280 CIRCULATION 



water to the reservoir to compensate for the outflow. If, how- 

 ever, the head of pressure is produced by the action of a piston 

 in a cylinder, it will not cause a continuous but an intermittent 

 flow in the main conduit. The pressures shown in the piezo- 

 meters will vary from a maximum to a minimum as the wave of 

 pressure passes down the system after each stroke. Such con- 

 ditions entail great loss of power. In order to reduce this loss 

 to a minimum, it is necessary to replace the rigid conduit by an 

 elastic tube. Such a tube would, of course, if rigid, permit a 

 certain flow of fluid per unit of time per unit of pressure, say with 

 a constant velocity of ( v). Now on the descent of the piston more 

 water tends to be forced into the conduit than can be passed out 

 with this velocity (v). The elastic walls distend till their elastic 

 power exactly counterbalances the extra energy, and the fluid 

 has an outflow velocity of (v). The influx of water having 

 ceased, the steady pressure of the distended walls of the tube as 

 they recoil keeps the fluid flowing at the constant velocity (v). 

 In this way the fluid is held under a continuous pressure and, 

 provided the pump has the proper frequency, the outflow remains 

 practically constant. That is, the elastic tube really converts an 

 intermittent inflow into a constant outflow, the property of elasticity 

 preserving normal conditions of flow even during periods when the 

 piston is not descending. 



Such a system of single stroke pump and elastic regulator does 

 not differ in essentials from the one contrived by nature to provide 

 a perfect transport service to every unit of a complex organism 

 like the human body. In Fig. 61 a simple force pump and its 

 circulating system is compared with the left ventricle, aorta, etc. 



The manner in which the contents are forced out from the 

 ventricle differs in some details from that obtaining in the water 

 pump. In the latter, a rigid piston descends within a rigid 

 cylinder and thus obliterates the space of the main chamber and 

 forces the water through the outflow pipe. The power necessary 

 to drive the plunger home is derived from an engine of some sort, 

 external to and independent of the pump itself. In the heart, 

 the elastic muscular walls of the ventricles contract as a whole, 

 deriving their force just as any other muscular structure does, 

 from the potential energy of materials brought to them by the 

 blood and liberated in their protoplasm. 



The value of the work done by a pump may be calculated approxi- 

 mately by the formula W 



-t 



