1068 EXCRETION 



osmosis, but is materially modified by the activity of the cells lining 

 the convoluted tubule. It is assumed that the glomerulus furnishes 

 the water and inorganic salts, while the distal convoluted tubule 

 produces the specific organic constituents, together with an inconsider- 

 able quantity of water. Thus, the character of this secretion depends 

 in reality upon the activity of both groups of cells and varies with the 

 differences in the composition of the blood, the blood pressure, and the 

 velocity of the capillary blood-stream. Th vS rather schematic presenta- 

 tion of these two theories, however, should not convey the idea that 

 they are directly opposed to one another. They are not, because 

 Heidenhain does not deny the occurrence of filtration, but merely 

 amplifies this process by the secretory activity of the cells. 



Facts Contradicting the Pure Mechanical Theory. — The kidney 

 is one of the most vascular organs in our body, receiving about 150 

 c.c. of blood per minute for each 100 grm. of substance; moreover, 

 its blood-supply is accurately controlled by a vasomotor mechanism 

 contained in the renal and suprarenal plexuses.^ The division of these 

 fibers gives rise to a relaxation and injection of the blood-vessels of 

 this organ, this change being associated as a rule with a copious flow 

 of urine and a slight albuminuria. - It is also a well-established fact that 

 urinary secretion is closely dependent upon the blood pressure, because 

 a fall in the latter is usually followed by a diminution in the quantity 

 of the urine, and vice versa. While this relationship is entirely in 

 accord with filtration, it can easily be shown that pressure is not the 

 only factor here at work, because if the renal vein is temporarily ob- 

 structed, a procedure which must necessarily raise the intraglomerular 

 pressure, the flow of urine stops altogether. In a similar way it has been 

 shown that a partial obstruction of the venous return produces only a 

 slight diminution in the rate of flow, which may immediately be in- 

 creased by the administration of a diuretic. This latter fact is of 

 importance, because SoUmann's experiments upon perfused excised 

 kidneys have shown that the stoppage of the flow of urine following 

 the ligation of the renal vein, may be caused by a mechanical obstruc- 

 tion of the uriniferous tubules caused by the distention of the entire 

 organ. A dissociation between the renal blood-supply and the flow of 

 urine may also be effected by the temporary ligation of the renal 

 artery, or by the stimulation of the vagus nerve. ^ Almost directly 

 thereafter the flow of urine ceases, as might be expected, but the flow 

 does not regain its former value immediately upon the reestablishment 

 of normal circulatory conditions, but in many instances only after an 

 interval of from 30 to 60 minutes. Consequently, while it may be 

 granted that the function of the kidney, like that of other organs, 

 is closely dependent upon the blood-supply, it is easily apparent 

 that some outside factor is here at work. In this connection, brief 



1 Bradford, Jour, of Physiol., x, 1889, 358, Asher and Pearce, Zeitschr. fur 

 Biol., Ixiii, 1913, 83, and Burton-Opitz, Jour. Exp. Med., xl, 1916, 437. 



2 Richards and Plant, Am. Jour, of Physiol., xlii, 1917, 592. 



