Final answer:
The difference between colloid osmotic pressures of plasma and interstitial fluid, key in fluid reabsorption, is known as oncotic pressure.
Step-by-step explanation:
The difference between colloid osmotic pressure of the plasma and the interstitial fluid is termed as oncotic pressure. This pressure is determined by the osmotic concentration gradients, which is the difference in solute-to-water concentrations between the blood and tissue fluid.
In the context of capillary exchange, the plasma proteins in the blood, which cannot cross the semipermeable capillary walls, create a higher blood colloidal osmotic pressure (BCOP) compared to the very low interstitial fluid colloidal osmotic pressure (IFCOP). This difference in osmotic pressures drives the reabsorption of water from the tissue fluid back into the capillaries, aiding in maintaining fluid balance within the body.