The net pressure that drives reabsorption—the movement of fluid from the interstitial fluid back into the capillaries—is called osmotic pressure (sometimes referred to as oncotic pressure).

Osmotic pressure plays a key role in biological systems. The cell membrane, for example, acts as a semipermeable membrane subject to osmotic pressure based on the cell’s external environment. A hypotonic environment outside the cell can cause the cell to swell or burst, whereas a hypertonic environment can cause the cell to shrink.