Capillaries & Tissue Fluid (AQA AS Biology): Revision Note

Capillaries & tissue fluid

Capillaries

• Capillaries are tiny blood vessels that carry blood from the larger blood vessels to the cells

• Capillaries form networks called capillary beds which function as exchange surfaces with the cells

• The features of capillaries allow effective exchange of substances with the surrounding cells

Tissue fluid

• As blood passes through capillaries, some plasma leaks out through gaps in the walls of the capillary to surround the cells of the body; the resulting fluid is known as tissue fluid

  • The composition of plasma and tissue fluid is very similar, though tissue fluid contains fewer large proteins as these are too large to pass out of the capillaries

• Exchange of substances between cells and the blood occurs via the tissue fluid

  • E.g. waste carbon dioxide leaves the cells, dissolves in the tissue fluid, and then diffuses into the capillary

Formation and return of tissue fluid

• Tissue fluid forms and returns to the blood due to the balance between the:

  • hydrostatic pressure

    • Hydrostatic pressure exerts an outward force on the contents of the capillaries

  • osmotic pull generated by dissolved solutes, e.g. plasma proteins

    • Dissolved substances in the blood lower the water potential, exerting an inward force on the tissue fluid due to the resulting water potential gradient

• Tissue fluid forms as follows:

  • at the arterial end of a capillary the hydrostatic pressure is greater than the osmotic pull

  • water and small molecules are forced out of the capillary down a hydrostatic pressure gradient, forming tissue fluid

    • Large molecules, e.g. large plasma proteins, remain in the blood as they are too large to pass out of the capillaries

• Tissue fluid returns to the capillaries as follows:

  • at the venous end the osmotic pull is now higher than the hydrostatic pressure

    • Hydrostatic pressure in the capillary has decreased due to loss of plasma volume and flow resistance in the narrow capillary

  • dissolved proteins in the blood lower the water potential and create a water potential gradient between the capillary and the tissue fluid

  • fluid is drawn back into the capillary down its water potential gradient

• Tissue fluid formation can be affected by factors such as:

  • high blood pressure: this can force extra fluid out of the capillaries, meaning that an increased volume of fluid is left behind after the effects of osmosis

  • low blood protein content: this can increase the water potential of the blood and reduce the effect of osmosis, also resulting in a build-up of fluid in the tissues