BRAINCAKEOncotic Pressure
Blood plasma contains large dissolved proteins, the most abundant of which is albumin. These plasma proteins are too large to pass through the gaps in the capillary wall, so they stay in the blood. Tissue fluid contains very few of them.
Dissolved proteins lower the water potential of the plasma. Because the plasma has a lower water potential than the tissue fluid, water tends to move from the tissue fluid into the capillaries by osmosis. The pressure created by this tendency is called oncotic pressure. It acts to pull water back into the capillaries.
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Key terms in this lesson
- water potential
- A measure of the tendency of water molecules to move out of a solution; water moves from a higher to a lower (more negative) water potential.
- osmosis
- The net movement of water molecules from a region of higher water potential to a region of lower water potential through a partially permeable membrane.
- oedema
- The build-up of excess fluid in the tissues, for example when too much tissue fluid is formed or too little is drained away.
More in Circulatory Systems
- Arterioles
- Capillaries
- Veins and venules
- Tissue Fluid
- Hydrostatic Pressure
- Arterial End of Capillaries
- Venous End of Capillaries
- Formation of Lymph
All 33 lessons in Circulatory Systems · All Edexcel AS-level Biology topics