Explain how the kidneys regulate blood osmolarity when a person becomes dehydrated.

IB DP Biology Standard Level (2023 syllabus) — D3.3 Homeostasis · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

During prolonged exercise in hot conditions, a person loses significant water through sweating, causing blood osmolarity to rise above the normal range.

Model answer (4 marks)

During dehydration the rise in blood osmolarity is sensed by osmoreceptors in the hypothalamus. This stimulates the posterior pituitary to release ADH into the bloodstream. ADH binds to receptors on the collecting duct (and distal convoluted tubule) and increases their permeability to water. Consequently more water is reabsorbed from the filtrate into the blood by osmosis, producing a smaller volume of more concentrated urine. The increased water reabsorption lowers blood osmolarity back to the normal range, completing the negative‑feedback loop.

Examiner tips

Mention the hypothalamic osmoreceptors first, then ADH release, then increased ductal permeability, and finally the osmotic reabsorption and urine concentration.
Use the exact terms: osmoreceptors, posterior pituitary, ADH, collecting duct, permeability, osmosis, negative feedback.
Show the sequence clearly to hit all four marks.

Common mistakes

Failing to state that ADH acts on the collecting duct (or distal convoluted tubule).
Confusing ADH with vasopressin or using the wrong hormone name.
Not linking the increased water reabsorption to the reduction in blood osmolarity (missing the negative‑feedback conclusion).

Mark scheme (4 marks)

Rise in blood osmolarity is detected by osmoreceptors in the hypothalamus
The posterior pituitary releases ADH (antidiuretic hormone) into the blood
ADH increases the permeability of the collecting duct (and distal convoluted tubule) to water
More water is reabsorbed by osmosis into the blood, producing a smaller volume of more concentrated urine, restoring blood osmolarity to normal (negative feedback)

Key terms in this question

osmolarity