236 HUMAN PHYSIOLOGY 



natural or artificial, it is turned to heat by friction within 

 the stream and along the enclosing walls. Friction is 

 greater at particular places and at these same places pres- 

 sure will be cut down. The great reduction "loss of 

 head," as the hydraulic engineer would call it occurs in 

 the blood-vessels where the extensive subdivision of the 

 stream creates the greatest resistance to progress. In the 

 large tubes, whether arteries or veins, there is little re- 

 sistance and only a gradual diminution of pressure. 

 When the returning blood nears the right auricle the 

 pressure is scarcely above that of the atmosphere and we 

 may say that the original impetus is spent. 



The student finds it hard to refrain from bringing to- 

 gether considerations of velocity and those of pressure 

 which are better kept separate. Note that reduction in 

 velocity means a widening of path, while reduction 

 pressure means resistance overcome. Observe also that 

 velocity may increase by the contraction of the channel 

 but we can never find pressure increasing as we follow the 

 direction of the flow in horizontal vessels; it can only 

 decrease. Extra pressure applied at the starting point 

 may indeed increase velocity, but the acceleration will 

 be in all parts of the circuit and it will remain true as 

 before, that the highest velocity is observed where the 

 path is narrowest and the lowest where it has the greatest 

 cross-section. 



The Intermittent and the Constant Flow. The blood 

 is discharged from the ventricles in successive gushes 

 with pauses between. Signs of an intermittent flow are 

 apparent all along the course of the arteries but the char- 

 acter is less marked as the small branches are observed. 

 In the capillaries the onward movement of the blood 

 seems steady and so it does, generally speaking, in the 

 veins. How is the original intermittency overcome? 

 The answer to this question is most easily discovered by 

 attending to the artificial devices which serve a similar 

 purpose. One of these is the air chamber used in con- 

 nection with force pumps. 



