ON FLUID FLOW; BLOOD 



217 



'synthetic plasmo" 



o 



synthetic plasmo 

 plus red blood cells 



AP (mmHgO/cm length) 



Figure 8-12. Fluidity (slope) of Synthetic 

 Plasma to Which Different Volume Percent- 

 ages of Cells Have Been Added. 



within the physiological range of operation both plasma and whole blood are 

 essentially Newtonian; that is, their curves are linear; Poiseuille's law of 

 laminar flow is obeyed. 



However, closer inspection of not only very low rates of flow but also very 

 high rates reveals that the fluidities in these ranges are lower than in the 

 intermediate range in Fig. 8-11: the fluidity is dependent upon flow rate in 

 these regions. Thus at low flow rates an elasticity due to the formation of 

 liquid crystals by hydrogen bonds makes flow more difficult and has to be 

 broken down; at high flow rates turbulence sets in and makes flow more 

 difficult. 



Figure 8-12 illustrates the first point. Notice how the fluidity (slope) 

 changes with flow rate, when flow is slow. On the other hand, turbulence 

 can actually be heard (or its effects can be heard) over the heart where very 

 high flow rates accompany the high pressure part of the beat .... The de- 

 pendence of viscosity (1/0) on tube radius (Figure 8-13), at first surprising, 

 resolves to a question of the interruption of laminar flow when the diameter 

 of the suspended particles (red blood cells) approaches the diameter of the 

 tubes through which the suspension is flowing. This is the condition which 

 exists in the blood capillaries — the process is more like an extrusion than a 

 laminar flow. The velocity gradient across the tube is the cause of Bernoulli 

 forces which not only make the cell spin, but also force it toward the center 

 (the bore) of the tube. Further, the blood vessels are somewhat elastic and 

 can increase their diameter under pressure. Thus the flow rate doubles for 

 a 16 per cent increase in radius! This fact, plus the probably great differ- 

 ence between the surface of glass tubes and the molecular-hair-lined** blood 



**These "molecular hairs" arc hydrated protein molecules, partly detached from the wall, 

 and jutting out into the tube. 



