Describe the process of ultrafiltration in the kidney and explain why not all substances filtered from the blood are excreted in urine.

WJEC A-Level Biology (Wales) — 3.7 Homeostasis and the kidney · Describe · 5 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (5 marks)

1. High hydrostatic pressure in the glomerular capillaries forces small molecules such as water, glucose, urea and ions across the filtration membrane into the Bowman's capsule.
2. Large molecules – proteins, blood cells – are too large to pass through the filtration barrier and remain in the bloodstream.
3. In the proximal tubule, glucose is selectively reabsorbed back into the blood via active transport.
4. Water is reabsorbed osmotically from the tubule back into the blood; the amount varies with the body’s hydration and osmotic needs.
5. Urea is not reabsorbed; it remains in the filtrate and is excreted in the urine as a waste product.

Examiner tips

  • Use the correct sequence: filtration, selective reabsorption, osmotic water reabsorption, urea excretion.
  • Mention the role of glomerular pressure and size exclusion.
  • Show that glucose is reabsorbed but urea is not.

Mark scheme (5 marks)

  1. High pressure in the glomerulus forces small molecules out of the blood / into the Bowman's capsule
  2. Small molecules such as water, glucose, urea and ions pass into the filtrate / large molecules such as proteins and blood cells are too large to pass through
  3. Glucose is selectively reabsorbed back into the blood from the tubules
  4. Water is reabsorbed by osmosis from the tubules back into the blood / amount reabsorbed varies according to body's needs
  5. Urea is not reabsorbed and remains in the filtrate to be excreted in urine as a waste product

Key terms in this question

ultrafiltration

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