SECTION XI 



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 hyperpncea. 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. Increased 

 contractions of voluntary muscles will in the first place quicken the return 

 of venous blood to the heart, and so will cause a greater diastolic 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. It must be 

 remembered that the heart cannot put out more blood than it receives. 

 Since during active exercise the output may be increased four to six times 

 above the normal, it is evident that the venous circulation must be corre- 

 spondingly increased, and this increase can only be ascribed to the pumping 

 action of the contracting muscles and to the movements of respiration. 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 is in itself sufficient to raise the blood -pressure, 

 and it seems probable that the increased activity of the motor centres actually 

 spreads to the medullary centres which preside over the heart and blood- 



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