Which pressure primarily governs water movement between body compartments?

• A. Hydrostatic pressure
• B. Osmotic/colloidal pressure
• C. Oncotic pressure
• D. Blood pressure

Answer: (B)

Water moves between intracellular, interstitial, and vascular spaces mainly in response to osmotic forces. Osmotic pressure is the force created by solute differences across a membrane; it pulls water toward the side with higher solute concentration to equalize osmolarity. In the body, plasma proteins contribute to this as colloid (oncotic) pressure, which specifically draws water from the interstitial space back into the blood, helping maintain circulating volume. While hydrostatic pressure can push water out of capillaries, the net distribution of water between compartments over time is driven by osmotic/colloidal pressure. This is why changes in plasma protein levels, like low albumin, can shift fluid into tissues and cause edema.