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.Because the plasma proteins remain in the blood along the whole length of the capillary, the oncotic pressure stays roughly constant from the arterial end to the venous end. A typical value is about 3.3 kPa (25 mmHg).Oncotic pressure opposes hydrostatic pressure, which pushes fluid out of the capillaries. Whether fluid leaves or returns at any point depends on which of the two pressures is larger.
If the concentration of plasma proteins falls, oncotic pressure falls too. Less tissue fluid is drawn back into the capillaries, so fluid accumulates in the tissues and causes oedema. This can happen in severe protein malnutrition (kwashiorkor, which causes a swollen abdomen) and in liver disease, because albumin is made in the liver.
If the concentration of plasma proteins falls, oncotic pressure falls too. Less tissue fluid is drawn back into the capillaries, so fluid accumulates in the tissues and causes oedema. This can happen in severe protein malnutrition (kwashiorkor, which causes a swollen abdomen) and in liver disease, because albumin is made in the liver.