Which statement best explains why blood flow velocity decreases when moving from arteries to capillaries?

• A. Viscosity increases dramatically
• B. Capillary beds have a very large total cross-sectional area
• C. Arteriolar smooth muscle constricts capillaries
• D. Pulse pressure compresses capillaries

Answer: (B)

Velocity falls as blood reaches the capillary beds because the capillary network presents a vastly larger total cross-sectional area than the arteries. With the overall flow rate (Q) staying roughly constant, the velocity (v) is determined by v = Q/A, so when A becomes enormous in the capillary bed, v drops. This slowdown is exactly what enables efficient exchange of gases, nutrients, and wastes between blood and tissues. Blood viscosity stays fairly constant and isn’t the driver of the drop, and while arterioles can regulate flow upstream, they don’t explain the fundamental velocity reduction once blood enters the capillaries. The slow, extended transit time in capillaries is what makes exchange possible.