162 ENERGY CHANGES INVOLVED IN SECRETION 



the case of the sodium chloride. As a result, taking the average 

 daily quantities to be 30 grm. in the case of urea, and 16 grm. 

 in the case of sodium chloride, and correcting for the almost com- 

 plete dissociation of the sodium chloride, a calculation of the work 

 done in the separation in the two ca&es 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 50 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 semi-permeable 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 Tl 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 7^7 = 75,000 c.c. of plasma and 



0-04 



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

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



U" 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 

 as a result, for example, in the case of the urine, the separation 

 of the urea involves more work than the separation of all the other 

 constituents combined. 



As an example of the method of calculating the work done in 

 secretion against osmotic pressure, we may give the calculation of 

 the amount done in secreting the normal daily amount of urea, 



