The alveoli in the lungs and the nephrons in the kidneys both perform filtration or exchange functions. Compare these two structures with respect to their design features that maximise efficiency, and the substances they deal with.
Generated by claude-sonnet-4-6 · 2026-06-15 06:45 · grounding rag
Model Answer
Alveoli (Lungs) vs. Nephrons (Kidneys) — Comparison
Structural similarity: Both alveoli and nephrons have clusters of very thin-walled blood capillaries as their basic functional unit. This thin wall allows efficient exchange/filtration across it.
Design features for efficiency:
- Alveoli have a large surface area and rich blood supply to maximise gas exchange. Thin walls allow rapid diffusion.
- Nephrons have a cup-shaped Bowman's capsule surrounding a capillary cluster to collect filtrate efficiently. Coiled tubules allow selective reabsorption, and large numbers of nephrons are packed together in each kidney.
Substances dealt with:
- Alveoli exchange gases — oxygen diffuses into blood; carbon dioxide diffuses out.
- Nephrons filter nitrogenous wastes (urea, uric acid) from blood. Useful substances like glucose, amino acids, salts and water are selectively reabsorbed; only waste is excreted as urine.
Both structures thus maximise efficiency through thin walls, rich capillary networks, and large numbers working together.
Source: Chapter 5, Section 5.5.1 (Excretion in Human Beings)
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Explanation
- Examiners look for a parallel comparison — don't describe alveoli and nephrons separately in isolation; link them explicitly.
- Key scoring points: thin-walled capillaries (common feature), large surface area (alveoli), Bowman's capsule + reabsorption in tubules (nephrons), gases vs. nitrogenous wastes.
- The textbook itself draws this comparison ("Just as CO₂ is removed in the lungs… nitrogenous waste is removed in the kidneys"), so use that framing — it signals to the examiner you've read the source carefully.
- For 5 marks, aim for ~5 distinct points — don't pad with irrelevant detail about the heart or ureter.