CHAPTER XIX. 



THE CIRCULATION (Cont'd). 



The Blood Flow Through the Vessels. 



Introduction. A clearer idea of the principles governing the 

 circulation of blood through the vessels can be had if the laws 

 governing the flow of water in a city water system are called to 

 mind. For example, a water-works system is arranged by means 

 of either special pumps or a standpipe, to furnish a stream of 

 water at constant rate and pressure into the city water mains. 

 The water is first forced into one large pipe and from this de- 

 livered to the consumer by means of much smaller pipes. By 

 simple mathematical calculation it can be shown that the total 

 cross-section area of the smaller pipes is many times that of the 

 main pipe ; for the sake of argument, let us say 800 times great- 

 er. Therefore the average rate of flow of water in the smaller 

 pipes must be 800 times less than in the main pipe, providing all 

 the outlets are open. However, if only one-half of the distribut- 

 ing pipes are in use, the flow of water would be only 400 times 

 less than in the main pipe, and the resistance offered by the walls 

 of the pipes to the flowing water is also halved. Thus the same 

 amount of water is delivered in the same unit of time but under 

 twice the pressure, since only one-half of the force used to deliv- 

 er the water through all the pipes is used in delivering it through 

 one half of them. In other words, it takes X force to overcome 

 the resistance offered by Y, therefore X equals Y. When X re- 

 mains constant and Y is halved, then X Y/2 equals X/2, leav- 

 ing X/2 as a remainder. To bring it home, there is less water 

 delivered from the garden hose and it has far less pressure be- 

 hind it when all the neighbors are also using the water, than 

 there is when only a few outlets are in use. Likewise, if the 

 amount and the pressure of water in the main pipe are varied 

 by changing the force of the pumps or the level of water in the 

 stand pipe, the amount of pressure of water delivered are also 

 varied in the same direction. 



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