7.20B - ADH and water balance
Explain how ADH changes collecting-duct permeability to regulate blood water content. This Higher Biology lesson connects hormone release, water reabsorption and the volume and concentration of urine.
7.20B - Higher: ADH and water balance
Higher tier only (7.20B). In a nephron, filtration transfers water and small solutes from blood into a tubule. Useful substances and much of the water return to nearby blood; the remaining fluid eventually becomes urine. The collecting duct receives this fluid and makes the final adjustment to how much
water is lost in urine. Its permeability to water means how easily water can cross its
wall.
ADH, or antidiuretic hormone, is made in the hypothalamus and released by the pituitary gland. It is carried in the blood to the kidneys. It regulates blood
water content by changing collecting-duct permeability:
| Condition | ADH response | Collecting duct | Result |
|---|---|---|---|
| Blood water content is too low; blood is too concentrated | More ADH is released | More permeable to water | More water moves from duct fluid into the blood by osmosis; a smaller volume of more concentrated urine is formed |
| Blood water content is too high; blood is too dilute | Less ADH is released | Less permeable to water | Less water returns to the blood; a larger volume of more dilute urine is formed |
For example, compare two otherwise identical situations. After heavy sweating, conserving water matters: more ADH leaves less water in the eventual urine. After a large drink, the body can lose surplus water: less ADH leaves more water in the urine. In both cases, separate the water remaining in the duct from the water returned to blood.
This is negative feedback. The response opposes the original change and moves blood water
content back towards its normal level. ADH does not absorb water itself: it changes the
permeability of the collecting duct, allowing a different amount of water to be reabsorbed.
Continue this lesson.
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