The big idea: Tissue fluid forms when blood pressure forces plasma out of capillaries, and most of it drains back in further along.
The points to remember
- Blood from an arteriole is at high hydrostatic pressure.
- At the arteriole end, this pressure forces plasma through the capillary wall: pressure filtration.
- Water and small solutes (glucose, oxygen, ions, amino acids) leave; plasma proteins and blood cells stay in.
- Pressure falls along the capillary; at the venule end it is lower, so most tissue fluid drains back in.
- The rest drains into lymph ducts (2.7.13).
Arteriole end
- Blood pressure higher than in tissue fluid
- Plasma is forced out
- Tissue fluid forms
Venule end
- Blood pressure lower
- Fluid drains back in
- Tissue fluid is reabsorbed
Remember it as: Pushed out at the start, drains back at the end.
Real example: swollen ankles. After hours sitting still on a long flight, more fluid leaves the capillaries in the legs than drains back, so ankles swell with tissue fluid.
Two traps: Tissue fluid forms because of pressure, not because the capillary wall is 'more permeable' at one end.
Proteins do not leave: they are too large, so tissue fluid has almost no protein.
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How it is asked: Expect a capillary-bed diagram with a region marked: what happens there, and why.
IB-style questionExplain[3 marks]
Explain how tissue fluid is formed and returned to the blood in a capillary bed.
Model answer plan
See the answer broken into clear points, with guidance on how each point earns marks.