THE SECRETION OF URINE 1067 



Theories of Urinary Secretion. — The kidney is the most important 

 excretory organ of the body. Its function is to separate the constitu- 

 ents of the urine from the blood— its watery part as well as its solids. 

 Upon it, in particular, rests the maintenance of the composition of the 

 body-fluids; and hence, it must keep up an almost continuous activity 

 which cannot be compensated for by any other organ. Thus, it is a 

 well-known fact that the removal of both kidneys is fatal, owing to 

 the accumulation in the blood of the end-products of protein metabo- 

 lism. The same result follows the ligation of botli renal arteries, 

 but the extirpation of only one organ usually produces no untoward 

 effects, because the opposite organ then enlarges and accomplishes 

 the work previously performed by the two organs together. 



The modern views regarding the manner in which the renal tubules 

 perform their work, is based upon the older theories of Ludwig^ and 

 Heidenhain.2 The former embodies the simple physical principles of 

 filtration and diffusion and the latter, these principles in conjunction 

 with a secretory activity on the part of the lining cells of the tubules. 

 The filtration theory of Ludwig holds that the glomerulus plays the 

 part of a filter, giving rise to a quantitatively and qualitatively com- 

 plete urine under the pressure of the blood. This structure, therefore, 

 constitutes the most important segment of the urinary tubule, while 

 the others fulfill merely the function of a conducting channel. In 

 substantiating this view, Ludwig laid particular stress upon the struc- 

 tural peculiarities of the glomerulus, emphasizing the fact that it 

 consists of a coil of capillaries which are suspended in a double-walled 

 capsule. Moreover, the narrowness of the efferent vessel tends to 

 augment the lateral pressure and to diminish the velocity of the blood- 

 flow. As far as the pressures are concerned, it will be noted that a 

 capillary blood pressure of 40 to 60 mm. Hg. is in this instance con- 

 trasted against a pressure of about zero, thus affording most favorable 

 conditions for a passive transfer of the constituents of the blood into 

 the capsule of Bowman. In the tubule this process is then augmented 

 by an endosmosis between the concentrated blood and the watery 

 urine which leads to a passage of the molecules of water from the urine 

 into the blood until the former has acquired its normal consistency. 

 This process of reabsorption of water from the urine Ludwig conceived 

 as purely physical diffusion, although he clearly recognized the fact 

 that this process may undergo decided changes in consequence of the 

 administration of diuretics, such as urea and sodium chlorid. 



In 1842 Bowman^ expressed the idea that the glomerulus serves 

 merely as the seat of the secretion of the watery part of the urine, 

 whereas its solid constituents are formed in the tubule itself. In 

 analogy with his work upon other glands, Heidenhain then promulgated 

 the theory that the urine is not produced solely by filtration and 



1 Wagner's Handworterb. der Physiol., ii, 1844, 628. 



2 Hermann's Handb. der Physiol., v, 1883, 279. 



3 Phil, transact., London, i, 1842, 57. 



