KIDNEY 49 



Mechanism of Urinary Secretion. Theories on this subject 

 group themselves, more or less definitely, about two points 

 of view, so that there are physical explanations and physio- 

 logical explanations. A tremendous amount of ingenuity has 

 been shown in experimentation in attempting to establish 

 one or the other point of view. At present the evidence is 

 most decidedly in favor of the physiological theories, which it 

 must be remembered do not exclude physical factors. 



Ludwig regarded the secretion of urine as due to simple 

 filtration and osmosis taking place in the glomeruli, and to 

 a concentration in the convoluted tubules, of the fluid thus 

 formed. 



Recent work has shown that the cells of the convoluted 

 tubes have a distinct secretory function in the elimination of 

 urea and related bodies. The evidence is: 



1. In birds, where uric acid takes the place of the urea in 

 mammals, the small solubility of the urates enables experi- 

 menters, by ligation of the ureters, to cause a deposit which 

 is always found in the cells of the convoluted tubes. 



2. Indigo carmine injected into the circulation of a living 

 animal may be precipitated by the injection of alcohol, when 

 the pigment is always found in the convoluted tubes. 



3. The inactive cells of the convoluted tubes are small, 

 granular, and toward the lumen show a striated border. During 

 activity they lose their striated border, project into the lumen 

 of the tubule, making it smaller, and a clear vesicular area is 

 formed near the nucleus. The vesicle ruptures and empties 

 its contents into the lumen. 



The excretion of water and salts takes place mainly through 

 the glomerular epithelium. There are no secretory nerves. 

 Section of the cord in the cervical region, which results in a 

 general fall of blood pressure, diminishes the secretion. In 

 general it has been found that the secretion of urine varies 

 both with the pressure of the blood in the glomeruli and the 

 quantity of blood flowing through the kidney, and Heidenhain 

 has insisted upon the latter factor as the essential one, giving 

 as evidence the fact that compression of the renal vein stops 

 the flow of the urine. This raises the pressure in the glomerulus, 

 but stops the flow of blood. Ligation of the vein for one-half 

 minute will stop the secretion for three-quarters of an hour, 

 4 



