EFFECTS OF EXERCISE 177 



number of the muscles at work. When many large 

 muscles are hard at work, as in running or walking 

 upstairs, a comparatively large amount of blood is 

 affected. If under these conditions a more rapid ex- 

 change of waste products were not possible, the blood 

 supply of the entire body would speedily become impure. 

 To avoid this, there is a nervous mechanism by which 

 the condition of the blood is constantly tested to deter- 

 mine its quality. If the quality falls below the stand- 

 ard, this nervous mechanism has the power of quick- 

 ening both the respiratory movements and the heart 

 beat. In consequence, the blood not only gets more 

 oxygen and is more quickly freed of its waste, but it is 

 also sent around faster to supply the larger amounts 

 demanded by the muscles. To aid the heart in the work 

 of pumping this increased amount of blood, the arte- 

 rioles in the active muscles are, as we have seen, relaxed, 

 the blood escapes more freely from the arterioles, the 

 blood pressure in the arterial reservoir is lowered and 

 the left ventricle of the heart has to overcome less re- 

 sistance in opening the aortic valve and emptying its 

 content of blood into it. The work of the heart in 

 pumping the increased amount of blood is thus dimin- 

 ished, and the heart itself is. saved from quick exhaustion. 

 Effect of training. In athletes and in those used to 

 hard work, the nervous control is so perfectly trained 

 for its work that it enables the muscles to exert their 

 greatest effort with little or no strain of the heart. In 

 untrained persons, on the contrary, the automatic mech- 

 anism lacks the skill which comes with practice and so 

 may overdrive the heart in attempting to bring about 

 the proper adjustment between the circulation and res- 

 piration and the muscular activity. This is especially 

 the case in youth, and as a result athletic contests are 



