The big idea: Cells never touch the blood: they exchange substances with the tissue fluid that bathes them.
The points to remember
- Oxygen, glucose, amino acids and ions diffuse from the tissue fluid into the cells.
- Carbon dioxide and wastes such as urea diffuse out of the cells into the tissue fluid.
- Tissue fluid is constantly renewed from the blood, so the concentration gradients stay steep.
Remember it as: The blood feeds the fluid; the fluid feeds the cells.
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Plasma and tissue fluid: Tissue fluid is plasma without its large proteins: pressure filtration lets small molecules out but holds the proteins and cells in.
| Component | Plasma | Tissue fluid |
|---|---|---|
| Water, glucose, amino acids, ions | Present | Present, at similar concentrations |
| Oxygen | Higher | Lower (cells use it) |
| Carbon dioxide, urea | Lower | Higher (cells release them) |
| Plasma proteins | Present | Almost none: too large to leave |
| Red blood cells, platelets | Present (in blood) | None |
The points to remember
- Similar: water and small solutes cross the capillary wall freely.
- Different: proteins and blood cells stay in the plasma.
- Near the cells, tissue fluid holds less oxygen and more carbon dioxide than the plasma.
Real example: a blood test. Doctors measure blood glucose in plasma because it is close to what the tissue fluid around the cells receives.
Trap: Tissue fluid is not identical to plasma: give the proteins as the main difference, then the gases.
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How it is asked: Expect a comparison of plasma and tissue fluid, or what passes between tissue fluid and a cell.
IB-style questionCompare and contrast[3 marks]
Compare and contrast the composition of plasma and tissue fluid.
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See the answer broken into clear points, with guidance on how each point earns marks.