CHANGES ACCOMPANYING MUSCULAR EXERCISE 411 



sure markedly falls, until the extra space in the veins has been occupied 

 by blood. It is for this reason that the arterial blood pressure is always 

 found to be little, if any, above normal when taken within a few seconds 

 after such exercise. It subsequently rises because the other factors 

 responsible for the increased pressure (quick heart and arteriole constric- 

 tion) are still in operation at the time the veins again become filled with 

 blood. The purely mechanical influence outlasts the exercise for a com- 

 paratively short time, whereas the nervous and hormone influences con- 

 tinue acting. This interpretation is supported by the observation that 

 the fall of blood pressure is greater when the subject is left standing 

 after a given amount of dumbbell exercise than when he is allowed to sit 

 with his elbows resting on his knees. In the standing position the pres- 

 sure on the abdominal veins is less and the hydrostatic effect of gravity 

 causes more blood to collect in the large veins (Cotton, Rapport and 

 Lewis 36 ). Being purely mechanical in its causation, the preliminary fall 

 following dumbbell exercise can always be demonstrated if the observa- 

 tions are made at close enough intervals of time. 



The mechanical response of the circulation to exercise acts therefore 

 through the rate of filling of the right heart with blood, and if this organ is 

 in a healthy condition, it will respond to the greater inflow by correspond- 

 ingly increased discharge. Like every other physiologic mechanism, the 

 heart works with a large factor of safety a reserve power and it is 

 the rate of venous filling that determines how much of this reserve must 

 be called upon to maintain the circulation. In isolated heart-lung prep- 

 arations Starling and his coworkers have very clearly demonstrated the 

 close dependence of cardiac output upon rate of venous filling and the 

 enormous range through which the systolic discharge can be made to 

 vary by altering this factor. As explained elsewhere, when the reserve 

 power of the heart is lessened, the rise in blood pressure following exer- 

 cise is longer in attaining its maximum, which is set at a higher level and 

 persists for a longer time. Observation of the extent of these changes 

 furnishes a most useful functional test of cardiac efficiency. 



Other mechanical factors that augment the cardiac output depend on 

 the increased respiratory movements. During each respiration the in- 

 crease in capacity in the thorax causes both an opening up of the thin- 

 walled veins, so that blood is aspirated towards them from the extra- 

 thoracic venous system, and a dilatation of the blood vessels of the lungs, 

 so that the blood finds its way from right to left heart more readily. 

 Although this dilatation will at first tend to cause more blood to collect 

 in the intrathoracic vessels and less to be pumped out of them, the expira- 

 tory act when it supervenes will, by compressing the veins, cause the 

 extra blood to be expelled into the left ventricle and thence into the 



