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 [1–6]. 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. I 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.
