How do plasma proteins help keep water inside blood vessels?

Plasma proteins, particularly serum albumin, play a vital role in maintaining the volume of circulating blood by regulating the movement of water across the walls of capillaries.12

Because these proteins are large molecules, they cannot easily pass through the semipermeable membranes of the endothelial cells that line blood vessels. Their presence within the blood creates osmotic pressure—often referred to as colloid osmotic pressure—which exerts an osmotic effect that draws water into the plasma and retains it within the bloodstream.132

When the concentration of these proteins is too low, this pressure gradient is disrupted. As a result, fluid is forced out of the blood vessels and into the surrounding tissue spaces, which can lead to edema, or swelling. While other proteins like globulins also contribute to this effect, albumin is the most significant factor due to its high concentration in the plasma.132