Water in balance.

Osmoreceptors respond when blood water potential changes. If the body is short of water, the posterior pituitary releases more antidiuretic hormone (ADH). ADH increases water reabsorption in the kidney collecting ducts.

More water returns to the blood and a smaller volume of more concentrated urine is produced. With excess water, ADH release falls and less water is reabsorbed. This is a negative-feedback loop.

INTERACTIVE MODEL

Water balance

ADH 50% · water reabsorption 50% · urine volume 50%Body hydration50%LowHigh

Explore: Change hydration and observe ADH, reabsorption and urine volume together.

\text{low body water}\to\uparrow ADH\to\uparrow\text{water reabsorption}

WORKED EXAMPLE

After prolonged exercise with little drinking, predict urine output.

Water shortage stimulates more ADH release.
The kidneys conserve water, so urine volume decreases and concentration rises.

Assumed knowledge

Water potential and feedback.

Learning checkpoints & source

YOUR LEARNING CHECKPOINT

Link water availability to ADH release.
Predict the effect on urine concentration.

QCAA Biology · Unit 2 · Homeostasis ↗

READY TO TRY IT?

Water in balance.

Water balance

After prolonged exercise with little drinking, predict urine output.

Put the idea to work.