RESISTANCE AND CAPACITANCE PHENOMENA IN VASCULAR BEDS 



957 



Normal 



Arterial occlusion 



Vosospasm 



Venous obstruction 



fig. 28. Averaged plethysmographic pulses from the digits of patients. A — normals; B — patients 

 with arterial occlusion, but with good collateral circulation; C — patients with vasospasm; D — pa- 

 tients with deep thrombophlebitis. All pulses are redrawn to the same amplitude and same time dura- 

 tion. Note : the flow to pulse ratios in the patients with arterial occlusion were 2 to 5 times those of 

 the normals. 



follows the notch, indicating a larger than normal 

 reflected wave component in the volume pulse (11). 



INTERPRETATION OF VASCULAR BEHAVIOR 

 FROM MEASUREMENTS OF FLOW, 

 PRESSURE, AND VASCULAR VOLUME 



Flow through a vascular bed is dependent upon 

 arterial pressure, arteriolar inflow and venous outflow 

 resistances, viscosity of the blood, and extravascular 

 pressure. Physiologic control of flow is exerted in many 

 vascular beds by local autoregulation as modified by 

 the influence of autonomic nerves and by circulating 

 constrictor substances. 



Analysis of the influences of autonomic nerves and 

 circulating vasoactive substances under conditions of 

 various physiological stresses is complicated, particu- 

 larly if there is an accompanying change of arterial 

 pressure. For instance, if there is a decrease in arterial 

 pressure to 50 per cent of the control, the measured 



peripheral resistance might increase in a nonreactive 

 bed such as the skin, whereas the resistance might 

 decrease in a reactive bed such as that of kidney, 

 brain, or skeletal muscle. These changes would occur 

 in the absence of extrinsic influence. Therefore, in 

 order to analyze the potency of extrinsic influences 

 upon the resistance vessels, it is necessary to obtain 

 previous data on the behavior of the resistance vessels 

 during changes of pressure per se in the absence of 

 extrinsic influences, and then compare the measured 

 changes in resistance with these pre-established 

 findings before drawing any conclusions as to the 

 influence of extrinsic factors on the vascular bed. 



Similar observations apply to measurements of 

 vascular volume. The latter measurements become 

 important, particularly in conditions such as shock 

 in which it is presumed that there is a stagnation and 

 pooling of blood in various vascular beds; however, 

 extensive data are not as yet available on such 

 changes in vascular volume. 



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