• Water moves freely between compartments and the distribution is determined by the osmotic equilibrium between them.
  • The dilution of plasma is described as plasma osmolality. The more diluted the plasma is, the lower the osmolality. Plasma osmolality is detected by osmoreceptors in the hypothalamus of the brain.
  • The normal plasma osmolality is 285-300mOsmol/kg.
  • Body water is controlled mainly by changes in the plasma osmolality. An increased plasma osmolality, sensed by osmoreceptors in the hypothalamus, causes thirst and release of ADH from the posterior pituitary.
  • ADH in turn increases water reabsorption from renal collecting ducts.
  • In addition, non-osmotic stimuli may cause the release of ADH, even if serum osmolality is normal or low. These include:
    • Hypovolaemia (irrespective of plasma osmolality)
    • Stress (surgery and trauma)
    • Nausea.
  • In contrast, at a plasma osmolality of <275mOsmol/kg, there is complete suppression of ADH secretion. This is because the fluid is very diluted, and thus no ADH is required to promote more water reabsorption in the kidney (which would make the serum even more diluted and potentially result in fluid overload).

N.B.
This is related to sodium content, which is also regulated by volume receptors; water content is adjusted to maintain a normal osmolality and a normal plasma sodium concentration. Disturbances of sodium concentration are usually caused by disturbances of water balance, rather than an increase or decrease in total body sodium.

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