Fig. 5.11
The relationship between plasma osmolality and plasma vasopressin (upper), and between plasma osmolality and thirst (lower). 
Thirst is measured in arbitrary units along a linear scale. Plasma vasopressin rises in a linear fashion in response to increasing plasma osmolality. The abscissal intercept represents the theoretical threshold for vasopressin release, and the slope of the line represents the sensitivity of the osmoreceptor-vasopressin-synthesising unit. Healthy individuals maintain their plasma osmolality within the range 282–295mosmol/kg, with plasma vasopressin concentrations varying from 0.3–4pmol/l. There is a similar progressive increase in thirst sensation with rises in plasma osmolality.