How is Tissue Fluid formed?
- At the start of the capillary bed, the arteriole end, there’s high pressure due to the contraction of the ventricle
- This pressure is called hydrostatic pressure and it forces fluid out of the capillaries into the spaces around the capillary wall, forming tissue fluid (consists of plasma with dissolved nutrients and oxygen – others are too big to be pushed out)
- As the fluid leaves the blood, there’s a lower pressure at the end of the capillary bed near the venules.
- The fluid surrounds body cells, so that gaseous and nutrients exchange can occur by diffusion
How does fluid return to the blood?
- Not only does the hydrostatic pressure force the fluid out, but the tissue fluid itself has some hydrostatic pressure, which will push fluid back into capillaries
- The water concentration of the blood (in the vessel) is lower than that of tissue fluid.
- This means water re-enters into the capillaries from the tissue fluid at the venous end (where there is a low conc. of water) by osmosis.
Not all the tissue fluid returns to the blood capillaries. There’s excess that is drained away into the lymphatic system. 20% of the tissue fluid returns to the circulation via the lymph system.
If B.P increases (hypertension) more fluid is forced out. The fluid accumulates in the tissues causing oedema. This is a sign of hypertension.