ENERGY CHANGES INVOLVED IN SECRETION 267 



chloride are 0-55 and 1-10 per cent.; and hence in the expressions 

 for the work done in secreting urea and sodium chloride respectively 



the factor log ? has quite a different value in the two cases, being 



log,50 in the case of the urea and log*2 in the case of the sodium 

 chloride. As a result, taking the average daily quantities to be 

 30 grms. in the case of urea, and 16 grms. in the case of sodium 

 chloride, and correcting for the almost complete dissociation of the 

 sodium chloride, a calculation of the work done in the separation in 

 the two cases shows that the amount of work done in separating 

 the urea is nearly six times as great as that done in separating the 

 sodium chloride. 



This is quite different from the usual type of treatment, in 

 which it is taken in calculating the work done merely from the 

 lowerings of freezing-point of serum and of urine respectively, 

 that the calculation may be based on the supposition that the 

 secretion may roughly be regarded as a concentration of sodium 

 chloride. 



The reason of the fallacy is not far to seek : the urea solution is, 

 roughly speaking, concentrated fifty times in the process of secretion, 

 while the concentration of the sodium chloride is barely doubled. 

 If then we imagine the urea and sodium chloride as being separately 

 removed from the plasma by the action of a semipermeable piston, 

 in the first case impermeable to urea and in the second case im- 

 permeable to sodium and chlorine ions and to sodium chloride; 

 then, to separate in each case 1500 c.c. of secretion containing in 

 one case 2 per cent, of urea, and in the other case 1-1 per cent, of 

 sodium chloride, from a plasma containing 0-04 per cent, of urea 

 and 0-55 per cent, of sodium chloride, we should require to take 



2 

 in the case of the urea 1500 x--.= 75,000 c.c. of plasma and 



compress down to 1500 c.c., while in the case of the sodium chloride 



we should onlv have to take 1500 x--^ =3000 c.c. of plasma 



O-oo 



and compress down to 1500 c.c. 



Hence to get the true expression for the work done against 

 osmotic pressure in secretion, each constituent must be treated 

 separately, and the work done depends in large degree upon the 

 pressures of the separated constituent in plasma and secretion 

 respectively, and the total molecular amount separated. So that 



