Science (086) — AI-generated practice question

Alveolus vs. Nephron — A Comparison

(i) What is filtered:
In the alveolus, gases are exchanged across thin-walled capillaries — CO₂ passes out of the blood into the air sac, and O₂ enters the blood. In the nephron, blood is filtered under pressure at the Bowman's capsule; the filtrate contains water, glucose, amino acids, salts, urea, and uric acid.

(ii) Fate of useful substances after filtration:
In alveoli, O₂ absorbed into the blood is carried by haemoglobin to all body cells — nothing is "reabsorbed" as such. In nephrons, useful substances — glucose, amino acids, salts, and a large amount of water — are selectively reabsorbed back into the blood as filtrate flows along the kidney tubule.

(iii) What the body expels:
From the lungs, CO₂ is breathed out as a gaseous waste. From the kidneys, nitrogenous wastes (urea, uric acid) dissolved in water are expelled as urine through the ureter, bladder, and urethra.

Source: Chapter 5, Section 5.5.1 (Excretion in Human Beings)

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• The examiner wants a direct comparison across three specific points — structure your answer clearly under each point (or make it obvious in flow).
• The key contrast: alveoli deal with gaseous exchange (respiratory waste = CO₂); nephrons deal with nitrogenous waste (urea/uric acid) via filtration + selective reabsorption.
• The phrase "selectively reabsorbed" is textbook language — use it for nephron; it earns marks.
• Don't forget that ~180 L is filtered daily in kidneys but only 1–2 L is excreted — this shows the scale of reabsorption and is a good detail if you have space.
• Alveoli have no reabsorption step — that's the fundamental structural difference the question is targeting.