RESISTANCE AND CAPACITANCE PHENOMENA IN VASCULAR BEDS 



937 



behavior of the cognate vascular bed (41). Analysis 

 of the rate of backflow may be useful, however, as a 

 measure of the effectiveness of existing communica- 

 tions between cognate and collateral arteries. 



Errors analagous to those on the arterial side may 

 occur when recording venous outflow, due to the 

 presence of postcapillary communications with 

 veins draining collateral vascular beds (fig. i-c, d). 



PASSIVE CURVILINEAR RELATIONSHIP OF PRESSURE-FLOW 



plots. The simplest relationships between pressure 

 and flow occur in vascular beds which do not show 

 autoregulation such as skin or "nonreactive" hind 

 limbs (46, 88, 115) or the pulmonary vascular bed 

 (78). All such curves are curvilinear with the con- 

 vexity toward the pressure axis (fig. 3). In such 

 studies changes of vasomotor tone have occurred 

 spontaneously (46) or have been induced by infusion 

 of epinephrine (88). With increase in vasomotor tone 

 the curves rotate toward the pressure axis so that for 

 any given level of pressure, flow is less (fig. 3). In all 

 such experiments, using blood as the perfusate, the 

 curves approach the zero flow axis asymptotically. A 

 'critical closing pressure," such as was described by 



Burton (6) and others (10, 37) was not noted in the 

 above studies (see also 32, 35, 64). Curves of this 

 type may be said to exhibit a "passive" relationship 

 between pressure and flow. 



mathematical relationships. When the data from 

 the above experiments are plotted on log-log paper, 

 approximately straight lines with varying foci and 

 slopes are obtained (fig. 3). It appears, therefore, that 

 the mathematical relationship between flow and 

 pressure is a power function 



F = c X P n 



where F = flow in ml per min, c is a constant, P = 

 arteriovenous pressure difference in mm Hg, and n 

 is an exponent having a value between 1 and 3 (46). 

 The lowest value of n and the highest value of c were 

 found at "low vasomotor tone" (fig. 4, point A) and 

 vice versa (fig. 4, point C). Levy & Share (74) have 

 confirmed these findings and demonstrated that with 

 maximal dilation induced by a 10-min period of 

 ischemia and subsequent perfusion with hypoxic 

 blood, the value of n is 1 .0. The relationship of c to n 



48 



50.0 



/ / 



1 /' ■ J 



80 120 160 



PRESSURE 



10 20 50 100 



PRESSURE 



300 



fig. 3. Plots of the arteriovenous difference pressure vs. the blood flow in a cutaneous (saphenous) 

 bed at three levels of spontaneous "vasomotor tone." Left half, plotted linearly; right half, plotted on 

 log-log paper. Triangles represent the lowest level of vasomotor tone; circles represent an intermediate 

 level and squares, the highest level of vasomotor tone. The figures in the upper left-hand corner of the 

 graph represent the parameters for the straight lines reproduced in the log-log plot and for the curvi- 

 linear lines reproduced in the linear plot. Flow — ml/min; pressure — mm Hg. [Modified after Green 

 et al. (46).] 



