The kidneys filter approximately 180 litres of blood plasma per day, yet the composition of urine is very different from the original filtrate. (a) Which substances are selectively reabsorbed from the filtrate back into the blood, and why is this reabsorption necessary? (b) What would be the consequences for the body if this selective reabsorption did not take place?
Generated by claude-sonnet-4-6 · 2026-06-26 01:01 · grounding rag
Model Answer
(a) As the filtrate flows through the nephron tubules, substances like glucose, amino acids, salts, and a major amount of water are selectively reabsorbed back into the blood. This reabsorption is necessary because these are useful substances that the body needs; excreting them would be wasteful and harmful to normal body functions.
(b) If selective reabsorption did not occur, the body would lose essential nutrients (glucose, amino acids), vital salts, and large amounts of water in urine. This would lead to dehydration, loss of energy sources, electrolyte imbalance, and ultimately failure of normal cellular activities — which could be life-threatening.
Source: Chapter 5, Section 5.5.1 – Excretion in Human Beings
---
Explanation
- (a) The textbook explicitly lists glucose, amino acids, salts, and water as reabsorbed substances. Examiners expect all four mentioned and a reason (they are useful/needed by the body).
- (b) Think logically from (a): if useful things aren't reabsorbed, they are lost. The key consequences are dehydration (water loss), energy loss (glucose), and ion imbalance (salts). The passage also notes that of 180 L filtered, only 1–2 L is excreted — underlining how critical reabsorption is.
- For a 3-mark question split into (a) and (b), aim for roughly 1.5 marks each — about 2–3 crisp points per part.