The plasma protein mainly responsible for maintaining colloidal osmotic pressure in vivo is

• A. Albumin
• B. Hemoglobin
• C. Fibrinogen
• D. α2-macroglobulin

Answer: (A)

Colloidal (oncotic) osmotic pressure in the plasma is generated by plasma proteins that stay in the blood vessels. Albumin is the most abundant of these proteins in plasma, so it provides the majority of this inward pull that draws water back into capillaries. This keeps intravascular fluid from leaking out into the interstitial space.

Hemoglobin is inside red blood cells and does not contribute to plasma oncotic pressure. Fibrinogen and α2-macroglobulin are present, but they are far less abundant than albumin (and α2-macroglobulin is both large and less plentiful), so their contribution to oncotic pressure is much smaller. Thus albumin is the primary determinant of maintaining colloidal osmotic pressure in vivo.