Set point for sodium homeostasis: Surfeit, deficit, and their implications

Abstract

In a cybernetic or feedback-controlled system, some variable is maintained around a predetermined level, variously known as the set point, null point, or the quiescent value. Perhaps the example most familiar to us is the role played by the thermostat in our home heating system. Any description of a cybernetic system that omits the set point is incomplete. Sodium homeostasis, especially the control of extracellular fluid volume, provides an excellent example of a feedback-controlled system. The afferent and efferent limbs of this system have received considerable attention, but the set point has received remarkably little emphasis - an omission that has led to inevitable confusion. The author will focus, therefore, on the definition of the normal set point for sodium balance and on the implications of the set point for a number of related disciplines. Only a few examples of the implications will be cited, but those interested in the field will find many more.