osmoreceptor

An osmoreceptor is a tiny biological “saltiness meter” in the brain. It does not measure how much water you have directly; instead it senses how crowded your blood is with dissolved particles. When the blood gets too concentrated — too salty — it sounds the alarm to drink and to save water.

These specialized neurons sit in the hypothalamus, near the third ventricle in regions with a leaky blood-brain barrier so they can sample the blood. When blood osmolality rises, water leaves the osmoreceptor cells and they shrink slightly; this shrinkage activates them. They then drive two responses at once: they make nearby neurons release more vasopressin to conserve water, and they switch on the sensation of thirst to bring water in.

Osmoreceptors are remarkably sensitive, responding to osmolality changes of only about 1 percent, which keeps blood concentration in a tight range. They are the “sensor” half of the body’s water-balance feedback loop; vasopressin and thirst are the “effector” half. Their set point can be reset in some conditions, such as pregnancy, where the body tolerates a slightly lower normal osmolality.