Which vessels are primarily responsible for regulating systemic vascular resistance?

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

Answer: (A)

The main concept here is that systemic vascular resistance is controlled mainly by the diameter of the small arteries and arterioles—the resistance vessels. Arterioles have a thick smooth muscle layer that can constrict or dilate in response to autonomic signals, local metabolites, and hormones. When they constrict, their radius drops, and resistance rises dramatically (because resistance scales roughly with 1/r^4). When they dilate, resistance falls. This powerful radius control lets arterioles set the overall resistance the heart must push against, shaping systemic blood pressure for a given cardiac output.

Capillaries are the sites of exchange; they are numerous and have a high total cross-sectional area, which influences flow characteristics but they are not the main regulators of systemic resistance. Veins and venules are capacitance vessels that mainly affect venous return to the heart and preload, not the steady-state resistance of the systemic circulation.

So the best choice is the arterioles, the primary regulators of systemic vascular resistance.