Read the following passage and answer the questions that follow.
Rohan is a 45-year-old man who was recently diagnosed with chronic kidney disease. His doctor explained that both his kidneys were functioning at only 20% of their normal capacity. As a result, harmful nitrogenous wastes were accumulating in his blood. The doctor recommended he undergo hemodialysis three times a week. During hemodialysis, Rohan's blood is passed through tubes with a semi-permeable lining immersed in a dialysing fluid that has the same osmotic pressure as normal blood but contains no nitrogenous wastes.
(i) Name the nitrogenous waste products that accumulate in Rohan's blood due to kidney failure. (1 mark)
(ii) By what process do the waste products move from Rohan's blood into the dialysing fluid during hemodialysis? (1 mark)
(iii) A healthy person produces about 180 L of initial filtrate in the kidneys each day but excretes only 1–2 L of urine. Explain the process responsible for this difference and where it occurs. (2 marks)
Generated by claude-sonnet-4-6 · 2026-06-26 01:00 · grounding rag
Model Answer
(i) The nitrogenous waste products that accumulate in Rohan's blood are urea and uric acid.
(ii) The waste products move from Rohan's blood into the dialysing fluid by the process of diffusion. Since the dialysing fluid contains no nitrogenous wastes, a concentration gradient exists, causing wastes to diffuse out of the blood across the semi-permeable membrane.
(iii) The process responsible is selective reabsorption, and it occurs in the kidney tubules (nephrons). As the initial filtrate passes through the tubules, useful substances like glucose, amino acids, salts, and a large amount of water are selectively reabsorbed back into the blood. The amount of water reabsorbed depends on the body's needs. Only the remaining waste-concentrated fluid (~1–2 L) is excreted as urine, out of the 180 L filtered daily.
Source: Chapter 5, Section 5.5.1 — Excretion in Human Beings; Artificial Kidney (Hemodialysis)
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Explanation
- (i) Always name both urea and uric acid — writing only one may cost you the mark.
- (ii) The keyword examiners look for is diffusion. Justify it briefly by mentioning the concentration gradient.
- (iii) Two things must be stated for 2 marks: the name of the process (selective reabsorption) and the location (kidney tubules/nephrons). Mentioning the 180 L vs 1–2 L figures from the passage strengthens the answer. Avoid confusing reabsorption with filtration.