A2 Biology 9700: Osmoregulation and the Action of ADH

A hiker becomes dehydrated after a long walk in hot conditions, causing the water potential of their blood plasma to fall (become more negative) below its normal value.

(a) Explain, with reference to osmoreceptors and the hypothalamus, how this fall in blood water potential leads to an increased release of antidiuretic hormone (ADH) into the blood. [3]

(b) Explain how an increase in the concentration of ADH in the blood affects the permeability of the collecting duct wall to water, and hence how water is reabsorbed from the fluid in the collecting duct. [4]

(c) Describe the resulting effect of this increased ADH concentration on the volume and concentration of the urine produced, and state what would happen to ADH secretion and to urine output if the hiker instead drank a large volume of water so that blood water potential rose above normal. [3]

Part (a): Detecting dehydration and releasing ADH

Osmoreceptor cells in the hypothalamus continually monitor the water potential of the blood as it flows past them. When the hiker becomes dehydrated, the blood plasma loses water relative to its dissolved solutes, so its water potential falls (becomes more negative/less dilute).

Because the tissue fluid around the osmoreceptors now has a lower water potential than the fluid inside these cells, water moves out of the osmoreceptor cells by osmosis, and the cells shrink. This change in cell volume/shape stimulates neurosecretory cells in the hypothalamus, which respond by signalling the posterior pituitary gland to release more ADH (antidiuretic hormone) into the bloodstream.

Part (b): How ADH affects the collecting duct

Rising ADH concentration acts directly on the collecting duct. ADH binds to specific receptors on the cell-surface membrane of collecting duct cells; via a second-messenger pathway inside the cell, this causes vesicles containing aquaporin (water-channel) proteins to fuse with the membrane facing the lumen of the collecting duct, making that membrane more permeable to water.

The tissue fluid of the surrounding medulla has a lower (more negative) water potential than the fluid inside the collecting duct. Once the wall is more permeable, water therefore moves out of the duct by osmosis, down this water-potential gradient, and is reabsorbed into the blood.

Part (c): Effect on urine, and the converse response to overhydration

Because the collecting duct wall is now more permeable to water, and the duct runs down through the low water-potential medulla, water moves out of the fluid inside the duct by osmosis into the surrounding tissue fluid and is reabsorbed into the blood. The fluid that reaches the renal pelvis therefore has a smaller volume and is more concentrated, a small volume of concentrated urine, an appropriate response to conserve water in a dehydrated body.

If the hiker instead drank a large volume of water, blood water potential would rise above normal. Osmoreceptors in the hypothalamus would then gain water by osmosis and swell (rather than shrink), reducing stimulation of the neurosecretory cells and so decreasing ADH release from the posterior pituitary. With less ADH circulating, the collecting duct wall becomes less permeable to water (fewer aquaporins inserted into its membrane), so less water is reabsorbed from the duct, producing a larger volume of more dilute urine instead.

Final answers

(a) Osmoreceptors in the hypothalamus shrink (lose water by osmosis) as blood water potential falls, stimulating the posterior pituitary to release more ADH.
(b) Rising ADH makes the collecting duct wall more permeable to water by inserting aquaporins, so water moves out of the duct by osmosis into the lower-water-potential medulla and is reabsorbed into the blood.
(c) More water is reabsorbed as the duct passes through the low water-potential medulla, giving a small volume of concentrated urine; after drinking a large volume of water, ADH secretion falls instead and a larger volume of more dilute urine is produced.