Explain how the lymphatic system contributes to the maintenance of tissue fluid balance in the body.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
Capillary blood pressure pushes fluid, water, dissolved solutes and small proteins out of the capillaries into the interstitial space, forming tissue fluid.
Not all of this fluid is re‑absorbed by the capillaries; the excess fluid and large proteins that cannot re‑enter the blood vessels are collected by blind‑ended lymph capillaries.
The lymph is then carried along lymphatic vessels, ultimately returning to the bloodstream at the subclavian veins.
If lymph drainage is impaired or blocked, the excess interstitial fluid accumulates, producing oedema – showing how the lymphatic system is essential for maintaining tissue fluid balance.
Not all of this fluid is re‑absorbed by the capillaries; the excess fluid and large proteins that cannot re‑enter the blood vessels are collected by blind‑ended lymph capillaries.
The lymph is then carried along lymphatic vessels, ultimately returning to the bloodstream at the subclavian veins.
If lymph drainage is impaired or blocked, the excess interstitial fluid accumulates, producing oedema – showing how the lymphatic system is essential for maintaining tissue fluid balance.
Examiner tips
- Use the word "explain" to describe the sequence of events; include the role of capillary pressure, lymph capillaries and return to blood.
- Mention the consequence of failure (oedema) to demonstrate understanding of the system’s importance.
Common mistakes
- Confusing lymph with blood or describing lymph as a waste product only.
- Omitting the step that large proteins cannot re‑enter capillaries or the return of lymph to the subclavian veins.
Mark scheme (4 marks)
- Capillary blood pressure forces fluid (water, dissolved solutes, small proteins) out of capillaries into the interstitial space, forming tissue fluid
- Not all tissue fluid is reabsorbed by the capillaries; excess fluid and large proteins that cannot re-enter capillaries are drained into blind-ended lymph capillaries
- Lymph is transported along lymphatic vessels back towards the heart / returned to the blood at the subclavian veins
- If lymph drainage fails / is blocked, tissue fluid accumulates causing oedema, demonstrating the essential role of the lymphatic system in maintaining fluid balance
Key terms in this question
Related
- All IB DP Biology Standard Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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