure cause 

 disproportionate increase in coronary flow. The blood supply of the 

 heart itself depends much more upon arterial blood pressure than upon 

 any other factor (Markwalder and Starling 40 ). Indeed the heart is not 

 the only organ in which a similar relationship between blood pressure 

 and bloodflow exists. The same is true for glands, for Gesell 41 has found 

 that a trivial fall in general pressure causes a marked .curtailment in 

 bloodflow. 



The known influence of changes in H-ion concentration of the blood on 

 the vasomotor centers is difficult to correlate with the changes which ac- 

 tually occur in muscular exercise. There is no doubt that an increase in 

 CH stimulates the vasoconstrictor centers, not only of the medulla, but 

 also although much more feebly, of the spinal cord, but this action, if 

 it occurs during exercise, must be confined to the splanchnic area, where 

 it would have the effect of bringing about a redistribution of the total 

 available blood by expressing it from the viscera and sending it to the 

 active muscles. 



The effect of increased H-ion concentration on the vagus center must 

 be insignificant. It is commonly believed that it would cause not what is 

 actually observed, a quickening, but rather a slowing of the heart rate. 

 But even this is doubtful. If increase in the H-ion concentration does 

 affect the heart during muscular exercise, it must act by inhibiting the 

 vagus tone, which is opposite to the action which it is usually believed 

 to have. 



The activity of the respiratory center is of course excited by in- 

 crease in H-ion concentration (page 352) and the resulting greater ac- 

 tivity of the respiratory pump will cause important changes in the cir- 

 culation. To this extent alterations in C H of the blood will assist in 

 bringing about a greater mass movement of blood during muscular ex- 

 ercise. 



In this connection we must consider the effect of change in the tem- 

 perature of the blood. The extent of this rise in temperature apart from 

 the amount of exercise depends on several factors such as the cooling 

 effect of the environment, the amount of subcutaneous fat, and whether 

 or not the person is in training. By observations on the temperature of 

 the urine in a group of soldiers Pembrey found that a march of seven 



