440 



THE RESPIRATION 



The value of oxygen inhalations is most marked in the early stages of 

 great exertion, rather than later, and it is no doubt particularly by 

 maintaining the vigor of the heart beat, that it acts. From what has 

 been said in the foregoing paragraphs of this chapter, it must be clear 

 that the limitations to muscular work are set by the ability of the heart 

 to maintain a circulation rate that is proportionate to the demands of 

 the muscles for oxygen, and the output of the heart depends on the oxy- 

 gen carried to it by its blood supply. It is probable also that the heart 

 does not require to perform as much work in order to maintain an ade- 

 quate oxygen supply to the tissues when these can use some of the ex- 

 cess stored in them, if such storage occurs. In this way its expenditure 

 of energy will be conserved. 



The After-effects of Exercise 



Much attention has been given in recent years to the study of the 

 after-effects of exercise, because of the valuable information concern- 

 ing the reserve power of the heart which can thereby be obtained. In a 

 normal person the blood pressure and pulse rate, as we have seen, are 

 both materially raised during the exercise, but they promptly return 

 to normal after it is terminated, unless the exercise has been both severe 

 and prolonged, when the pulse rate may decline fairly rapidly at first, 

 but later only slowly, so that it still remains excessive even after an 

 hour. This delay in return to the normal pulse rate is possibly asso- 

 ciated with the prolonged increase in energy metabolism, which a bout 

 of strenuous exercise always stimulates (Benedict and Cathcart 65 ). In 

 order to standardize methods for testing these effects in the clinic, 

 Cotton, Rapport and Lewis 36 have adopted the practice of causing the 

 patients to lift dumbbells of 20 Ib. weight from the floor to the shoulders 

 at a rate of every 2 seconds for periods of time varying between 20 and 

 80 seconds. The pulse rate and blood pressure are taken immediately 

 before and at varying periods after the exercise, and the results are 

 tabulated as shown in the accompanying table. 



