Which vessels have the highest velocity of blood flow and why?

• A. Pulmonary arteries because of thin walls
• B. Capillaries due to large total surface area
• C. Aorta and large arteries due to large radius and direct propulsion from the heart
• D. Venules due to low resistance

Answer: (C)

Velocity of blood flow is governed by flow rate divided by the total cross-sectional area of the vessel bed. Cardiac output is roughly constant as blood moves through the circuit, so the speed depends mainly on how much area the blood has to travel through. Capillaries collectively present a huge cross-sectional area, so blood slows to allow exchange of nutrients and gases. In contrast, the aorta and other large arteries form the main pathways leaving the heart and have relatively large lumens but far less total cross-sectional area than the capillary network, so the blood is propelled with the highest velocity there. The pulmonary arteries operate under different pressures and are part of a different circuit, and venules (and veins) have high capacitance and larger total cross-sectional areas, leading to slower flow.