THE FLOW OF A LIQUID IN ELASTIC TUBES 



199 



portions, the fall in pressure is the same, for the velocity and hence the internal 

 friction also, are the same in these. 



When a main-delivery tube is divided up into a number of small branches, 

 whose total cross section area is greater than that of the stem tube, and these 

 branches then reunite into a single tube of smaller cross section than the 

 branches, so as to imitate roughly the relationships of arteries, capillaries and 



l> 



FIG. 73. The flow of a liquid through a rigid tube of varying diameter. 



veins, the same quantity of liquid will flow through every total cross section of 

 the system in unit time, and the velocity again will be inversely proportional to 

 the total cross section area. In such an enlargement of the total cross section by 

 branching the superficial contact between the liquid and the walls of the tubes 

 becomes greater as the diameter of the branches becomes smaller, and the resist- 

 ance therefore also becomes greater. Now increased resistance acts against the 

 favorable influence which the mere widening of a current bed produces. Con- 

 sequently the effect on the flow of the current produced by any particular branch- 

 ing of the bed will be the resultant of these two opposing factors. 



2. THE FLOW OF A LIQUID IN ELASTIC TUBES 



The laws which apply to a constant current in rigid tubes holds also for 

 tubes with elastic walls. But an important difference exists between rigid and 

 elastic tubes, w r hen the fluid is driven into them intermittently. We leave out 

 of account here for the present wavelike movements in elastic tubes. 



If a fluid be pumped rhythmically into one end of a rigid tube, it will flow 

 out of the other end in jets of the same rhythm. But if we use an elastic tube 

 for such an experiment, and if the resistance in the tube is sufficient and the 

 rate of inflow rapid enough, the outflow may become continuous. This conver- 

 sion of an intermittent to a constant flow is explained by the fact that the elastic 

 wall of the tube is put on the stretch by the injecting force so that a part of 

 the energy is stored in the wall. Then when the inflow ceases for a moment, the 

 stretched wall exerts pressure on the contained fluid in consequence of which 

 the latter flows during the pause between jets. 



These conditions are realized in the vascular system. The blood is driven 

 by the heart into the arteries in spurts; the arterial walls are elastic; the 

 smaller arteries and capillaries present a high resistance. Consequently the 

 arterial wall is stretched by the blood at every systole of the heart, and dur- 



