Which type of vessels would be most numerous due to branching and thus contribute most to peripheral resistance?

• A. Arteries
• B. Arterioles
• C. Capillaries
• D. Venules

Answer: (B)

Arterioles are the main site that sets peripheral resistance because they are the small-diameter, muscular vessels formed by extensive branching from arteries. Resistance rises sharply as radius shrinks (a small change in radius produces a large change in resistance), so each arteriole contributes a substantial amount of resistance. There are many arterioles feeding each tissue, so their collective network creates the bulk of the resistance the heart must overcome to push blood through the systemic circulation. Capillaries, though extremely numerous, are arranged in parallel across tissues and have little smooth muscle to actively constrict, so their overall resistance is relatively low despite their tiny size. Large arteries are fewer and wider, contributing much less to resistance, and venules mainly serve as capacitance vessels with modest resistance.