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George A. Sacher 



II. PHYSIOLOGIC REGULATIONS 



The ability to maintain the physiologic steady state in the face of an unfavor- 

 able environment is called homeostasis (1). A number of quantitative indices 

 of homeostatic capacity are in use. In ecological studies the tolerable range 

 of an environmental variable, such as ambient temperature or salinity (of 

 sea waters), is widely employed (2). Resistance to transient stresses is a more 

 common measure in experimental physiology. If the response can be followed 

 continuously, measures such as the amplitude of displacement of function, 

 and rate of return to normal may be obtained. The above may be referred 

 to as determinate measures of homeostatic capacity, for they reflect the fact 



Fig. L Schematic representation in two dimensions of the probabihty distri- 

 bution of physiologic states and their relation to the boundary delimiting viable 

 from non-viable states. The probability distribution is indicated by elliptical 

 contours of equal probabihty. The dehmiting boundary (L-L), called the lethal 

 bound, is indicated as a sharp line, and is treated as a precise value in the derivation 

 of equation 15. A more realistic representation is given in Fig. 2. 



that the homeostatic mechanism, even if it functions perfectly and without 

 error, has but a finite regulatory capacity, set by the physical limitations of 

 the mechanism. 



In summary, there is a closed region in the physiologic configuration space 

 within which some degree of stable physiologic function may persist, and beyond 

 which stable function is impossible. This is indicated schematically in Fig. 1. 

 The boundary surface of this region will be called the lethal bound, and denoted 

 byL. 



The quantitative properties of the lethal bound diff'er for different physiologic 

 processes. In the case of white blood cells, the lethal bound on the low side, 

 either for number of circulating cells or for number of proliferative cells, 

 is only a small fraction of the normal level. Similarly the lethal range on the 

 high side is considerably above normal levels. The boundary values for erythro- 

 cytes lie somewhat closer to the normal values. Blood glucose is rather more 



