436 EXCRETION 



THE METHOD OF EXCRETION OF URINE. 



The secretion of urine is an act the complexity of which can be profitably 

 discussed only after a clear understanding of three main factors which have 

 already been presented, viz., the chemical composition of the urine secreted, 

 the structure of the kidney tubule as a secreting organ, and, finally, the chem- 

 ical composition of the blood which supplies the materials to the kidney for 

 the formation of the urine. The substances found in the urine are, for the 

 most part, also to be found in the blood plasma. But the relative percent- 

 age composition is very different. The amount of urea in the blood is only a 

 fractional part as concentrated as in the urine, while albumins and sugars, 

 which are so plentiful in the blood, are normally present in the urine only 

 in traces. The presence of the glomerulus with its special vascular supply, 

 and the different loops of the tubule, with its gland-like epithelial wall, 

 would, a priori, lead one to suspect special functions for each. 



Theories of the Secretion of Urine. Bowman in 1842, wholly on 

 structural grounds, advanced a theory of urinary secretion. This theory was 

 given an experimental basis and elaborated by Heidenhain and generally 

 bears his name. 



Heidenhain' 's theory is stated as follows: 



i. The secretion in the kidney depends upon the physiological activity of 

 special secreting cells which are of two kinds. 2. The first type of cell is 

 represented by the single layer of epithelium covering the glomerular capil- 

 laries. These cells secrete especially water and salts. 3. The second type of 

 cell is represented by the gland-like epithelial cells which form the convo- 

 luted tubules and the loop of Henle. These cells secrete the urea, uric acid, 

 and other specific constituents of the urine. 4. The activity of each kind of 

 cell is influenced by the chemical composition of the blood and by the flow 

 of blood through the kidney. 5. The relative secretory activity of the glomer- 

 ular cells and the tubule cells is sufficient to account for the variation in the 

 chemical composition of the urine. 



Ludwig's theory, advanced in 1844, is a strictly mechanical theory of 

 urinary secretion based on experiments which he presented, i . He considered 

 the glomerulus and Bowman's capsule as a filtering apparatus in which 

 substances present in the blood are driven through the epithelium of the 

 capsule into the renal tubule by the positive pressure of the blood in the 

 glomerular capillaries. 2. This very dilute urine in the capsule is supposed 

 to be concentrated by the resorption of water as it flows down the tubule. 

 Ludwig originally considered this resorption of water an imbibition process 

 in which the greater saturation of salts in the blood caused water to be taken 

 up through the renal tubule walls, an osmotic process. At present most 

 observers who accept the view that filtration takes place at the glomerulus 

 explain the resorption of water down the tubules as an act of cellular 

 resorption or secretion. 

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