Regulation and the ponderostat.

Control theory is concerned mainly with the treatment of signals. This article takes into account that living beings not only treat information, but they are open systems traversed by flows of energy and mass. A block diagram of the regulation process is proposed, taking into account this fundamental difference between engineered and living systems. This new diagram is used to describe the stability of body weight. The ponderostat is hypothesized to work as a regulation of blood glucocorticoids with a hypothalamic set-point related to CRH concentration.