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.
What a learner can do afterwards
- 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.
1 · Read
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.
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.
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.
Hypothetical volumes in litres: 6 = 4 + 2.
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.
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.
Readings and flashcards written for this dataset. Anatomy and Physiology sections 20.3, 21.1 and 26.1 were used for factual study; their prose is not reproduced.
More to read
What a capillary model predicts
Pressure inside a capillary tends to move fluid towards the tissue spaces. Retained plasma proteins provide an opposing osmotic effect. Conditions on the tissue side and the properties of the vessel wall also influence exchange.
A common classroom model compares falling capillary hydrostatic pressure with constant protein osmotic pressure. It leaves out tissue-side pressures. Where the outward pressure is larger, the model predicts filtration. Where osmotic opposition is larger, it predicts reabsorption.
That comparison is useful when its assumptions are stated. It is not a complete description of every tissue. In many tissues at steady state, little fluid is continuously reabsorbed at the venous end. The lymphatic system returns the net filtered fluid to blood.
When interpreting a question, distinguish a result calculated from an idealised model from a claim about all capillaries. Both the stated assumptions and the tissue being considered matter.
Following fluid into lymph vessels
Fluid occupying spaces between tissue cells can enter lymphatic vessels. It is then called lymph. The lymphatic network carries this fluid, including proteins that escaped from blood vessels, back to venous blood near the heart.
A hypothetical balance can divide filtered fluid between direct return and lymph return. If no fluid accumulates and those are the only return routes, subtract direct return from filtration to find lymph return. The numbers describe that model, not a fixed daily total for every person.
Drainage has limits. If more fluid enters the tissue spaces than can leave them, the tissue-fluid volume increases. This accumulation is oedema. Impaired lymph drainage can cause oedema even when capillary pressure has not risen.
The return pathway therefore matters alongside the pressures driving filtration. Knowing only how much fluid leaves a capillary is not enough to establish whether swelling will develop.
The barriers between fluid spaces
A cell membrane separates fluid inside a cell from the tissue fluid outside it. A capillary wall separates tissue fluid from blood plasma. These are different barriers, so a substance that crosses one does not automatically cross the other at the same rate.
Ordinary tissue capillaries retain red blood cells and platelets and restrict most large plasma proteins. Water and small dissolved substances can cross. Tissue fluid usually contains less protein than plasma, but it is not completely protein-free. Capillary permeability also differs between tissues.
Higher capillary hydrostatic pressure can favour greater filtration. Lower plasma protein concentration can weaken the opposing osmotic effect. Either change may contribute to oedema if drainage cannot match the net filtered volume.
These mechanisms describe possible changes in fluid distribution. They do not mean that every pressure change causes swelling or that the body's fluid compartments always maintain fixed volumes.
2 · Watch
Take it off screen
Where it sits
Where this leads
Jobs that lean on this skill. Follow one to see everything it is built on.
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.