RESISTANCE AND CAPACITANCE PHENOMENA IN VASCULAR BEDS 



953 



sciatic nerve stimulation. Incidentally, they noted that 

 retrograde flow could be induced in the distal small 

 vessels from vein to artery when these were not being 

 stimulated, and the arterial pressure was low. 



It should be noted that Davis and Hamilton speak 

 of recording "'small vessel" pressures, but it is our 

 interpretation that the data indicate the role of 

 intermediate vessels as well as arterioles and venules. 

 It remains to be demonstrated whether the above 

 observations represent a response that might occur 

 under reasonable degrees of physiologic stress or a 

 change that would occur only under unusual condi- 

 tions such as intense electrical stimulation of the 

 sympathetic chain. Effects such as these were signifi- 

 cantly less marked in the intermediate vessels supply- 

 ing the rabbit ear during stimulation of the third 

 cervical nerve or the ipsilateral cervical sympathetic 

 trunk (14). 



INDICATOR CONCENTRATION (lo»er) INTEGRATOR (upper) 



CONTROL 166 mm Hg 



ACETYLCHOLINE 



fig. 23. Records obtained from the dog's paw showing the 

 time course of the concentration of an indicator (indocyanine 

 green) recorded in the venous outflow following an intra- 

 arterial injection of the indicator, C. A . curve recorded from 

 the output of an integrator whose input was the output from 

 the indicator concentration recorder. Time lines at the top — 5 

 sec. B and D: similar records obtained during an intra-arterial 

 infusion of acetylcholine, which were recorded after a stable 

 rate of flow had been attained. Perfusion pressure 1 16 mm Hg 

 for all four curves. In each case, 0.125 m g 01 indicator was 

 injected as a square -wave pulse during a i-sec period. The 

 upper asymptote was drawn parallel to '"integrator base 

 line." The slight inclination of the latter was due to a failure 

 to balance the integrator to the densitometer output when 

 control blood was flowing through the cuvette. A 2- to 3-sec 

 delay is noted between the beginning movement of the densi- 

 tometer and the beginning movement of the integrator pens. 

 This is due in part to the lag of the recorder and possibly to 

 slight delay in the integrator. It is corrected in the calculations 

 since the true mean transit time is obtained by subtracting 

 the delay time from the apparent mean transit time. The 

 delay time is obtained by making a second injection at the 

 sampling site in the venous outflow. 



BLOOD VOLUME IN VASCULAR BEDS 

 (VASCULAR CAPACITY) 



Methods 



The volume of the various components of the 

 vascular bed has been studied by injection techniques 

 (77). The volume in a given vascular bed can be 

 computed also by ligating abruptly the artery and 

 vein, washing out the contained blood, measuring the 

 hemoglobin content of the washout and comparing 

 this with the hemoglobin content of the blood per- 

 fusing the bed. 



Estimates of vascular capacity in an intact bed can 

 be made by intra-arterial injection of an indicator 

 with determination of the time course of the indicator 

 concentration in the venous effluent (42). A typical 

 record (fig. 23) shows an abrupt rise in the indicator 

 concentration, beginning about 5 sec after the 

 injection; the concentration reaches a peak in 10 sec 

 and then returns to control level at approximately 90 

 sec. The rate of flow can be computed by dividing 

 the milligram of indicator injected by the area under 

 the indicator concentration curve, expressed in 

 mg • sec/ml (fig. 23, C and D). 



Curves A and B in figure 23 show the integration 

 of the indicator concentration curve. By integrating 

 the area above and to the left of the upper curve and 

 dividing this area by the height of the curve, the mean 

 transit time of the indicator through the vascular bed 

 can be obtained. Multiplication of the mean transit 

 time by the rate of flow gives the volume in the 

 vascular bed. In this experiment on the paw, flow was 

 26.5 ml per min (0.44 ml/sec), mean transit time 

 21.2 sec, and vascular volume 9.35 ml. Comparison 

 of the flow measured with the indicator concen- 

 tration curve with that measured simultaneously with 

 the electromagnetic flowmeter shows the former to 

 average 103 percent (sd 4.6) of the latter. Correlation 

 of the last determination of vascular volume with the 

 vascular volume determined by the hemoglobin 

 washout method shows that the former averaged 94 

 per cent (sd 38.5) of the latter (48). 



Effects of Various Factors on I'ascular Volume 



In a set of experiments on the dog's paw, changes 

 in perfusion pressure produced approximately propor- 

 tional changes in flow (fig. 24), whereas mean transit 

 time varied inversely. As a consequence neither vas- 

 cular volume nor conductance (1 /resistance) was 

 altered to any significant extent. Acetylcholine, 



