SECTION X 



THE EFFECT OF MUSCULAR EXERCISE ON 

 THE CIRCULATION 



ANY muscular exercise, even moderate, produces rise of blood 

 pressure and acceleration of the pulse, associated with an increase of 

 pulmonary ventilation hyperpnoea. These effects can be readily 

 shown by running up and down stairs for half a minute. The following 

 Table by Pembrey and Todd shows the effect of such a form of exercise 

 on the pulse-rate and systolic blood pressure in two individuals, one 

 trained and the other untrained : 



Several factors may concur in the production of these effects. In- 

 creased contractions of voluntary muscles will in the first place quicken 

 the return of venous blood to the heart, and so will cause a greater dias- 

 tolic distension of this organ and therefore a greater output of blood 

 by the left ventricle. The increased respiratory movements will also 

 aid the venous circulation and have a similar effect in increasing the 

 systolic output. Moreover the increased exercise will raise the tension 

 of carbonic acid in the blood in consequence of the increased produc- 

 tion of this gas by the contracting muscles. If the exercise is very 

 violent, lactic acid may be also present in the circulating blood. We 

 have already seen that a moderate increase in the H ions in the blood, 

 whether due to carbonic acid or lactic acid, gives rise to increased 

 diastolic relaxation of the ventricles and raises its output into the 

 arteries. All these factors will thus concur in producing a rise of 

 pressure even when the heart and blood-vessels are cut off from the 

 central nervous system. The latter also is concerned in the rise 



of pressure. The mere act of attention preparatory to muscular effort 



1099 



