THE SECRETION OF URINE 1073 



ules. It is held that a large quantity of plasma is filtered through the 

 glomerular vessels under the pressure of the blood and under exclusion 

 of the colloidal proteins. The non-colloidal material being allowed to 

 pass, owing to the permeability of the vessel-wall, imparts to the urine 

 a concentration approximately equal to that of the blood. Conse- 

 quently, the blood leaving the glomeruli, may be compared to a con- 

 centrated colloid solution which requires salts and water to reconvert 

 it into its original form. ' This end the blood attains as it traverses 

 the tubule by absorbing the constituents required by it from the 

 glomerular filtrate. Those substances which the plasma must again 

 obtain, are called threshold substances, while those which it does not 

 need again, are designated as non-threshold substances. The latter 

 remain in the urine to be excreted. Thus, urea must leave the body 

 as long as any of it is present in the blood, whereas the urinary sugar 

 must again pass into the blood, provided its concentration remains 

 below the physiological limit. 



This theory may well be employed to explain several perplexing 

 points regarding the pathology of the kidney, particularly such as 

 concern diuresis, albuminuria and the phenomena associated with the, 

 stagnation of the urine in consequence of urethral obstructions. In 

 spite of this fact, however, it cannot be said that it rises above the 

 dignity of a mere working hypothesis, because in view of the uncertain 

 and contradictory character of the evidence presented in its favor, 

 it seems risky to accept it as a truity. None seems sufficiently definite 

 to allow of no other and, possibly, more correct interpretation. It 

 appears, therefore, that the student who accepts Heidenhain's theory 

 which does not wholly exclude the factor of glomerular filtration, can- 

 not be considered as less " modern" than the one who adheres to the 

 absorption-hypothesis. Thus, it may be said that water and salts, 

 and even such substances as sugar, egg-albumin, peptone and hemo- 

 globin when injected into the blood-stream, are mainly excreted by 

 the glomeruli, whereas urea, uric acid, and the other organic constitu- 

 ents, together with small amounts of water and salts, are excreted 

 by the epithelium of the uriniferous tubule. Neither process is accom- 

 plished by filtration alone, but embraces a definite vital element 

 consisting of unknown physicochemical factors resident in the renal 

 cells. Both processes are closely dependent upon the pressure and 

 velocity of the renal blood flow. In this connection, it should also 

 be mentioned that the existence of separate secretory nerves to the 

 kidney has not been proved, 1 although it must be granted that the 

 stimulation of the fibers constituting the renal plexus, profoundly 

 affects the quantity and quality of the urine. These results, however, 

 may be due wholly to vasomotor influences. 



Diuresis. The diuretics produce their characteristic effect in two 

 ways, namely,' by augmenting the secretory pressure and concentra- 



1 Asher and Pearce, Zeitschr. fur Biol., Ixiii, 1913, 83; and Pearce and Carter, 

 Am. Jour, of Physiol., xxxviii, 1915, 350. 



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