THE COURSE AND PHYSICS OF THE CIRCULATION 235 



This feature may be seen in a fire engine. There is a 

 large dome of metal which contains a quantity of air and 

 is in communication with the channel which conducts 

 the outflow from the pumps. The air inside is an elastic 

 body or cushion. It is under compression to an extent 

 determined by the pressure which is applied to the 

 water by the rapidly working pistons. The tendency of 

 the confined air to expand maintains the discharge 

 during the brief interruptions of the delivery from the 

 pumps. If the air chamber were not provided, the 

 stream sent into the hose would be a pulsatile one. 



In the circulatory system there is no large and localized 

 organ to serve the purpose of an air chamber. But the 

 elastic property is represented throughout the arterial 

 tree. The larger trunks, in particular, swell when blood 

 is thrust out of the heart and exert an extra pressure upon 

 their contents in proportion as they are distended. By 

 their contraction when the discharge from the heart 

 ceases they continue the flow in their smaller branches 

 and thus in the capillaries and veins. 



Economy of power is secured by the conversion of the 

 intermittent to a constant flow. Much more force would 

 be required to drive the same volume of blood through a 

 rigid system for there would be a dead loss of momentum 

 after every heart-beat and all the blood would have to 

 be started afresh from a state of arrest. This would 

 severely rack the pump. Something approaching this 

 condition occurs in arterio-sclerosis. The hardened 

 arteries of the aged do not yield readily to the inrush of 

 blood from the ventricle. Less of it can be accommodated 

 by lateral enlargement and more has to be pushed straight 

 ahead. This requires the development of greater pres- 

 sure than would be called for in a more elastic system. 

 Thus the heart is subjected to increasing demands when 

 it is naturally declining in reserve strength. It is one of 

 the gravest of "vicious cycles." 



Gravity and the Circulation. In our discussion of blood 

 pressure we have proceeded thus far as though gravity 



