7.21—ADH and osmoregulation

Osmoreceptors detect blood water potential and the hypothalamus/pituitary adjusts ADH release. ADH increases aquaporins in collecting-duct membranes, so more water returns to the blood and urine becomes concentrated.

When blood is dilute, less ADH leaves the collecting duct less permeable and more dilute urine is produced. The response is negative feedback around a regulated water balance.

After dehydration, high ADH increases water reabsorption even though the filtered load may be similar; the final urine changes because permeability changed.

ADH does not “add water” to urine and does not act equally on every nephron segment. Separate sensor, hormone, membrane change and final urine effect.