(HT only) The water level in the body is controlled by the hormone ADH…

The idea

The water level in the body is regulated by negative feedback involving ADH (antidiuretic hormone). ADH is produced by the pituitary gland and acts on the kidney tubules, specifically the collecting duct, to control water reabsorption. When the water level in the blood is too low (blood too concentrated), more ADH is released, making the tubules more permeable to water so more water is reabsorbed, restoring the water level. When the water level is too high, less ADH is released, less water is reabsorbed, and more dilute urine is produced. For example, after drinking a large volume of water, ADH secretion decreases, urine volume increases, and blood water level returns to normal. This is an example of homeostasis.

What earns the marks

• State that ADH is the hormone that controls the water level in the body.
• Identify the pituitary gland as the source of ADH.
• Explain that ADH acts on the kidney tubules to increase water reabsorption.
• Describe the negative feedback mechanism: changes in blood water concentration trigger changes in ADH release.
• Link the action of ADH to the maintenance of a constant internal environment (homeostasis).

Common mistakes and how to put them right

• Thinking that ADH is produced by the kidneys.

  Put it right: ADH is produced by the pituitary gland and acts on the kidneys.

• Believing that ADH directly controls ion balance.

  Put it right: ADH primarily controls water reabsorption.

• Confusing the effect of ADH with that of insulin or glucagon.

  Put it right: ADH is specific to water balance.

Exam tips

• Use the term 'negative feedback' when describing how ADH maintains water balance.
• Make clear that ADH travels in the blood to reach the kidney tubules.
• When explaining, always state the stimulus, the hormone response, and the effect on water reabsorption.