Which blood vessels offer the greatest resistance to systemic blood flow?

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Resistance to systemic blood flow is determined primarily by vessel radius, and arterioles are the main site where this regulation happens. Arterioles have small radii and thick, muscular walls that can actively constrict or dilate, allowing rapid and substantial adjustments to systemic vascular resistance and arterial pressure. Because resistance scales roughly with the inverse fourth power of radius, even small changes in arteriolar diameter produce large changes in resistance, making them the dominant source of systemic resistance.

Capillaries, while individually tiny, are arranged in vast networks in parallel, giving a huge total cross-sectional area. This parallel arrangement lowers the overall resistance of the capillary beds even though each capillary has a small radius. Venules and veins have relatively large diameters and thinner walls, so their resistance is low; they function mainly as capacitance vessels to store blood and regulate venous return rather than limit flow.