Explain how the body maintains blood osmolarity when a person becomes severely dehydrated.

IB DP Biology Higher 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, significant water loss through sweating can raise blood osmolarity well above the normal range of approximately 285–295 mOsm kg⁻¹.

Model answer (4 marks)

Osmoreceptors in the hypothalamus detect the rise in blood osmolarity caused by water loss. They send a signal to the posterior pituitary to release ADH (vasopressin) into the bloodstream. ADH acts on the collecting ducts and distal convoluted tubules of the nephron, increasing their permeability to water by inserting aquaporin channels into the tubular cell membranes. Consequently, more water is reabsorbed from the filtrate back into the blood by osmosis, producing a smaller volume of more concentrated urine. This reduces blood osmolarity back towards the normal set point, completing the negative feedback loop.

Examiner tips

Mention osmoreceptors and ADH release first to show the trigger and effector. Explain the role of aquaporins in increasing water permeability. Show the outcome: smaller, concentrated urine and reduced osmolarity. Use the term "negative feedback" to link the response to the stimulus.

Common mistakes

Failing to specify that ADH increases water permeability via aquaporins. Confusing ADH with other hormones or forgetting the posterior pituitary. Not linking the response back to the set point or describing the outcome as a smaller, concentrated urine.

Mark scheme (4 marks)

Osmoreceptors in the hypothalamus detect the rise in blood osmolarity / loss of water from osmoreceptor cells by osmosis
The hypothalamus signals the posterior pituitary to release ADH (antidiuretic hormone / vasopressin) into the blood
ADH increases the permeability of the collecting ducts (and distal convoluted tubules) of the nephron to water by inserting aquaporins into the tubule cell membranes
More water is reabsorbed from the filtrate back into the blood by osmosis, producing a smaller volume of more concentrated urine, reducing blood osmolarity back towards the set point (negative feedback)

Key terms in this question

blood osmolarity