510 THE EXCRETION OP URINE 



nic and the vagus. Other branches from plexuses in the region of the 

 suprarenal body and the aorta join with those coming from the celiac 

 ganglion to form what is known as the renal plexus, which is arranged 

 in a network along the blood vessels and on the walls of the pelvis of 

 the kidney. These fibers are distributed to the very smallest blood ves- 

 sels, and nerve fibers have been observed among the cells of the tubules. 



THE MECHANISM OF THE EXCRETION OF THE URINE 



The great number as well as the variety of substances which are pres- 

 ent in both the blood and the urine makes it appear improbable that 

 urine excretion is dependent upon chemical combinations within the 

 renal cells, and leads us to seek a physicochemical mechanism to explain 

 the phenomenon. Can we discover the processes by which the kidney 

 fabricates a highly concentrated solution of salts from a very dilute 

 solution of the same salts in the blood plasma ? The problem is compli- 

 cated by the fact that the ratios existing between the concentration of 

 each urinary salt in the urine and the concentration of the same salt 

 in the blood are different. In other words, the urine is not merely 

 concentrated blood plasma freed from protein. 



The passage of Avater and salts through the capillary wall and through 

 the basement membrane surrounding the renal cell probably takes place 

 by simple diffusion. If it were otherwise, an expenditure of energy 

 would be required, and it is difficult to understand how a basement 

 membrane could bring about energy changes. Any substance to which 

 the cell membrane is permeable will diffuse into the cell until an equi- 

 librium is established between its concentration within the cell and 

 that of the lymph or blood plasma. A nondiffusible substance will not' 

 enter the cell because it can not pass through the cell membrane, and 

 if it exerts an osmotic pressure, it will also tend to keep the water in 

 which it is dissolved from entering. If water does pass into the cell 

 under these conditions, it is due to the expenditure of energy opposed 

 to and greater than that which is offered by the osmotic pressure of the 

 nondiffusible substances. Possible sources for such energy are the pres- 

 sure of the blood in the renal capillaries, which would exert a force op- 

 posite to that of its osmotic pressure, and the presence within the cell of 

 a concentration of salts greater than is present in the blood, and able to 

 exercise a sufficient osmotic force to draw fluid into the cell against the 

 osmotic force of the nondiffusible salts. The passage of the urinary 

 constituents through the cell might also be due to- simple diffusion, the 

 substances passing through the cell to be extruded on the other side in 



