The resistance of the arterioles accounts for _____ of the total resistance in the systemic system.

• A. 1/3
• B. 1/2
• C. 1
• D. 2/3

Answer: (B)

Arterioles are the main control points for systemic vascular resistance because their small radius and muscular walls make them highly effective at restricting flow. The resistance in a series of vessels adds up, and although many segments contribute, the arterioles provide the greatest single contribution to overall resistance. Their numerous branches and ability to constrict or dilate in response to autonomic signals let them set the baseline and the adjustments of systemic vascular resistance.

Because capillaries, arteries, and veins each contribute differently, the arterioles together account for roughly half of the total systemic resistance. Their small diameter gives them a large resistance per segment (and many such segments in series throughout the vascular tree), so they dominate the total resistance, even though other portions contribute as well. Some texts quote a range around 60%–70%, but in this question the best-fit answer is about half, reflecting their major role in determining systemic resistance.