Lesson 6 of 10 / Homeostasis and response
Water balance and ADH
How does the body conserve water when blood water potential falls?
In this lesson: Trace the ADH pathway from detection to a change in urine volume and concentration.
About 6 min
The key ideaMore ADH increases the water permeability of kidney collecting ducts, so more water is reabsorbed and a smaller volume of concentrated urine is produced.
Blue tube: collecting duct. Arrows across its wall: water reabsorbed by osmosis through surrounding tissue into blood. More arrows mean more reabsorption, not active water pumps.
ADH is made in the hypothalamus and released from the posterior pituitary. The kidney collecting duct is a target.
Low blood water potential -> more ADH -> more permeable collecting ducts -> more water reabsorbed -> smaller volume of concentrated urine.
Explanation
Losing water without replacing it can lower the water potential of blood. Receptors in the hypothalamus detect the change. ADH, antidiuretic hormone, is made in the hypothalamus and released into the blood from the posterior pituitary gland.
When blood water potential is too low, more ADH is released. It makes the walls of the kidney collecting ducts more permeable to water. Water leaves the filtrate by osmosis, down a water-potential gradient into the surrounding tissue, and returns to the blood.
Greater reabsorption leaves less water in the urine. A smaller volume of more concentrated urine is produced. Retaining water helps blood water potential rise towards its normal range, reducing the stimulus for additional ADH release.
When blood water potential is high, less ADH is released. Collecting ducts become less permeable to water, less water is reabsorbed and a larger volume of more dilute urine is produced. ADH changes reabsorption after filtration; it does not create water or actively pump water molecules.
Step by step
- 1
Compare blood water potential
Decide whether it has risen or fallen relative to normal.
- 2
Link ADH to permeability
More ADH makes collecting ducts more permeable to water; less ADH does the reverse.
- 3
Explain urine and feedback
Link water reabsorption to urine volume and concentration, then to the correction in blood.
Worked example
After substantial sweating
A person loses water in sweat and has not replaced it. Predict the ADH response and urine change.
One way to explain it
As blood water potential falls, more ADH is released. Collecting ducts become more permeable to water, increasing water reabsorption into blood. A smaller volume of more concentrated urine is produced.
Why this answer works
- Start from water loss and lower blood water potential.
- Specify the collecting-duct change.
- Distinguish urine volume from urine concentration.
Is this true? "ADH is made by the kidneys and pumps water into the blood."
ADH is made in the hypothalamus and released by the posterior pituitary. It changes kidney permeability; water moves by osmosis.