The velocity of blood flow is ________.

• A. Slowest in the capillaries because the total cross-sectional area is the greatest
• B. Fastest in the capillaries due to low resistance
• C. Constant throughout the circulatory system
• D. Slowest in the arteries due to high resistance

Answer: (A)

Velocity in the circulatory system is determined by the total cross‑sectional area of the vessel network in a given segment. Even though the heart pumps the same volume of blood per unit time, the speed of that flow changes as it moves through different parts of the circuit because the combined cross-sectional area changes. Capillaries have by far the largest total cross-sectional area because there are many tiny vessels in parallel, which spreads the blood flow thin and slows it down. This slower velocity gives more time for exchange of gases, nutrients, and wastes with the surrounding tissues. In contrast, arteries have a much smaller total cross-sectional area, so the blood moves faster there. Hence, the velocity is slowest in the capillaries due to their vast total cross-sectional area.