CHANGES ACCOMPANYING MUSCULAR EXERCISE 415 



as the H-ion concentration of the perfusion fluid is increased by adding 

 C0 2 to the air ventilating the lungs. 



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

 vagus and vasomotor centers is usually believed to be stimulatory. 

 There is no doubt that an increase in C H stimulates the vasoconstrictor 

 centers, not only of the medulla, but also, although much more feebly, 

 of the spinal cord. But it is a question whether any part of the rise in 

 systolic pressure during muscular exercise can be attributed to this 

 cause, for the enormously increased bloodflow which is known to occur 

 makes it problematical whether any vasoconstriction really occurs. If 

 it does so, it 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. The slowing of the heart that is observed in 

 asphyxia, for example, is in part at least due to the increased intra- 

 cranial pressure, for when the carotid artery is connected with a mer- 

 cury valve so that the blood escapes as the pressure rises above the 

 normal level, no slowing of the heart is said to occur in asphyxia. As 

 Leonard Hill and Flack 37 have shown, however, a part of the slowing is 

 due to the direct effect of CO,. 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 be- 

 lieved to have. The activity of the respiratory center is of course ex- 

 cited by increase in H-ion concentration, and this, as we have seen, will 

 cause important changes in the circulation because of the mechanical 

 effects which follow. 



Along with hormones we must consider the effect of change in the 

 temperature of the blood. That this rises during muscular exercise is 

 well known, but that it should be responsible for many of the cardio- 

 vascular adjustments that occur is quite commonly overlooked. It is, 

 for example, very likely that rise in blood temperature is responsible 

 for the acceleration of the heart that occurs during exercise when both 

 vagi have been severed, and it no doubt is responsible for a part at 

 least of the vasodilatation and respiratory acceleration. 



Finally, it is interesting to speculate as to the nature of the changes 

 that occur when the "second wind" is acquired during strenuous mus- 

 cular exercise. In running, for example, considerably more distress is 

 experienced a short time after the start than some time later. Three 



