How does the body measure dehydration?

Question

What, physically, does the body do to measure it? I assume it's measured by how how turgid or plasmolysed certain 'sample cells' are, or water concentration in the blood,

• What is the way they use to measure it?
• What do the receptor cells that do the measuring actually do that is variable with water concentration, etc.

Answer 1

From Wikipedia entry on thirst:

In the mammalian brain, the posterior surface of the hypothalamus forms the front wall of the third ventricle (a cerebrospinal fluid-filled cavity) and clusters of cells (osmoreceptors) on this surface, notably in the organum vasculosum of the lamina terminalis (OVLT) and subfornical organ (SFO), signal this cellular dehydration to other parts of the brain, and thirst is experienced. Destruction of this part of the hypothalamus in humans and other animals results in partial or total loss of desire to drink even with extremely high salt concentration in the extracellular fluids.1

The entry on osmoreceptor says in part:

When the osmotic pressure of blood changes (i.e. it is more or less dilute), water diffusion into and out of the osmoreceptor cells changes. That is, they expand when the blood plasma is more dilute and contract with higher concentration.

It also describes how the kidney measures chlorine anion flow through some nephrons, which triggers a cascade of messenger molecules resulting in increased blood levels of the hormone angiotensin, which also results in thirst messages originating in the hypothalamus.

1Derek A. Denton (8 June 2006). The primordial emotions: the dawning of consciousness. Oxford University Press. pp. 118–19. ISBN 978-0-19-920314-7.

2Walter F., PhD. Boron (2005). Medical Physiology: A Cellular And Molecular Approach. Elsevier/Saunders. ISBN 1-4160-2328-3. Page 872

Answer 2

Dehydration results in decreased blood volume (hypovolemia) and in slight decrease of blood pressure. The body recognises dehydration by these ways:

1. Baroreceptors in the aorta detect the drop of blood pressure - this results in activation of the sympathetic nerves and the release of adrenaline from the adrenal medulla - this leads to constriction of the arteries ad increased heart rate.
2. Volume receptors in the heart atria detect hypovolemia - this result in the secretion of the antidiuretic hormone from the pituitary gland - this results in water retention in the kidneys.
3. Osmoreceptors in the hypothalamus detect increased blood osmolality - this triggers the release of the hormone ADH.
4. Juxtaglomerular apparatus in the kidneys detect decreased perfusion, what results in the renin-angiotensin-aldosterone release cascade, which results in water retention in the kidneys.

All these mechanisms are activated in the compensated hypovolemic shock.