Changes in blood tonicity are recognised by osmotically sensitive cells that are situated in the circumventricular organs of the anterior hypothalamus, probably the subfornical organ and/or the organum vasculosum of the lamina terminalis. There is evidence for the existence of two distinct osmoreceptors: one controls vasopressin secretion, principally from the supraoptic nucleus; the other that serves thirst appreciation (Fig. 5.3).
