THE SECRETION OF URINE. 421 



ents 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 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 of the glornerular loop is covered over 

 and wrapped around, so to speak, by an adherent layer of cells, which 

 though reduced and thin are still epithelial cells ; the materials which go 

 to form urine have to pass through these cells as well as through the film 

 of epithelioid plates. It seems to be this layer of cells which determines 

 what shall pass and what shall not. 



Obviously the passage through this epithelium is of a peculiar nature. 

 The necessary condition for the due accomplishment of the passage is as we 

 have seen a full and rapid stream of (arterial) blood ; the high pressure 

 which accompanies that full and rapid stream, though probably under 

 normal circumstances an adjuvant, is by itself helpless. Thus when the 

 pressure is raised by venous obstruction, in which case the high pressure is 

 accompanied by a slow stream or by actual arrest of the flow, even the 

 passage of mere water is retarded. Seeing that many of the constituents 

 of urine are diffusible substances certainly preexisting in the blood, inor- 

 ganic salines for instance, and seeing that diffusible abnormal constituents 

 of blood, such as peptone and sugar, pass into the urine not by the tubular 

 epithelium but by the glomeruli, we might expect that diffusion, in contrast 

 to filtration (see 265), played an important part in the passage ; and a 

 full rapid stream would undoubtedly favor diffusion. But diffusion by 

 itself will not explain matters. Egg-albumin differs very slightly as regards 

 diffusibility from, serum-albumin, and yet while at the most a minute quan- 

 tity only of the latter passes into the urine in normal circumstances, the 

 former when injected into the blood at once makes its way into the urine, 

 presumably by the glomeruli. On the other hand urea is an eminently diffus- 

 ible body, and yet if we can trust the experiments on the amphibian kidney, 

 the main mass at all events of the urea of the urine passes by the epithelium 

 of the tubules. 



The important part played by the epithelium is shown when the epithe- 

 lium is deranged. If the renal artery be temporarily ligatured or other- 

 wise obstructed, so that the glomeruli are shut off from the blood-supply for 

 some little time, the secretion of urine is stopped ; on reestablishment of the 

 circulation the secretion of urine slowly returns, and the urine is then found 

 to be albuminous, remaining so for some little time. The serum-albumin 

 and globulin which could not pass through the intact epithelium, can pass 

 through when the epithelium is damaged by interference with its nutrition. 

 The appearance of albumin in the urine (albuminuria) is a not infrequent 

 symptom of kidney disease, and its presence in other than minute quantities 

 indicates imperfections in the glomerular epithelium. But even under un- 

 healthy conditions that epithelium still governs to a certain extent the pas- 

 sage of material ; for the proteids of the blood-plasma do not pass through 

 bodily or in a proportion which corresponds either to the relative proportion 

 in which they exist in the plasma or to the relative ease (or difficulty) with 

 which they pass through membranes. Though the " albumin" of albumin- 

 ous urine frequently consists of both serum-albumin and globulin, these do 

 not necessarily occur in the same proportion as in blood ; they vary in urine 

 much more than they do in blood ; and indeed the one or the other may be 

 absent ; moreover, fibrin factors are very rarely found. 



