CHAP, in.] ELIMINATION OF WASTE PRODUCTS. 669 



the glomeruli and not pass to the other branches of the renal 

 art TIT, in which case the circulation of the glomeruli would be 

 exclusively (or nearly so) affected; but of this at present we 

 know nothing, and the general argument remains good if we speak 

 simply of the branches of the renal artery as a whole. 



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 and to the capillaries themselves. 



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. (Cf. 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 constituents 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 

 fibrin factor, but only water accompanied by some, and apparently 



