194 D. RiTTENBERG 



a labelled amino-acid added to the diet, we must evaluate 

 the parameters which govern the kinetic aspects of the urea 

 pool. We have done this by direct measurements. A small 

 amount of ^^N labelled urea (~50 mg.) was injected intra- 

 venously into a normal adult. It is possible to predict the 

 fate of this material. 



Let y=mg. ^^N injected as urea. 



Au=mg. ^^N in urea pool at any time 



^^Nu = ^^N concentration (atom per cent excess) in urea 

 pool at any time. 



It is relatively easy to show that 



and that 



E 



_100Au _ lOOye-u* (2) 



if we assume mixing of the exogenous and endogenous urea 

 to be fast. When t=0 



("N„) t-o=^ (3) 



°^ ^=pNiro (*) 



In Fig. 2 is shown the results of three experiments. In 

 each case you observe a rapidly declining curve which then 

 becomes almost linear. Extrapolation of the linear portion 

 of the curve to zero time gives the isotope concentration which 

 would have existed had mixing of the injected urea been 

 instantaneous. These values shown on the left hand side of 

 the figure are (^5Nu)t=o- Inserting this value in equation 4 

 gives U. In normal adults U is approximately 6 g. nitrogen. 

 Since the rate of excretion of urea in urine is easily measured, 

 all the constants in equations 1 and 2 are known. 



