Q.ADH is controlled by both positive and negative feedback mechanisms. Explain the conditions for these two mechanisms.

ADH is usually regulated by negative feedback tied to blood osmotic pressure, but in conditions of severe blood volume loss, its release can follow a positive feedback pattern instead.

Antidiuretic hormone (ADH, also called vasopressin) is secreted by the posterior pituitary (having been produced in the hypothalamus) and its principal job is to control how much water is reabsorbed by the kidney tubules, thereby regulating blood osmotic pressure and volume. Negative feedback mechanism (the normal, everyday regulation): when the body becomes even mildly dehydrated (through sweating, low water intake, etc.), the blood's osmotic pressure rises. Osmoreceptors in the hypothalamus detect this rise and stimulate increased secretion of ADH. ADH then acts on the kidney's collecting ducts to increase water reabsorption, producing a smaller volume of more concentrated urine and returning water to the blood. As blood osmotic pressure falls back toward normal, the stimulus for ADH secretion is removed and ADH release decreases again — this is a classic negative feedback loop, since the hormone's effect works to cancel out (negate) the original stimulus. Positive feedback mechanism (an emergency condition): in cases of severe fluid loss, such as major haemorrhage (heavy blood loss) or severe dehydration, blood volume and blood pressure fall sharply. This drop is sensed by baroreceptors (pressure receptors) and triggers a strong release of ADH aimed at conserving water and raising blood pressure by promoting vasoconstriction and water retention. However, if the underlying blood/fluid loss is not stopped, blood pressure continues to fall despite (and partly regardless of) the ADH response, which in turn keeps stimulating even greater ADH secretion — the response escalat …