The Physiology of Chance

The previous chapters of this book have presented rheostasis in terms of the phenomena observed rather than in terms of underlying mechanisms. They have described changes in regulated level and said that there has been a change in set-point when it appears "as if" such a change had taken place. Taking this approach, one may find enough examples of rheostasis to become convinced that the body is not always trying to control variables within particular narrow ranges. There is more to physiology than maintaining constancy. Examples of rheostasis have been classified on the basis of the circumstances in which they occur, with an attempt to discover the possible survival value of forsaking regulatory constancy. Of course, to a large extent lack of knowledge makes classification in terms of mechanisms premature. However, even if we knew much more about the underlying mechanisms, I am not sure that I would take a different approach or advocate abandoning a descriptive use of set-point changes. To other investigators this stance may seem highly unsatisfactory. Many of those with an appreciation of control systems would undoubtedly prefer (e.g., Toates, 1983), or even insist on, reserving a term llke rheostasis for cases where it is clear that changes in response to feedback are causal in changing the level of the regulated variable. Some specific examples, the increase of body temperature during exercise and the dependence of body weight on palatability of the diet, illustrate the problems in a descriptive use of the term selpoint.