652 THE SECRETION OF URINE. 



in diabetes insipidus, the urine secreted may have A = 0'16 C., i.e. a 

 difference between A of blood and of urine of 4 C. Hence he con- 

 cludes that the production of urine by the glomeruli is also attended 

 with the doing of work, and must therefore be looked upon as a process 

 of secretion. We might, however, still adhere to the theory of glomer- 

 ular nitration, if we assumed either that the cells of the tubules could 

 absorb water or solids according to the needs of the organism, or that 

 they were able to secrete pure water and so dilute the glomerular 

 trans udation. 



Heidenhain's criticism of the theory of Ludwig. The difficulties 

 in the way of accepting Ludwig's hypothesis have led Heidenhain, after 

 a long series of researches on the subject, to reject this theory abso- 



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Heidenhain sums up his objections to the mechanical theory under 

 the following headings : 



1. The hypothesis that a rise of arterial pressure causes increased 

 transudation through the vessel walls, is not confirmed by our experience 

 in other parts of the body (lymphatics of the limbs, salivary glands). 



2. This hypothesis is rendered the more improbable for the kidney, 

 since in this situation the glomerular capillaries are covered by a second 

 layer of epithelium, and we know, from Leber's researches on the cornea, 

 that such a simple epithelial layer can afford great resistance to 

 nitration. 



3. If we assume that all the constituents of the urine are filtered 

 off in the glomeruli, the small amount of urea in the blood renders it 

 necessary that in man about 70 kilos, of fluid should be filtered through 

 and reabsorbed, in order that the urea produced in the course of 

 the day may be excreted in the urine, an amount which is highly 

 improbable. 



4. According to the filtration hypothesis, the amount of urine formed 

 must always increase with increased capillary pressure, whereas we find 

 that, on increasing capillary pressure by ligature of the renal vein, the 

 urinary flow is abolished. 



5. The hypothesis that the glomerular transudate is concentrated by 

 a process of osmosis or diffusion, on its way through the glomeruli, is 

 rendered impossible by the fact that the osmotic pressure of the 

 urine may be, and generally is, much higher than that of the lymph 

 or blood. 



6. The filtration hypothesis does not explain why the amount of 

 urine is increased by the presence of water or crystalloid (hmmfaMg) 

 substances in the blood. 



From his own researches on the subject, Heidenhain conies to the 

 following conclusions with regard to the mechanism of secretion : 



1. In the kidney, as in all other glands, the secretion depends on the 

 active intervention of special secretory cells. 



2. The first type of these cells is represented by the simple layer 

 of epithelium covering the glomerular loop of capillaries. The office of 

 these cells is to secrete water and such salts of the urine as are found 

 in all other parts of the body in watery solution (e.g. sodium chloride). 



3. Another system of secretory cells, forming the lining investment 

 of the convoluted tubules and ascending tubule of Henle, secrete the 

 specific constituents of urine (urea, uric acid, etc.). Under some con- 

 ditions they may at the same time secrete a certain amount of water. 



