198 



D. RiTTENBERG 



In Fig. 4 are shown a plot of equations 7, 8 and 9 with 

 arbitrary values of the constants. You notice that the isotope 

 concentration of the metabolic pool drops rapidly. Indeed, 

 after 3-4 hours it drops almost to zero. The isotope concen- 



Curve b 



Simmation of Fraction 



Excreted as 



Urea Nitrogen 



Curve c 

 Fraction Remaining 

 in Urea Pool 



lax 



TIME 



Fig. 4. Representative curves showing distribution of isotope 

 following intravenous injection of labelled amino-acid. 



tration in the urea pool rapidly reaches a maximum value 

 and then slowly declines. The total excretion steadily 

 increases along an S-shaped curve. 



If sufficient data are available, equations 7, 8 and 9 can be 

 solved and used to determine S and P. 



