THE CIRCULATION OF THE BLOOD 



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more blood flows through the lungs and that blood flows 

 more rapidly out of the arterial reservoir, but it does not 

 mean that blood flows more rapidly through all organs, for 

 the digestive and other internal organs at such times actually 

 receive less blood. Indeed, we may say that the quickening 



FIG. 74. Simple apparatus to illustrate the relation between the output of 

 the heart, the peripheral resistance, and the general arterial pressure 



The amount delivered by the faucet represents the output of the heart, and is one 

 factor in keeping up arterial pressure ; two alternative routes of outflow, each 

 capable of regulation, represent the arterioles to different organs. Compensatory 

 constrictions and dilations and other hydraulic conditions described in the text 

 may readily be imitated 



of the circulation during exercise is chiefly confined to three 

 important organs the muscles, the skin, and the lungs ; in 

 other organs the change is relatively slight, as, for example, in 

 the brain ; while in still others, notably those of the digestive 

 system and the kidneys, the speed is diminished. 



18. The circulation during sleep. An adequate blood sup- 

 ply is necessary to the full activity of the brain; when the 



