Free Tissue Fluid Formation revision notes for OCR A Level Biology – covering specification point 3.1.2(d).
Tissue Fluid
Tissue fluid is the liquid that surrounds body cells, and enables the exchange of substances (e.g. oxygen, glucose, carbon dioxide, urea).
Tissue fluid forms at the arteriole end (where there is a higher pressure) of the capillary bed, and then is ‘reabsorbed’ at the venous ends (where the osmotic pull of water pulls it back in).
The table below compares the composition of blood plasma, tissue fluid and lymph:
| Component | Plasma | Tissue Fluid |
|---|---|---|
| Water |  |
|
| Ions (e.g. Na+, Cl–) | |
|
| Glucose | |
|
| Amino acids | |
|
| Plasma proteins | |
 * |
| RBCs | |
|
| WBCs | |
|
| Function | Transports substances in the blood. |
Surrounds cells for substance exchange. |
*Some proteins may be present, such as antibodies from lymphocytes.
Tissue Fluid Formation
The formation and return of tissue fluid is determined by two opposing pressures:
- Hydrostatic pressure: The outward force exerted by the blood on capillary walls caused by heart contractions (blood pressure).
- Oncotic pressure: The inward osmotic pull caused by plasma proteins (mainly albumin) that cannot leave the capillaries, causing a low potential.
The table below outlines the formation and return of tissue fluid.
| Location | Key Events |
|---|---|
| Arterial end |
– Hydrostatic pressure (from heart contraction) is higher than oncotic pressure. – Water and small solutes are forced out of the capillaries into tissue spaces. – Large plasma proteins and red blood cells remain inside. |
| Venous end |
– Hydrostatic pressure is lower than oncotic pressure. – Oncotic pressure draws water (and dissolved solutes) back in by osmosis down a water potential gradient. – Around 90% of the fluid is reabsorbed. |
| Lymphatic system | – About 10% of fluid is left over and it enters the lymphatic system, becoming lymph, which eventually returns to the blood. |
