Osmotic pressure in the capillary bed is generally:

• A. Increases toward the venule end
• B. Remains relatively consistent throughout the capillary bed
• C. Decreases markedly toward the venule end
• D. Fluctuates with filtration rate

Answer: (B)

Osmotic (colloid osmotic) pressure in the capillary bed is driven mainly by plasma proteins that stay inside the blood vessels. Because these large proteins do not cross the capillary wall easily, their concentration—and thus their pulling force on water—stays roughly the same along the length of the capillary. In contrast, the hydrostatic pressure inside the capillary is highest at the arterial end and falls toward the venous end. So with a fairly constant oncotic pressure, the higher hydrostatic pressure at the arterial end pushes fluid out (filtration), while the lower hydrostatic pressure at the venous end allows the constant oncotic pressure to pull water back in (reabsorption). In healthy tissue, this balance means osmotic pressure remains relatively constant throughout the capillary bed, and the net fluid movement is governed mainly by the changing hydrostatic pressure. If plasma protein levels drop or capillaries become leaky, this constant osmotic pull can weaken, altering fluid movement.