THE SECRETION OF URINE 695 



of concentrating the dilute glomerular filtrate. This process of 

 concentration will be carried out by the tubule cells being readily 

 permeable to some molecules, e.g. water, sugar, chlorides, &c., 

 and relatively impermeable to others, e.g. urea, uric acid, pigments, 

 &c. The quantity of urine passing away from the kidney will 

 be determined by the difference between the rates of filtration 

 and of absorption. The composition of the urine will depend on 

 three factors (a) the relative amounts of the various constituents 

 in the blood, (b) the relative permeability of the tubule cells to 

 them, and (c) the rate at which the glomerular filtrate passes 

 down the tubule. For, this last factor will determine the amount 

 of change which the tubule cells can produce in the glomerular 

 filtrate, and we should expect that the greater the rate of flow 

 through the tubule the more the urine would resemble the 

 glomerular filtrate. 



On the Bowman-Heidenhain view the cells covering the 

 glomerulus are normally impermeable to all the constituents of 

 the blood excepting water and inorganic salts, which are passed 

 out by the vital activity of these cells and appear in the same 

 proportion in the glomerular filtrate as in the urine. The 

 glomerular filtrate will therefore be a very hypotonic fluid as com- 

 pared with the blood plasma, and much work will have to be done 

 by the cells of Bowman's capsule in effecting the separation of 

 this extremely dilute fluid. The cells of the tubule will be per- 

 meable to and excrete urea, uric acid, &c., but practically imper- 

 meable to dextrose and most salts, and to a less extent to water. 

 The tubule cells must also do much work, for, they have to pass 

 out solid constituents in extremely concentrated solution, and so 

 raise the osmotic pressure of the dilute glomerular filtrate up to 

 that of the urine. The quantity and composition of the urine 

 will depend upon (a) the quantity and proportion in the blood 

 of substances which have to be excreted, and (b) the rate at which 

 these substances are brought by the blood stream to the ex- 

 creting cells. 



Urine is secreted in order to get rid of material, and so help 

 to keep the quantity and composition of the blood constant. On 

 Heidenhairi's view an excess or decrease of any constituent in the 

 blood will lead automatically to its increased or diminished excre- 

 tion. But on Ludwig's view, in which all materials are passed 

 out through the glomerulus in the same relative proportion in 



