Major Stimulus for ADH Release

The major stimulus for release of ADH is:

Correct Answer: Hyperosmolarity of blood

The major stimulus for ADH (Antidiuretic Hormone / Vasopressin) release is hyperosmolarity of blood (increased plasma osmolality).

Regulation of ADH Release

• Primary stimulus — Hyperosmolarity: Osmoreceptors in the anterior hypothalamus detect elevated plasma osmolality (e.g., dehydration → blood becomes concentrated → plasma osmolarity rises) → trigger ADH release from posterior pituitary. Even a 1–2% rise in osmolarity effectively stimulates ADH release.
• Secondary stimulus — Hypovolemia: Volume receptors (atrial stretch receptors, arterial baroreceptors) detect reduced blood volume → stimulate ADH release → vasoconstriction + water retention.

ADH Action

• Acts on V2 receptors in DCT and collecting duct → inserts aquaporin-2 channels → water reabsorption → concentrated urine, diluted plasma.
• At high concentrations: acts on V1 receptors → vasoconstriction (hence also called 'Vasopressin').

Example

• A cow deprived of water for 24 hours → plasma osmolarity rises → ADH released → kidneys produce concentrated urine → water conserved → plasma osmolarity normalizes.