The balance between the direction and amount of fluid flow across capillary walls is determined by which pressures?

The balance between the direction and amount of fluid flow across capillary walls is determined by which pressures?

• A. Hydrostatic and osmotic pressure
• B. Blood viscosity
• C. Oxygen tension
• D. Temperature

Answer: (A)

Fluid movement across capillary walls is governed by the balance of hydrostatic pressure pushing fluid out of the capillary and osmotic (colloid osmotic) pressure pulling fluid back in. The capillary hydrostatic pressure drives water into the interstitial space, while plasma proteins maintain an oncotic pull that draws water back toward the blood. Along the capillary, hydrostatic pressure tends to decrease, and the oncotic pressure from plasma proteins remains relatively constant, so the net flow can shift between filtration and reabsorption depending on which force predominates. Other factors like viscosity, oxygen tension, or temperature don’t set this exchange across the capillary wall.

Fluid movement across capillary walls is governed by the balance of hydrostatic pressure pushing fluid out of the capillary and osmotic (colloid osmotic) pressure pulling fluid back in. The capillary hydrostatic pressure drives water into the interstitial space, while plasma proteins maintain an oncotic pull that draws water back toward the blood. Along the capillary, hydrostatic pressure tends to decrease, and the oncotic pressure from plasma proteins remains relatively constant, so the net flow can shift between filtration and reabsorption depending on which force predominates. Other factors like viscosity, oxygen tension, or temperature don’t set this exchange across the capillary wall.