Which vascular bed primarily regulates systemic vascular resistance and thus afterload?

• A. Large arteries
• B. Capillaries
• C. Arterioles
• D. Veins

Answer: (C)

Afterload is the pressure the left ventricle must overcome to eject blood, and the resistance controlling that load comes mainly from the small arteries and arterioles. Arterioles have thick smooth muscle in relation to their size, so small changes in their radius produce large changes in resistance. This makes arteriolar tone the primary determinant of systemic vascular resistance and thus of afterload. When arterioles constrict, resistance rises and the heart faces a higher afterload; when they dilate, resistance falls and afterload decreases. Neural and local factors modulate this tone, with sympathetic activity typically increasing constriction and various metabolites causing region-specific adjustments. Large arteries mainly serve as conduits and dampen pulsatile flow, not as the main regulators of overall resistance. Capillaries provide exchange surfaces and, collectively, have a huge cross-sectional area that reduces flow velocity, but they don’t set the overall resistive load. Veins hold most of the blood and influence venous return and preload rather than systemic resistance.