THE RENAL CIRCULATION 



1493 



40 80 120 160 200 240 

 RENAL ARTERY PRESSURE ( mmHq ) 



fig. 25. Renal circulatory autonomy in the dog as a func- 

 tion of tissue pressure (needle puncture). Symbols 4A and 4C 

 represent curves obtained with blood perfusions before and 

 after 4% dextran (4B). [After Hinshaw et al. (143).] 



increase in pressure. It was concluded that the auto- 

 regulation occurred because of increased accumula- 

 tion of extravascular fluid resulting from enhanced 

 filtration at high pressure, which compressed low 

 pressure vessels. Significantly, blood volume estimated 

 from mean transit time (T-1824 X mean blood flow) 

 was shown to decrease slightly at pressures in the 

 range 100-200 in the autoregulating kidney (188), 

 but volume increased in the K.CN poisoned kidney. 

 Hence the weight change is likely due largely to ex- 

 travascular fluid accumulation. 



Analysis of regional resistance changes has been 

 attempted by Hinshaw et al. (145), on the basis of 

 certain assumptions. The first was that a stabilized 

 ureteral pressure after occlusion was a measure of 



the Bowman's capsule extravascular pressure. Then, 

 glomerular capillary pressure should equal this pres- 

 sure plus the plasma oncotic pressure (20 mm Hg 

 in this series). Another assumption was that intra- 

 renal venous pressure (postperitubular capillary seg- 

 ment) was equal to tissue pressure (IRP), (which has 

 been shown to be correct for the arcuate veins at 

 elevated venous pressure) plus the plasma oncotic 

 pressure. The authors have formulated the regional 

 resistances as follows [reprinted with permission from 

 Hinshaw et al. ( 1 45)] : 



PRE -GLOMERULAR SEGMENT 

 = RA-UP- COP 



POST -GLOMERULAR SEGMENT 



(a) EFFERENT ARTERIOLAR SEGMENT 



UP-TP 



F 



(b) post- peritubular capillary segment 



(venous segment) 



_ tp -i- cop -rv 



f 



RA= RENAL ARTERY PRESSURE UP" URETERAL PRESSURE 



TP. TISSUE PRESSURE COP 'COLLOID OSMOTIC PRESSURE 



RV- ORIFICE RENAL VENOUS PRESSURE 



Autoregulation was shown to persist during occlu- 

 sion of the ureters (144), as indeed it does during 

 venous pressure elevation (119, 123, 281). The above 

 estimates of regional resistance are applicable, then, 

 to the autoregulation manifested during ureteral 

 occlusion in the isolated perfused kidney. In a range 

 of 100 to 191 mm Hg renal arterial pressure the fol- 

 lowing average changes occurred : preglomerular 

 resistance, —4 per cent; postglomerular, +101 per 

 cent (in the latter value, most is attributable to the 

 postperitubular capillary segment). Under these spe- 

 cial circumstances, afferent arteriolar control seems 

 unimportant, and it is the influence of increased 

 IRP on compressible postglomerular vessels that 

 appears to dominate. 



Although this hypothesis is ingenious in its applica- 

 tion, the special circumstance of the measurements 

 will make it difficult to apply to the normally func- 

 tioning kidney. It is well to recall that the fundamental 

 precept, i.e., that IRP varies with arterial pressure, 

 has not been uniformly accepted by all investigators. 



If the above hypothesis is correct, decapsulation of 

 the kidney should have a significant influence on the 

 autoregulatory mechanism. In this, investigators are 

 not in agreement. Bounous et al. (28) after careful 



