The Renal Blood Flow 77 



value) , and in view of the fact that the mean brachial blood 

 pressure was not lowered, it must be concluded that this re- 

 duction in renal plasma flow was due to constriction of the 

 arterioles of the kidney, either on the afferent or efferent side 

 of the glomeruli. The locus of the constriction can, we believe, 

 be identified from the available data. Had the afferent arter- 

 ioles been constricted the glomerular pressure would have 

 been reduced, and consequently the filtration fraction would 

 have fallen, while efferent constriction would have raised the 

 glomerular pressure and increased the filtration fraction. The 

 fact that the filtration fraction rose rather than fell is there- 

 fore evidence that it was the efferent rather than the afferent 

 arterioles which were constricted. Since the filtration frac- 

 tion increased almost in the same proportion as the plasma 

 flow was reduced, the filtration rate remained almost un- 

 changed, and had we observed the filtration rate alone we 

 might have concluded that adrenin has no effect upon the 

 kidney. Here we have a demonstration of an important fact 

 about the glomerular circulation. The construction of the 

 glomeruli is such that when the renal blood flow is altered in 

 consequence of changes in efferent arteriolar tone, the simul- 

 taneous and opposite changes in glomerular pressure tend to 

 maintain the filtration rate at a constant level. The filtration 

 rate in normal man is remarkably constant, even more con- 

 stant than the renal blood flow; and this constancy may be 

 attributed to the circumstance that changes in renal blood 

 flow appear to be mediated by changes in the tone of the ef- 

 ferent, rather than the afferent, glomerular arterioles.*^ This 

 unique feature of the glomerular apparatus is of considerable 

 importance since extreme changes in renal blood flow may 

 occur in the diseased kidney with practically no change in 

 the filtration rate. 



