Explain how the body detects a drop in blood water concentration and responds to restore normal water balance.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
The body constantly monitors and adjusts its internal conditions. One important variable that is regulated is the water concentration of the blood.
Model answer (5 marks)
The hypothalamus senses the drop in blood water concentration and acts as a receptor. It signals the posterior pituitary to secrete more antidiuretic hormone (ADH) into the bloodstream. ADH travels to the kidneys, where it increases the permeability of the collecting ducts to water. Consequently, more water is reabsorbed back into the blood, producing a smaller volume of more concentrated urine. This restores the blood water concentration to normal, completing the negative feedback loop.
Examiner tips
- Mention the hypothalamus as the receptor first; link pituitary ADH release; state ADH’s effect on kidney permeability; describe urine concentration; show negative feedback. Use exact terms: hypothalamus, posterior pituitary, ADH, collecting duct, reabsorption, urine concentration, homeostasis.
Common mistakes
- Failing to identify the hypothalamus as the receptor; confusing ADH with other hormones; omitting the role of the collecting duct; not mentioning negative feedback or urine concentration.
Mark scheme (5 marks)
- The hypothalamus detects the drop in blood water concentration (acts as a receptor / monitors blood water concentration)
- The pituitary gland secretes / releases more ADH (into the bloodstream)
- ADH travels in the blood(stream) to the kidneys (as its target organ)
- ADH increases the permeability of the kidney tubules / collecting duct to water, so more water is reabsorbed (back into the blood)
- A smaller volume of more concentrated urine is produced, restoring blood water concentration to normal (homeostasis / negative feedback)
Related
- All Eduqas A-Level Biology revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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