14.1.8—Osmoregulation with ADH

Osmoregulation keeps the water potential of body fluids within a functional range. When blood water potential falls, osmoreceptors detect the change and the ADH pathway makes the collecting duct more permeable to water, allowing more water to be recovered.

ADH control chain:

1. A fall in blood water potential is detected by osmoreceptors in the hypothalamus.
2. Nerve impulses stimulate the posterior pituitary gland to release antidiuretic hormone (ADH) into the blood.
3. ADH reaches the kidney and binds to receptors on collecting-duct cells.
4. Signalling causes aquaporin-containing vesicles to add water-permeable channels to the luminal membrane, increasing collecting-duct water permeability.
5. Water moves from the filtrate through aquaporins into the surrounding medullary tissue fluid and blood down a water-potential gradient.
6. The filtrate loses water and becomes a small volume of concentrated urine, reducing water loss.

When blood water potential is high, less ADH is released and fewer aquaporins remain in the collecting-duct membrane. Less water leaves the filtrate, so a larger volume of dilute urine is produced. The medullary gradient supplies the osmotic pull; ADH changes permeability so the existing gradient can act rather than directly pumping water or creating the gradient.

This card is the ADH/osmoregulation control chain, not the general nephron-segment map in 4682. It also does not cover blood-glucose test strips or other homeostatic systems.