Explain how the lymphatic system contributes to the maintenance of fluid balance in body tissues.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
Fluid (tissue fluid) leaks out of capillaries because hydrostatic pressure at the arteriolar end exceeds the osmotic pressure, so fluid enters the interstitial space.
Not all of this fluid is re‑absorbed at the venular end – about 10 % remains in the tissues.
The excess interstitial fluid is taken up by blind‑ended lymph capillaries, forming lymph.
Lymph is carried through the lymphatic vessels and ultimately returned to the bloodstream via the thoracic duct (or subclavian veins), thereby preventing oedema.
Not all of this fluid is re‑absorbed at the venular end – about 10 % remains in the tissues.
The excess interstitial fluid is taken up by blind‑ended lymph capillaries, forming lymph.
Lymph is carried through the lymphatic vessels and ultimately returned to the bloodstream via the thoracic duct (or subclavian veins), thereby preventing oedema.
Examiner tips
- Use the terms ‘hydrostatic pressure’, ‘osmotic pressure’, ‘interstitial fluid’, ‘lymph capillaries’, ‘thoracic duct’
- Show the sequence: capillary leakage → incomplete re‑absorption → lymph formation → return to blood
Common mistakes
- Confusing lymph with blood or mistaking lymph vessels for veins
- Omitting the 10 % figure or the role of the thoracic duct
Mark scheme (4 marks)
- Fluid (tissue fluid / interstitial fluid) leaks out of capillaries due to hydrostatic pressure exceeding osmotic pressure at the arteriolar end
- Not all of the fluid that leaves capillaries is reabsorbed back into the blood at the venular end (approximately 10% remains in tissues)
- Excess interstitial fluid enters blind-ended lymph capillaries, forming lymph
- Lymph is eventually returned to the blood (via the thoracic duct / subclavian veins), preventing oedema
Key terms in this question
Related
- All IB DP Biology Higher Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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