310 THE LANGUAGE AND CONCEPTS OF CONTROL 



an aid in diagnoses is the determination of rates of specific steps within an 

 over-all process from measurements of those variables which are susceptible 

 to measurement. It will be recalled that in Chapter 8, in the discussion on 

 the steady-state, we emphasized how necessary it is that all the small steps 

 of a process should proceed at some well-defined rate if the over-all steady- 

 state is to be maintained. Further, we discussed at length the factors upon 

 which rates depend. The use of radioactive tracers to examine the steady- 

 state was described in Chapter 5. 



The topical and interesting, if not classical, study of the biochemical 

 kinetics of iron metabolism in the red blood cells, work which was reported 

 by Huff and Judd 1 in 1956, ties many of these ends together. It is a very 

 instructive work because (a) measurements were made of iron turnover rate 

 by a radioactive tracer technique, using the hard gamma emitter, Fe 59 ; (b) 

 they were analyzed by means of an analog computer programmed to a 

 model based on known and suspected biochemical kinetics of iron; (c) the 

 comparison was made between normal human beings at atmospheric and at 

 reduced pressure; and those with polycythemia vera, aplastic anemia, and 

 other blood diseases; (d) both the factual information and the results of the 

 analyses have unquestioned clinical importance; and (e) the report is written 

 clearly and concisely, and is an excellent source of the detail which cannot 

 be given here. 



Kinetics of Iron Metabolism 



The study by Huff and Judd was on the kinetics (rates and mechanism) 

 of iron in human blood plasma, as followed by measuring turnover rates of 

 Fe 59 . The iron exchanges with various "pools" (Figure 1 1-6), which are not 

 precisely specified because they are not precisely known. Two possibilities 

 are shown in the figure; but many other pools of iron-containing pigments, 

 such as peroxidase, catalase, cytochrome, and myoglobin are ignored. Also 

 the iron may exchange with that from the intestine as well as that recircu- 

 lated from the bile. Therefore this work must not be considered complete. 



A microcurie dose of tagged iron was administered intravenously in the 

 chemical form in which it naturally occurs in the blood. From time to time 

 after injection, blood samples were taken and the plasma's radioactivity 

 measured. At the same time the body was surveyed outside with a highly 

 collimated Geiger counter which would pick up the flow pattern by detecting 

 Fe 59 's hard gamma rays. 



For the first few hours the loss follows the "natural" law that the rate is 

 proportional to the amount present, or 



da , 

 = ka 



dt 



