--- title: "Tissue Fluid and Its Return to the Circulation" description: "Capillary hydrostatic pressure favours filtration, while retained plasma proteins provide an opposing osmotic effect. Lymph vessels return net filtered fluid to blood. A simplified model helps compare" canonical: https://lightmysky.com/learn/science/tissue-fluid-and-its-return-to-the-circulation-mt_0jw7uXpK3C source: https://lightmysky.com/learn/science/tissue-fluid-and-its-return-to-the-circulation-mt_0jw7uXpK3C.md retrieved: 2026-11-12 --- > **Agent view.** This is the Markdown twin of the page, for tools and assistants. > When to use this site, and the call that answers each job: https://lightmysky.com/agent-instructions.md > API description (OpenAPI 3.1): https://lightmysky.com/openapi.json · Authentication: https://lightmysky.com/auth.md > Pricing: https://lightmysky.com/pricing.md · Catalog: https://lightmysky.com/llms.txt · Full catalog: https://lightmysky.com/llms-full.txt > Every machine-readable file on this domain: https://lightmysky.com/.well-known/ai-catalog.json > Ask for Markdown with `Accept: text/markdown`, a `.md` address, or `?mode=agent`. # Tissue Fluid and Its Return to the Circulation Capillary hydrostatic pressure favours filtration, while retained plasma proteins provide an opposing osmotic effect. Lymph vessels return net filtered fluid to blood. A simplified model helps compare pressures along a capillary. Subject: Science · Area: The Human Body · Ages 17 to 18 Page: https://lightmysky.com/learn/science/tissue-fluid-and-its-return-to-the-circulation-mt_0jw7uXpK3C ## Ready when they can - Compares hydrostatic and protein osmotic effects at the two ends of a capillary in a simplified model. - Describes substances that cross ordinary tissue capillaries and those mainly retained in blood. - Explains how raised capillary pressure, reduced plasma protein or impaired lymph drainage can cause oedema. ## Lesson: How tissue fluid leaves and returns Tissue fluid surrounds cells outside blood vessels. Capillary hydrostatic pressure favours outward filtration. Retained plasma proteins lower the solute contribution to plasma water potential. Their colloid osmotic effect opposes outward filtration. Hydrostatic pressure and this osmotic effect must be considered together, along with conditions outside the capillary. A simplified model ignores tissue-side pressures and keeps opposing protein osmotic pressure at 24 mmHg. Capillary hydrostatic pressure falls from 34 mmHg at the arterial end to 16 mmHg at the venous end. Compare the pressures: 34 exceeds 24, predicting outward flow; 16 is below 24, predicting inward flow. These illustrative values do not describe every tissue. *(drawing: Model pressures in mmHg. Compare hydrostatic push with osmotic opposition at each end.)* Real exchange does not always follow this model. In many tissues at steady state, little fluid is continuously reabsorbed at the venous end. Lymph vessels return the net filtered fluid to blood. Tissue pressure and the capillary barrier also affect exchange. The model's venous-end prediction is not a rule for every capillary bed. Water, ions, glucose and amino acids can cross capillary walls. Small solutes also move by diffusion or transport through the wall. Ordinary tissue capillaries retain red blood cells and platelets and restrict most large plasma proteins. Tissue fluid has much less protein than plasma, rather than none. Permeability differs between capillary types. **Example.** Tissue fluid becomes lymph when it enters lymph vessels, which return it to veins near the heart. For a hypothetical balance, subtract direct return from filtration. If 6 litres filter out and 4 litres return directly, lymph carries 2 litres when no fluid accumulates. These are invented model values, not daily measurements. Oedema is excess fluid in tissue spaces. Higher capillary hydrostatic pressure can increase filtration. Less plasma protein weakens the osmotic opposition. Either can cause swelling if lymph drainage cannot keep up. Blocked lymph drainage can also cause oedema. A rise in systemic blood pressure alone does not establish the pressure change in a particular capillary. **Recap.** Hydrostatic pressure favours filtration, protein osmotic effects oppose it, and lymph returns net filtered tissue fluid to blood. *Lesson, questions and flashcards written for this dataset. Anatomy and Physiology sections 20.3, 21.1 and 26.1 were used for factual study.* ## Practice 8 questions on this page, each with its working shown. ## Needs first - [Water Potential and Osmosis Quantified](https://lightmysky.com/learn/science/water-potential-and-osmosis-quantified-mt_dFBz-cE6W0) - [The Cardiac Cycle and Pressure in the Heart](https://lightmysky.com/learn/science/the-cardiac-cycle-and-pressure-in-the-heart-mt_iOSVyoV3Pj) ## Opens up - [Ultrafiltration and Selective Reabsorption in the Nephron](https://lightmysky.com/learn/science/ultrafiltration-and-selective-reabsorption-in-the-nephron-mt_vSKfoJN6zh)