A patient has damage to their pituitary gland meaning it can no longer secrete ADH. Explain how this would affect the patient's urine and why this change in urine occurs.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
ADH is a hormone secreted by the pituitary gland which increases water reabsorption in the kidney by making the kidney tubules more permeable to water.
Model answer (5 marks)
The patient would produce a large volume of urine.
The urine would be more dilute and less concentrated.
Without ADH, the kidney tubules and collecting duct are less permeable to water.
Consequently, less water is reabsorbed from the tubules back into the blood.
More water passes out of the body in the urine, lowering blood water concentration and disrupting water balance (homeostasis).
The urine would be more dilute and less concentrated.
Without ADH, the kidney tubules and collecting duct are less permeable to water.
Consequently, less water is reabsorbed from the tubules back into the blood.
More water passes out of the body in the urine, lowering blood water concentration and disrupting water balance (homeostasis).
Examiner tips
- Use the exact phrase "large volume of urine" and "more dilute" to match the mark scheme.
- Explain the sequence: loss of ADH → decreased permeability → reduced reabsorption → increased urine volume and dilution.
- Include the consequence for homeostasis – a drop in blood water concentration.
Common mistakes
- Writing that the urine is more concentrated instead of more dilute.
- Failing to mention the reduced permeability of the collecting duct.
- Omitting the link to disrupted water balance/homeostasis.
Mark scheme (5 marks)
- The patient would produce a large volume of urine
- The urine would be more dilute / less concentrated
- Without ADH, the kidney tubules / collecting duct are less permeable to water
- Less water is reabsorbed from the kidney tubules / collecting duct back into the blood
- More water passes out of the body in the urine / blood water concentration falls / water balance is not maintained (homeostasis is disrupted)
Key terms in this question
Related
- All Eduqas A-Level Biology revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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