THE CIRCULATION 123 



from a pumping-station to distant points there is a similar 

 decline in pressure along each line as it is followed farther 

 from the fountain head. (The assumption is made that 

 we have to do with pipes which are on the same level 

 throughout.) 



In any such system the cutting down of the pressure will 

 be abrupt at any point on the route where there is an 

 unusual impediment to the flow. This is the case in the 

 body where the blood-steam is so extensively subdivided 

 to enter the smallest vessels. Subdivision of channel 

 means multiplied surface for friction. Thus there occurs 

 a radical drop in pressure between the smallest arteries in 

 which we can measure it and the veins of similar size. 

 The highest pressure developed anywhere must be in the 

 left ventricle when it is discharging the blood, for it is 

 then to be regarded as the starting-point of the circulation. 

 The lowest pressure which is ever registered is probably in 

 the same ventricle when it is beginning to fill, for now it is 

 the terminus of one of the two circuits. 



The swelling of the arteries which promptly follows each 

 ventricular contraction is what we recognize as the pulse. 

 It is the sign of a newly introduced portion of blood dis- 

 tributing itself in the vessels. We have said that the veins 

 show no such rhythmic enlargement. We have, therefore, 

 to account for the conversion of the intermittent flow in the 

 arteries into the constant flow in the veins. The principal 

 factor concerned is the marked elasticity of the arterial 

 trunks. It will be helpful to refer to the artificial devices 

 which serve the same purpose in connection with force- 

 pumps. A familiar one is the air-chamber. This is a 

 large container communicating with the outflow-pipe. 

 Whenever a stroke of the pump drives water along this 

 pipe a certain share proceeds directly toward the outlet, 

 while another fraction turns aside into the air-chamber. 

 During the return stroke when no water is issuing from the 

 pump barrel the air which a moment before underwent 

 compression in the chamber tends to regain its original 

 volume, and in so doing forces water through the lateral 





