Explain how changes in blood pressure along the length of a capillary bed lead to the formation and reabsorption of tissue fluid.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
Blood enters the arterial end of a capillary bed at a relatively high hydrostatic pressure.
At the arterial end, the hydrostatic pressure exceeds the plasma osmotic pressure, so water and small solutes are forced out of the capillary into the surrounding tissues, forming tissue fluid.
Along the length of the capillary, hydrostatic pressure falls because of resistance and loss of fluid.
At the venous end the hydrostatic pressure is lower than the plasma osmotic pressure, so the net inward osmotic pressure causes most of the tissue fluid to be re‑absorbed back into the capillary.
At the arterial end, the hydrostatic pressure exceeds the plasma osmotic pressure, so water and small solutes are forced out of the capillary into the surrounding tissues, forming tissue fluid.
Along the length of the capillary, hydrostatic pressure falls because of resistance and loss of fluid.
At the venous end the hydrostatic pressure is lower than the plasma osmotic pressure, so the net inward osmotic pressure causes most of the tissue fluid to be re‑absorbed back into the capillary.
Examiner tips
- Use the terms ‘hydrostatic pressure’ and ‘osmotic pressure’ explicitly.
- Show the direction of net movement at each end of the capillary.
- Keep the answer concise – one sentence per point.
- Mention that the change in pressure is due to resistance and fluid loss.
Common mistakes
- Confusing hydrostatic and osmotic pressure or using the wrong direction of flow.
- Omitting the fact that the hydrostatic pressure falls along the capillary.
- Using vague language such as ‘pressure changes’ without specifying which pressure dominates where.
Mark scheme (4 marks)
- Blood enters the arterial end of the capillary bed at relatively high hydrostatic pressure.
- At the arterial end, hydrostatic pressure exceeds the osmotic pressure of the blood plasma, so water and small solutes are forced out of the capillary into the surrounding tissues, forming tissue fluid.
- Hydrostatic pressure drops along the length of the capillary due to resistance/fluid loss, so at the venous end hydrostatic pressure is lower than the osmotic pressure of the plasma.
- At the venous end, the net inward osmotic pressure causes most of the tissue fluid to be reabsorbed back into the capillary.
Key terms in this question
tissue fluid · capillary bed · reabsorption
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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