386 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 chemi- 

 cal 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 percentage 

 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 which has more 

 recently been restated and given an experimental basis by Heidenhain. This 

 view as given by Heidenhain is 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 in 1844 advanced a strictly mechanical theory of urine secretion 

 based on his own experiments. 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. 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. 



Experimental Observations. There are numerous nerves to the 



