700 GLOMERULAR SECRETION. [BOOK n. 



In dealing with the vascular system we saw that relaxation 

 of a small artery, taking place without any marked change in 

 the general blood-pressure and in neighbouring arteries, leads to 

 a fuller and more rapid stream of blood through the capillaries 

 supplied by the artery, and that at the same time the pressure 

 in the capillaries themselves is increased ; owing to the decrease 

 of peripheral resistance through the widening of the artery, the 

 great fall of pressure (see 116) so characteristic of the peripheral 

 region is shifted from the arterial side of the capillaries towards 

 the venous side. 



Hence, as we have already said, when the renal artery dilates 

 two things happen in the loops of the glomeruli : a fuller, more 

 rapid stream of blood passes through them, and that blood as it 

 flows through them is exerting a greater pressure than before on 

 their walls. How does each of the events stand towards the 

 secretion of urine ? 



We have not at present the means of inducing a fuller and 

 more rapid flow without increasing the pressure ; but we may 

 easily obtain increase of pressure without the fuller and more rapid 

 flow. If we hinder or obstruct the outflow through the renal vein 

 we at once increase the pressure in the glomerular loops as in the 

 other capillaries of the kidney. Now, when the blood-pressure in 

 the glomeruli is thus raised by partial obstruction to the venous 

 outflow, the flow of urine so far from being increased is diminished. 

 Obviously then the passage of water and material through the 

 walls of the glomerular loops, to go to form the urine, is not the 

 result of mere pressure, and cannot therefore be spoken of properly 

 as a process of filtration. (Of. 302.) And we may here draw a 

 comparison between the passage of water and material through 

 the wall of a capillary in an ordinary situation to form lymph and 

 the passage through the wall of the glomerular loop to form urine 

 or part of urine. The former as we have seen ( 302) appears to 

 be directly dependent on pressure, though influenced as we have 

 also seen in a very material way by the condition of the vascular 

 wall ; and hindrance to venous outflow, so inefficient in promoting 

 a flow of urine, is as we have seen especially favourable to the 

 transudation of lymph. In the former case the substances which 

 pass through the capillary wall may be described as the con- 

 stituents of the blood generally, proteids as well as salts and other 

 soluble and diffusible matters. Through the wall of the glomerular 

 loop there pass, so long as that wall is sound and intact, neither 

 albumin nor globulin nor any other proteid, but only water ac- 

 companied by some, and apparently a selection of some, of the 

 soluble diffusible constituents of the blood ; for, as we have said, 

 the presence of proteids in normal urine is contested, and, at most, 

 there is present a very small quantity only (which moreover may 

 come from the tubular epithelium). This difference in the material 

 which passes through may be referred to the differences in the 



