364 CIRCULATION 



pulled along by their cohesion to the next inner layer. As a 

 result, their velocity is decreased. The net result is that a whole 

 series of cylindrical layers is produced each with a different rate 

 of flow — ranging from the almost stationary outer layer to the 

 central axial column, which is retarded least of all and, therefore, 

 possesses the greatest kinetic energy. In a straight tube of 

 uniform bore, such as is under consideration, this retarding 

 influence reduces the average rate of flow to half that of the axial 

 stream. It is obvious, therefore, that a considerable amount of the 

 potential energy of the liquid in the reservoir is absorbed in over- 

 coming the peripheral resistance caused by pure friction. Resist- 

 ance to flow also depends on the area of cross-section of the tube — 

 the wider the tube the larger the number of cylindrical layers 

 over which the adhesive resistance spends itself and, therefore, 

 the less the resistance met by the axial stream. Liquid, in a tube 

 so narrow that only an outer layer and a central column could 

 pass along it, woidd move with infinite slowness. Except in 

 instances in which the conducting tube has a very large or a very 

 small diameter, the rate of flow is proportional to the area of cross- 

 section. Further, the resistance in a tube of uniform diameter is 

 proportional to its length. Therefore, the energy of the fluid must 

 decrease gradually from the reservoir to the outlet of the tube. 



{b) Viscosity. The internal friction or viscosity depends on the 

 nature of the fluid, and, as indicated in Table XLIII., on the size 

 and concentration of the bodies suspended in it. The mean 

 viscosity of blood compared with water is 4-45, and, therefore, it 

 would require 4*45 times as much pressure to force blood along 

 a tube at the same rate as an equal volume of water. The blood 

 cells do not materially affect viscosity till they occupy about 

 two-thirds of the total volume, i.e. till the haematocrite reading is 

 about 66 per cent. When that concentration has been reached, the 

 value of the viscosity rises owing to the friction of one erythrocyte 

 against another. The values of the viscosity of blood vary at 

 different parts of the circulation, due principally to alterations in 

 the size of the corpuscles {q.v.). The total frictional loss (friction + 

 viscosity) reaches a maximum in the capillaries, where not only do 

 the corpuscles tend to increase in volume but the bore of the 

 individual vessel becomes so small that the corpuscles undergo 

 considerable distortion in their passage along it. 



Pressure. The energy of the fluid is shown by the pressure it 

 exerts. Pressure may be measured by some form of manometer. 

 It is sufficient to insert a number of vertical glass tubes, of uniform 

 bore and open to the air, at various points of the conducting tube 

 to see the fall of pressure with distance from the source of power. 



