RESISTANCE AND CAPACITANCE PHENOMENA IN VASCULAR BEDS 



939 



CC 00 



£ II 



5 6 



4 - 



Relotive 

 Pressure 



(| -"r7 ' 01 0.09 



4 



Pressure 

 (mm Hg) 



Per Cent 

 Extension 



II 

 30 



I 



33 



102 



256 



10 



0.5 

 181 

 565 

 1470 



100 



1.0 

 362 



Vessel 



-Aorto 

 1130 -Med. Art. 

 3940— Arteriole 



»/r. 



(mm) 



2 'l2 5 

 '/2 

 ° 2 /0,5 



Relative 



Conduct. I 104 1.46 



(Flow/Press.) 



16 



fig. 5. Relationship of internal pressure to radius in elastic tubes. Radius is expressed as ratio of 

 the radius at any pressure to the unstretched radius (r/r ). Pressure is plotted over the range from 

 zero to 1 , 1 representing that pressure at which the vessel extends indefinitely, and zero the pressure 

 at which the vessel is unstretched. Corresponding values of actual pressure, opposite the bracket, are 

 given in mm Hg; below this is given the per cent extension of the vessel at each of the levels of rela- 

 tive pressure, and below that is given the conductance which the vessel would have relative to that in 

 its unstretched state. Conductance is expressed as the ratio of Mow to pressure drop along unit length 

 of the vessel. These plots arc calculated from data in Burton (7). 



It is of interest that c' vs. n' and c" vs. n" (table 1) 

 both plotted as straight lines on log-log paper, as did 

 c vs. n in figure 4. It is of interest also that the lines on 

 the log-log plot in figure 3 approach each other at 

 high pressures and flows. As a result, if the data 

 could be extrapolated to such values, a point would 

 be found at which the resistance in state B would 

 equal that in A (i.e., P = 5,000, F = 16,000) and 

 another point at which the resistance in state C 

 would equal that in A (i.e., P = 2,950, F = 5,400). 

 In a log-log plot these two points are so close together 

 that a common point of intersection could be assumed 

 for all three lines — A, B, and C. This suggests that a 

 rise of perfusion pressure acts to overcome the con- 

 strictor tone, and that this effect is proportionally 

 greater the higher the vasomotor tone. 



From the above data it appears that the most satis- 

 factory method for defining "vasomotor tone" in 

 passive vascular beds is by means of a pressure-flow 

 plot, or by means of the equation for such plot. The 

 best quantitative expression for the comparison of 

 vasomotor tone at one moment with that at another 

 is to determine the plot of the pressure-flow relation- 

 ship during a control phase of vasomotor tone and to 



compare this with a similar plot obtained in the 

 experimental period (see fig. 3 and table 1, columns 

 B/A, C/A, lines II and III). Often this mode of ex- 

 pression is impractical because of the difficulty in 

 maintaining vasomotor tone constant, particularly 

 in the experimental period. A more practical com- 

 promise for expressing change of vasomotor tone may 

 be to determine the pressure-flow relationship over a 

 suitable range of pressures and/or flows during the 

 control period and to compare isolated experimental 

 observations of pressure and flow with this control 

 curve (see figs. 21 and 22 and table 1, columns 

 B/A, C/A, lines IV, V). 



From an inspection of figures 3 and 4 and table 1 , 

 it appears that comparison of the experimental per- 

 fusion pressure with that required in the control 

 period to induce the same rate of flow (table 1 , 

 lines III and V) may provide a ratio which approxi- 

 mates the apparent separation of the lines more 

 nearly than does the ratio of pressures at constant flow 

 (table 1, lines II and IV). This would provide merit 

 in perfusing passive (nonautoregulating) vascular 

 beds at a constant rate of flow while recording the 



