A patient is diagnosed with kidney failure. Their kidneys can no longer carry out excretion properly. Explain why the build-up of urea in the blood is harmful, and describe how healthy kidneys normally remove urea from the body to maintain a safe concentration in the blood.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Urea is a toxic waste product produced in the liver from the breakdown of excess amino acids. It is transported in the blood to the kidneys, where it is removed and excreted in urine. If urea is not removed, it accumulates in the blood to dangerous levels.
Model answer (5 marks)
Urea is a toxic waste product that must be removed from the body.
If urea accumulates in the blood it disrupts normal cellular function, damaging tissues and organs.
Healthy kidneys remove urea by filtering the blood in the glomerulus.
During reabsorption in the renal tubules, useful substances such as glucose and water are returned to the bloodstream.
The remaining urea, together with excess water and ions, is excreted as urine, keeping blood urea concentration safe.
If urea accumulates in the blood it disrupts normal cellular function, damaging tissues and organs.
Healthy kidneys remove urea by filtering the blood in the glomerulus.
During reabsorption in the renal tubules, useful substances such as glucose and water are returned to the bloodstream.
The remaining urea, together with excess water and ions, is excreted as urine, keeping blood urea concentration safe.
Examiner tips
- Use the exact terms "toxic waste product", "filter the blood", "reabsorb", "excrete as urine" to match the scheme.
- Show the sequence: filtration → reabsorption → excretion.
- Keep each point brief – 1–2 sentences per mark.
Common mistakes
- Calling urea a "nutrient" or ignoring its toxicity.
- Forgetting that reabsorption occurs in the renal tubules.
- Mixing up filtration and secretion, or omitting the step of reabsorption.
Mark scheme (5 marks)
- Urea is a toxic / poisonous waste product that must be removed from the body
- A build-up of urea would disrupt the body's normal functioning / damage cells / organs
- The kidneys filter the blood to remove urea (filtration)
- Useful substances (such as glucose and water) are reabsorbed back into the blood
- The remaining urea (and excess water / ions) is excreted as urine
Key terms in this question
Related
- All OCR A-Level Biology B — Advancing Biology (H422) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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