94-0 HANDBOOK OF PHYSIOLOGY -" CIRCULATION II 



table i. Mathematical Relationships Between Resistance and Pressure and Between Resistance and Flow 



The columns correspond to the curves: A = solid triangles; B = solid circles, and C = solid squares in fig. 3. PRUp = 

 resistance in mm Hg/(cc/min) expressed as a function of the arteriovenous difference in pressure in mm Hg; PRUf = resistance 

 expressed as a function of the rate of flow; PRUp 1M = resistance computed at an arteriovenous pressure difference of 100 mm 

 Hg; PRUf b = resistance calculated at a flow of 5 cc/min; B/A and C/A = ratios of values in columns B and C, respectively, 

 to those in column A. Note — columns B/A and C/A: for lines II and IV these values are also reciprocal ratios of flow at con- 

 stant arteriovenous pressure difference and for lines III and V they are also direct ratios of arteriovenous difference of pressure 

 at constant rate of flow. 



F = c X P" 



P 

 PRUp 



PRUf = - = 



F 



The data in this table were computed from results reported by Green et al. (46). 



arteriovenous difference of pressure during experi- 

 mentally induced changes of vasomotor tone. 



On the other hand the convergence of the pressure- 

 flow plots at high pressures, discussed above, suggests 

 a secondary influence of change of perfusion pressure 

 on vascular distensibility and measured resistance. 

 On this basis there is merit in using a constant per- 

 fusion pressure and allowing the flow to vary with 

 experimentally induced changes of vasomotor tone 

 rather than keeping the flow constant and allowing 

 the perfusion pressure to be the dependent variable. 

 At present we cannot find sufficient grounds for a 

 decision between the two methods of perfusion when 

 studying "passive" vascular beds. 



In those vascular beds which show autoregulation 

 (p. 948) anything which induces a change of flow, 

 i.e., alteration of perfusion pressure, tends to be count- 

 ered by an active change of vasomotor tone which 

 will tend to maintain flow constant. In this type of 

 bed it is particularly desirable to have data on the 

 control steady-state pressure and flow relationships 

 in order to make adequate quantitative comparisons 

 with experimental data. 



Effects of Viscosity on Pressure-Flow Relationships 



The viscosity of blood relative to that of water 

 increases nonlinearly with red cell concentration, 



20 40 60 80 100 



CORPUSCULAR CONCENTRATION (P C) 



fig. 6. Mean and probable error of the apparent viscosity 

 of blood relative to saline in a glass viscometer and in the 

 hind limb of a dog at different corpuscular concentration. 

 [Redrawn after Whittaker & Winton (115).] 



