Tissue Fluid and Its Return to the Circulation
Hydrostatic pressure forces fluid out of a capillary at the arteriole end, and water potential pulls most of it back at the venule end. Whatever is left returns through the lymph vessels.
What a learner can do afterwards
- Compares hydrostatic pressure and water potential at the two ends of a capillary.
- Says what leaves the capillary and what stays in the blood.
- Links a build-up of tissue fluid to a change in one of the two pressures.
1 · Read
Two forces fight across your capillary walls. Hydrostatic pressure is the push of blood against the wall, and it forces fluid out. The water potential gradient is set by plasma proteins too big to escape, and it pulls water back in by osmosis. One force pushes, the other pulls.
The winner changes along the capillary. At the arteriole end the push is strong and wins, so fluid filters out into the tissues. By the venule end the push has faded while the pull stays steady, so the pull wins and most fluid is reabsorbed. Your cells therefore bathe in fresh fluid delivered at one end and tidied away at the other.
Only small cargo crosses the wall. Water, ions, glucose and amino acids slip through the gaps. Red cells, platelets and big plasma proteins stay trapped inside the blood. That sorting is why tissue fluid looks like plasma with the big stuff removed.
Not all fluid returns directly, and the leftover needs a drain. Lymph vessels collect the excess and carry it back to the blood near the heart. Each day about 20 litres filter out, about 17 litres return directly, and about 3 litres travel home in the lymph. If hydrostatic pressure rises or plasma protein falls, fluid strands in the tissues and the area swells, which is called oedema.
Fluid leaves where the push wins, returns where the pull wins, and lymph clears the rest.
2 · Watch
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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.