536 GLOMERULAR SECRETION. [Book ii. 



the capillaries themselves is increased ; owing to the decrease of 

 peripheral resistance through the widening of the artery, the great 

 fall of pressure (see § 98) so characteristic of the peripheral 

 region is shifted from the arterial side of the capillaries towards 

 the venus 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 obstruc- 

 tion 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. § 244.) 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 Avail of the glomerular loop to form urine or part of urine. 

 The former as we have seen (§ 244) appears to be dependent on 

 pressure, though influenced as we have also seen in a very mate- 

 rial 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. Moreover, the substances which pass through the capil- 

 lary wall to form lymph 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, bat only water accom- 

 panied 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 an insignificant 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 nature of the partition. The transudation of lymph takes 

 place through the capillary wall ; between the blood on one side 

 and the lymph in the lymph-space on the other is only the thin 

 film of conjoined epithelioid plates. But the corresponding wall 



