Anti-diuretic hormone (ADH) - essential facts (i)

• small peptide (9 amino acids).
• short half-life (10 minutes), metabolised in liver and kidneys.
• secreted when blood osmolality increases due to dehydration.
• a change in osmolality is sensed by osmoreceptors (specialised hypothalamic neurones) which synapse mainly with supra-optic neurones.

Additional Information

Half-life
Half-life (T½) is the time taken for the plasma concentration of the hormone to be reduced by 50%

Osmolality
Osmolality is the osmotic pressure of a solution (osmols or millosmols per kg of water). Blood has an osmolality of 285-295 mOsm/kg due to the concentrations of osmotially active electrolytes (e.g. Na⁺, Cl^-) and non-electrolytes (e.g. glucose, urea).

Osmoreceptors
Osmoreceptors are specialised neurons found mainly in the anterior hypothalamus. When plasma osmolality rises above 285 mOsm/kg the frequency of action potentials generated by the neurones increases which in turn causes secretion of ADH. They are sensitive to changes in plasma osmolality of as little as 1%