Which two forces determine capillary blood flow?

• A. Capillary permeability and diffusion.
• B. Vascular tone and heart rate.
• C. Pressure and resistance.
• D. Osmotic pressure and hydrostatic pressure.

Answer: (C)

The rate of capillary blood flow is set by a driving pressure and the resistance to flow. Blood moves through a capillary where the pressure difference from one end to the other pushes it along, and the flow is limited by the resistance of the vessels it travels through. This relationship is described by Q = ΔP / R, where ΔP is the pressure gradient across the capillary bed and R is the resistance determined by factors like vessel diameter, length, and blood viscosity. If the pressure gradient increases or resistance decreases, flow increases; if the opposite occurs, flow decreases.

Other concepts here describe how blood exchanges substances with tissues rather than how quickly blood moves through the capillary bed. Capillary permeability and diffusion govern exchange; osmotic and hydrostatic pressures across the capillary wall influence filtration versus reabsorption (Starling forces); and heart rate or overall vascular tone can affect global perfusion but are not the direct pair that determines capillary flow.