What is tissue fluid?
As blood passes through capillaries, some of the plasma moves through gaps between endothelial cells and surrounds the body cells. The fluid outside the capillaries is called tissue fluid.
Its composition is similar to blood plasma, but tissue fluid contains far fewer large plasma proteins because these molecules are too large to pass readily through the capillary wall.
Exchange between the blood and body cells takes place through this fluid. For example, carbon dioxide released by cells can dissolve in tissue fluid before diffusing into the capillary.
Formation at the arterial end
Tissue-fluid formation depends on the balance between two opposing effects:
- Hydrostatic pressure inside the capillary pushes fluid outwards.
- Dissolved substances in the blood, particularly plasma proteins, lower its water potential and create an osmotic pull drawing water towards the blood.
At the arterial end of a capillary, the outward hydrostatic pressure is greater than the inward osmotic pull.
- Blood enters the capillary under relatively high hydrostatic pressure.
- Water and small dissolved molecules are forced through gaps in the capillary wall.
- These substances form tissue fluid around the cells.
- Large plasma proteins remain in the blood because they are too large to leave through the gaps.
| Component | What happens? |
|---|---|
| Water | Can leave the capillary and become part of tissue fluid. |
| Small molecules | Can pass out with the fluid through gaps in the capillary wall. |
| Large plasma proteins | Remain mainly within the blood because they are too large to pass through. |
Exam Tip: For tissue-fluid formation, give the pressure comparison explicitly: at the arterial end, hydrostatic pressure is greater than the osmotic pull, so water and small molecules are forced out while large plasma proteins remain in the blood.