Explain how the kidneys help to maintain homeostasis in the body.

WJEC A-Level Biology (Wales) — 3.7 Homeostasis and the kidney · Explain · 5 marks · View as Markdown

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

The kidneys play a vital role in keeping the internal environment of the body constant. They filter the blood and produce urine, adjusting its composition depending on the body's needs.

Model answer (5 marks)

1. Homeostasis is the maintenance of a constant internal environment.
2. The kidneys filter the blood, removing waste products such as urea.
3. Urea is excreted in the urine, preventing its accumulation in the blood.
4. The kidneys regulate the water content of the blood, adjusting urine volume to maintain fluid balance.
5. They also control ion and salt concentrations, ensuring proper electrolyte balance.

Examiner tips

Use the exact terms: ‘filter the blood’, ‘excrete urea’, ‘regulate water content’, ‘control ion/salt concentration’.
Show the logical sequence: filtration → waste removal → fluid & electrolyte balance.
Keep each point brief but complete – 1 mark each.

Common mistakes

Confusing filtration with reabsorption; both occur but only filtration removes waste. Failing to mention urea specifically. Using vague phrases like ‘clean the blood’ instead of ‘filter the blood’.

Mark scheme (5 marks)

Homeostasis is the maintenance of a constant internal environment
The kidneys filter the blood / remove waste products from the blood
Urea is removed / excreted by the kidneys
The kidneys regulate/control water content of the blood / body
The kidneys regulate ion / salt concentration in the blood

Key terms in this question

homeostasis