Oncotic pressure is:

Oncotic pressure is:

• A. The pressure driving diffusion of gases.
• B. The hydrostatic pressure within arteries.
• C. The osmotic pressure due to electrolytes.
• D. The pressure exerted by proteins in a fluid compartment, influencing fluid movement.

Answer: (D)

Oncotic pressure is the osmotic pressure created by proteins in a fluid, mainly albumin in plasma. These large proteins stay inside the blood vessels and pull water from the surrounding tissue back into the capillaries, helping to maintain blood volume and tissue fluid balance. This is different from hydrostatic pressure, which pushes fluid out of capillaries, and from osmotic pressure due to small solutes like electrolytes. Because it reflects the influence of proteins on fluid movement, it best explains why fluid shifts toward the vascular compartment. Clinically, lower oncotic pressure (as in hypoalbuminemia) can lead to edema due to reduced water reabsorption into vessels.

Oncotic pressure is the osmotic pressure created by proteins in a fluid, mainly albumin in plasma. These large proteins stay inside the blood vessels and pull water from the surrounding tissue back into the capillaries, helping to maintain blood volume and tissue fluid balance. This is different from hydrostatic pressure, which pushes fluid out of capillaries, and from osmotic pressure due to small solutes like electrolytes. Because it reflects the influence of proteins on fluid movement, it best explains why fluid shifts toward the vascular compartment. Clinically, lower oncotic pressure (as in hypoalbuminemia) can lead to edema due to reduced water reabsorption into vessels.