268 ENEEGY CHANGES INVOLVED IN SECKETION 



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 

 viz., 30 grms. in a 2 per cent, solution, measuring accordingly 1500 c.c. 

 The molecular weight of urea is 60, and it is not dissociated, so 

 that there is no correction for dissociation, also the usual figure of 

 0-04 per cent, may be taken for the concentration in the plasma. 



The expression for the work done is 



If we express this amount of work as heat energy in small 

 calories the value of the constant II becomes 1-98, if T be taken 

 at 40 0., the value of T in absolute scale becomes 273+40=313, 

 and hence the value of ET is 620 at this temperature; 1 Q is 30 

 grms., and the value of M, the molecular weight in grms., is 60, 



Q p' 



so that becomes 0-5; the value of the ratio of is the same 

 M p 



as that of the two concentrations of the urea in secretion and 



2 

 plasma respectively = ^ =50, and for Iog e 50, we can substitute 



Iog 10 50; on dividing by the Briggs modulus for transference from 

 Napierian to common logarithms, the value of Iog 10 50 is very 

 closely 1-7, and the value of the modulus is 0434; so that we 



1 The value of 5-8 rational calories or 580 small calories, given in a previous 

 chapter, was the usual value based on a temperature of 15 C.; the value 

 620 small calories used above is that which the expression RT has at a tem- 

 perature of 40 C., the approximate temperature of secretion of the urine. 

 The value of 1-98 for R is obtained by using the formula PV = RT, or 



PV 



R= , and then substituting the values for P, V, and T for a grm. molecule 



at any given values of pressure, volume, and temperature corresponding to 

 one another. Thus a grm. molecule at C. has a volume of 22,330 c.c., a 

 pressure of 76 cms. of mercury = 76 x 13-4 x 981 dynes, and T is 273 on absolute 

 scale. Also 1 small calory =42 x 10 6 dynes, and on substituting these values 

 in the above equation we obtain for the value of R in small calories : 

 _22330x 76 x 13-4 x 981 

 273x42xl0 6 



