Mechanisms which maintain extracellular fluid volume

Sensing the loss of tonicity

loss of tonicity

Water loss results in the increase of ECF tonicity.

This is sensed by hypothalamic osmoreceptors.

This mechanism is predominantly triggered by a rise in sodium concentration, as sodium contributes most to tonicity.

Sensing the loss of pressure

loss of pressure and baroreceptor response

Volume loss results in a decrease in venous pressure

This is sensed by Low Pressure Baroreceptors.

They are present in the atria and the large veins.

Volume loss also results in a decrease in arterial pressure

This is sensed by High Pressure Baroreceptors.

They are present in the carotid sinus and aortic arch.

The baroreceptor response is more potent that the osmoreceptor response.

If there happens to be hypotonicity, you would expect water retention to decrease (after all, you would want less dilution of the solutes in the ECF).

However, if there also happens to be hypovolemia, the loss of volume overrides the loss of tonicity, and water will be retained.

Tonicity will actually drop as the result. This is because the osmolar threshold for vasopressin release is set lower.

The hypothalamus will tolerate a more dilute ECF if it helps maintain euvolemia.

Readdressing the homeostasis

water is concerved and thirst is generated

ADH (antidiuretic hormone, vasopressin) is released.

Water is reabsorbed in the collecting duct.

Dilution of the ECF occurs.

Thus tonicity returns to normal.

Furthermore, free water is retained.

Thus volume expansion results.

 water redistributes from other compartments

Redistribution of water between compartments occurs very quickly to redress the homeostasis.

About 1000ml of interstitial fluid can migrate back into the bloodstream every hour to restore vascular volume.

References

anaesthesiamcq.org, as always; my source for most of this information has been the Kerry Brandis Anaesthetic Viva book.  Everyone should have one.