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A TEXTBOOK OF PHYSIOLOGY 



to the hiluin of the kidney, and together form the renal vein 

 (Fig. 231s). 



Such an anatomical arrangement of tubules and blood-supply 

 suggested to Bowman, who first described it, in 1840, that the 

 urine had a double source of origin glomerular and tubular. He 

 suggested that the water and salts of the urine were filtered from the 

 blood in the glomeruli, while the organic constituents of the urine 

 were secreted by the tubules, especially by the convoluted portions. 

 Ludwig laid stress on the vas efferens leaving the glomerulus being 

 narrower than the vas afferens, and put forward the view about the 

 same time (1844) that water and crystalloids of the blood filtered 

 through the glomeruli, and the urine was concentrated from this by 

 resorption in the tubules. 



A controversy arose around these two views, which resolved itself 

 into a question of principle. The Ludwig view was the more mechani- 

 cal one. It sought to minimize the unknown forces of the living 

 cells, and to make the excretion of urine a question of filtration through 

 the glomeruli and concentration by physical means in the tubules. 

 The adherents of the Bowman view attributed special selective activity 

 to the glomerular and tubular cells. The secretion of urine, to them, 

 was a vital function, and not to be explained by known physical 

 processes, such as filtration and osmosis. The glomerular epithelium 

 secreted water and salts (NaCl, etc.), the tubules the specific urinary 

 substances (urea, etc.) and some water. The activity of the kidney 

 was regulated by the amount of water and urinary substances in the 

 blood, and the velocity of flow of the blood through the kidney. This 

 is the view which has steadily gained support. 



In the course of the controversy two salient facts have emerged: 

 (1) That a concentration by resorption of dilute urine in the tubules 

 cannot be brought about by such physical forces as osmosis; (2) that 

 in the secretion of urine the kidney cells are performing work. 



In regard to the first point, it soon became clear that those physical 

 forces which had been evoked by the Ludw r ig school, such as diffusion 

 and osmosis, could not concentrate the weak urine supposedly filtered 

 through the glomerulus. When blood and urine are placed on either 

 side of a parchment membrane, water passes by dialysis from the 

 blood to urine. The electrolytes in normal urine are more concen- 

 trated than in the blood. In the case of the urine passed after drinking 

 a quart or two of water the opposite condition pertains. The kidney 

 works either way against osmotic force. It may secrete a urine with far 

 more salt than in the blood, or a urine with almost no salt in it at all. 



In regard to the second point, it has been demonstrated that 

 during the process of active secretion the amount of oxygen taken up 

 by the kidney from, and the amount of C0 2 given up to, the blood 

 is greatly increased. This is seen from the following figures: 



O 2 per minute 

 COo per minute . . 

 Urine per minute 

 Work per minute 



4-35 c.c. 



1-88 c.c. 



0-05 c.c. 



327-0 



o-58 c.c. 

 3-93 c.c. 

 1-53 c.e. 

 JCol-Og.cms. 



