(HT only) Students should be able to describe the effect of ADH on the…

The idea

ADH (antidiuretic hormone) controls the permeability of the kidney tubules to water. When ADH concentration in the blood increases, the tubules become more permeable to water, so more water is reabsorbed by osmosis into the blood. This produces a smaller volume of more concentrated urine. When ADH concentration decreases, the tubules become less permeable, less water is reabsorbed, and a larger volume of dilute urine is produced. For example, after sweating heavily, blood becomes more concentrated; the pituitary gland releases more ADH, making the collecting duct more permeable to water, so water returns to the blood and urine volume falls. You should describe this cause-and-effect relationship clearly.

What earns the marks

• State that ADH increases the permeability of the kidney tubules to water.
• Explain that increased permeability leads to more water reabsorption by osmosis into the blood.
• Describe that a decrease in ADH reduces the permeability of the kidney tubules to water.
• Link changes in ADH concentration to the volume and concentration of urine produced.
• Use the term 'osmosis' correctly when describing water movement from the tubule into the blood.

Common mistakes and how to put them right

• Saying that ADH makes the tubules permeable to glucose or ions rather than specifically to water.

  Put it right: ADH primarily affects water permeability.

• Confusing the effect of ADH with the effect of aldosterone or other hormones.

  Put it right: Focus only on ADH and water reabsorption.

• Stating that ADH is released by the kidney.

  Put it right: ADH is released by the pituitary gland.

Exam tips

• Use comparative language such as 'more permeable' or 'less permeable' rather than just 'permeable'.
• Link each change in ADH to a change in water reabsorption and then to urine volume or concentration.
• Remember that this is higher tier only, so expect questions that require a clear causal chain.