# Explain how changes in blood pressure along the length of a capillary bed lead to the formation and reabsorption of tissue fluid.

> IB DP Biology Standard Level (2023 syllabus) — B3.2 Transport · Explain · 4 marks

## Mark scheme (4 marks)

1. Blood enters the arterial end of the capillary bed at relatively high hydrostatic pressure.
2. 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.
3. 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.
4. At the venous end, the net inward osmotic pressure causes most of the tissue fluid to be reabsorbed back into the capillary.

## Key terms

- [tissue fluid](https://www.gradenine.co.uk/glossary/tissue-fluid)
- [capillary bed](https://www.gradenine.co.uk/glossary/capillary-bed)
- [reabsorption](https://www.gradenine.co.uk/glossary/reabsorption)

## Related

- [Revision notes for IB DP Biology Standard Level (2023 syllabus)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
- [Practice this with AI marking (free)](https://www.gradenine.co.uk/start)

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Source: [GradeNine](https://www.gradenine.co.uk/q/explain-how-changes-in-blood-pressure-424a7bfc) · Published by Druglandscape Ltd.