CHAPTER XX 

 THE PHYSICAL FACTORS OF THE CIRCULATION 



SOME of the physical laws which govern the circulation may be 

 illustrated by means of schemata made of rigid cylindrical tubes. 



Flow of Fluid in Cylindrical Tubes. In a cylindrical tube, the fluid 

 particles, flowing under constant pressure, move parallel with the 

 axis, but with varying velocity. In the axial layer the velocity is at 

 its greatest; at the wall it is almost nil. The wall is wet with the 

 fluid, and there is friction between the moving particles of fluid. The 

 fluid may be considered as consisting of an infinite number of concentric 

 cylindrical surfaces, which glide over one another, and move the 

 more rapidly the smaller their radius. The velocity, which is reckoned 

 from the outflow per second per sectional area of the tube, yields us 

 the mean velocity of all these cylinders of fluid. Poiseuille has laid 

 down the law that the mean velocity is directly proportional to the 

 sectional area of the tube and pressure gradient. We can find the 

 mean velocity by the product of three factors sectional area, pressure 

 gradient, and a constant coefficient, which depends on the viscosity 

 or physico-chemical nature of the fluid in the conditions of experiment. 

 This coefficient can be defined as that mean velocity which a current 

 would have with a unit pressure gradient in a tube of unit sectional 

 area. 



The coefficient at one and the same temperature varies for different 

 fluids, and is found to be smaller in proportion to the viscosity of the 

 fluid. 



The viscosity of blood is found to be three and a half to five times 

 that of distilled water. A mixture of blo.od and water is less viscous 

 than blood. Thus, the velocity of the circulation is increased by the 

 injection of Ringer's solution. The viscosity of the blood is increased 

 when there is great loss of water e.g., in cholera. Alterations in 

 viscosity can be compensated for by the vaso-motor system, which 

 regulates the peripheral resistance, and are therefore of minor im- 

 portance. 



By experiments upon the flow of distilled water in capillary glass 

 tubes, 0-65 to 0-15 millimetre in diameter, Poiseuille reached the 

 following conclusions: 



1-. That the amount of outflow is proportional to the head of 

 pressure. 



2. That the time spent in the outflow of a certain volume of fluid 



179 



