154 THE HUMAN MECHANISM 



17. The circulation during muscular activity. During mus 

 cular activity the arterioles of the muscles and of the skin 

 are dilated, the former in order to supply more blood to the 

 working organ, the latter to aid in the discharge of the ex- 

 cess of heat produced by the contracting muscles. The heavy 

 drain upon the arterial reservoir by these two large areas 

 (among the largest in the body) is compensated to some 

 extent by the constriction of the arteries of the digestive and 

 other internal organs. This alone, however, would not suffice 

 to keep the arterial reservoir filled ; and we accordingly find 

 that the heart beats more rapidly and more powerfully, 

 pumping more blood into the aorta in a given time. 



It is very important to remember that muscular activity 

 is the one condition of life which materially increases the 

 work of the heart ; at other times the greater demand of 

 blood for the working organ is met more or less success- 

 fully by withdrawing blood from a resting organ, while the 

 supply to the whole arterial system, and hence the work 

 of the heart, remains approximately unchanged. During 

 muscular exercise, and then only, is the heart called upon 

 for decidedly increased work ; and, as with skeletal muscles, 

 its" strength, its ability to meet strain and emergencies and 

 to withstand fatigue, depend to a great extent upon the 

 training given it in this way. 



Muscular activity also influences the circulation indirectly 

 by increasing the action of its secondary driving forces 

 the suction action of the respiratory movements and the 

 pumping action of the contracting muscles on the veins. 

 These are among the most important effects of this agent 

 upon the flow of blood, but they are too complicated for 

 detailed discussion here. 



It is sometimes stated that muscular exercise " quickens " 

 the circulation. This is true in the sense that the heart 

 pumps more blood into the pulmonary artery and the aorta 

 than during rest. From this it follows that during exercise 



