Why are arterioles referred to as resistance vessels?

• A. Because they contain the most blood
• B. Because the smooth muscle in their walls can change their diameter
• C. Because they have the thickest walls
• D. Because they lack smooth muscle

Answer: (B)

Arterioles regulate blood flow by changing their diameter through smooth muscle control in their walls. This ability to constrict or dilate alters the radius, and since resistance to flow changes very steeply with radius, even small diameter changes produce large shifts in resistance. That makes arterioles the main site of vascular resistance, shaping blood pressure and directing flow to where it’s needed. They aren’t the main blood reservoir—that role belongs to the veins—and their walls aren’t the thickest overall, and they do have smooth muscle, so the other statements don’t fit.