NATIONAL SEMIFINAL STAGE 2026
Mfantsipim School: 56
Presbyterian Boys' Sec. Sch: 53
Osei Tutu SHS: 27
QUESTION
What is the effect of dehydration on the kidneys' function, and the resultant appearance of the urine produced?
ANSWER: The kidneys retain more water and release less into the urine, making it appear darker and more concentrated than normal.
EXPLANATION
Dehydration raises blood osmolarity, which triggers the release of antidiuretic hormone (ADH). ADH increases water reabsorption in the kidney's collecting ducts, so less water leaves the body in urine — concentrating whatever waste (like urochrome pigment) remains, which is why the urine looks darker.
PRACTICE QUESTIONS
1. Which hormone controls the reabsorption of water in the kidneys?
ANSWER: Antidiuretic hormone (ADH).
2. Which gland releases ADH into the blood?
ANSWER: The posterior pituitary gland.
3. Where is ADH made?
ANSWER: In the hypothalamus.
4. Which parts of the nephron does ADH act on?
ANSWER: The distal convoluted tubule and the collecting duct.
5. How does ADH make the collecting duct more permeable to water?
ANSWER: It causes more water channels (aquaporins) to be inserted into the membrane.
6. Which receptors detect a rise in blood osmotic pressure?
ANSWER: Osmoreceptors in the hypothalamus.
7. What does the urine look like after a person drinks plenty of water?
ANSWER: Large in volume, pale and dilute.
8. Which pigment gives urine its yellow colour?
ANSWER: Urochrome.
9. Which type of control system regulates the water content of the blood?
ANSWER: Negative feedback.
10. What is osmoregulation?
ANSWER: The control of the water and salt content of the body fluids.
11. What happens to the amount of ADH when the blood is too dilute?
ANSWER: Less ADH is released.
12. What disease results from a lack of ADH?
ANSWER: Diabetes insipidus, with the production of large volumes of dilute urine.