576 



VETERINAKY PHYSIOLOGY 



of concentration of the various urinary constituents, compared 

 with their amount in the blood, varies very greatly (p. 570), 

 some of these substances would also have to be absorbed along 

 with the water. 



It is of course possible that both these processes are in 

 operation. 



In carrying out either process the epithelium has to do an 

 equal amount of ivork. 



This may be rendered clearer by the figure 228 — 



Fig. 228. — To illustrate the two views of the mode of action of the renal 

 tubules. The figures 16'2 and 6'7 give the osmotic pressures of the 

 urine and of the blood plasma respectively. 



On the secretion theory, electrolytes have to be piled up 

 from the point of low concentration in the blood to the point of 

 high concentration in the urine ; while on the reabsorption 

 theory water has to be taken from a point of high osmotic 

 pressure and passed to a point of low osmotic pressure. 



It is well to understand what the reabsorption theory 

 implies. 



To produce the average 30 grms. of urea excreted per diem 

 by a man from the plasma containing 0'03 per cent, would mean 

 the filtration through the glomeruli of some 30,000 cc. But 

 since only 1500 cc. of urine are generally secreted, this would 

 mean the reabsorption of no less than 28,500 cc. — about 95 

 per cent, of what was filtered off! The work of filtering off 

 some 30,000 cm. has superimposed upon it the work of reabsorb- 



