428 THE RESPIRATION 



called into play ; that is to say, mechanical, nervous and hormone factors 

 cooperate to an extent which is dependent upon the type of work being 

 performed. 



Besides the changes in pulse rate and blood pressure which are evi- 

 dently designed to supply more blood to the acting muscles, changes 

 dependent upon a secondary effect of the muscular movements have also 

 to be considered. Although the various factors work together and are 

 more or less interdependent, the final effect can be understood only after 

 we have studied the relative influence of each separately. 



The Mechanical Factor. It is particularly with regard to this factor 

 that the circulatory changes may be an unavoidable consequence of, 

 rather than a useful adjustment to, the muscular effort. The effects vary 

 with the type of exercise performed. In repeatedly lifting and lowering 

 dumbbells from the floor to above the head, the contracting muscles of 

 the back and extremities and of the abdomen compress the veins and 

 cause the blood to flow more rapidly into the heart, so that the arterial 

 pressure suddenly rises. So long as this compression exists, the veins 

 remain relatively empty and the arteries overfilled, but whenever it 

 -ceases and the muscles relax, the veins fill up again and the arterial pres- 

 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 physiological mechanism, the 

 heart, therefore, 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 



