Role of ADH in maintaining body fluid volume.

Regulation of water balance (Osmoregulation)

It is regulated by two mechanisms

1)     ADH Mechanism: ADH regulate water balance by tuning renal free water excretion or retention. ADH secretion is stimulated by plasma hyperosmolarity and hypovolemia.

2)     Thirst mechanism: It is stimulated by plasma hyperosmolarity and hypovolemia.

In water deficit ECF hyperosmolarity and hypovolemia develop. Hypovolemia stimulates sympathetic nervous system (via inhibitions of baroreceptors) and activate rennin-angiotensin-aldosteron system, both of which finally increase angiotensin-II production.

Hyperosmolarity stimulates the osmoreceptors in hypothalamus. Both activated osmoreceptors and increased angiotensin-II stimulate thirst center as well as increase ADH secretion leading to water intake and renal water retention to normalize the plasma osmotic pressure.

1-2% rise of plasma osmolarity and more than 10-15% reduction of ECF volume stimulate the ADH release. Stimulation of ADH release starts at plasma osmolarity above 285 mosmol/l (at Na⁺ >140mmol/l) and reaches maximum at plasma osmolarity 295 mosmol/l (at Na⁺ =145mmol/l) when one shows maximum antidiuresis with obligatory urine production only. Inhibition of ADH release starts at plasma osmolarity 280 mosm/l at Na⁺ <140mmol 280="280" and="and" at="at" inhibition="inhibition" l="l" maximum="maximum" mosm="mosm" na="na" osmolarity="osmolarity" plasma="plasma" reaches="reaches" sup="sup">+

=135 mmol/l, when one shows maximum diuresis with renal water excretion upto 23L/day.

2-3% rise of plasma osmolality and more than 10-15% reduction of ECF volume stimulates the thirst center. Stimulation of the thirst center starts at plasma osmolarity 295 mosm/l (at Na⁺=145mmol/l)  and reaches maximum at plasma osmolarity 300 mosm/l (atNa⁺=148mmol/l) who can drink upto 20-25l/day.