Why are arterioles considered the primary site of peripheral resistance?

• A. They have substantial smooth muscle relative to size, allowing precise control of radius and hence resistance and flow
• B. They are closest to heart
• C. They lack smooth muscle
• D. They have the thinnest walls

Answer: (A)

The key idea is that small arteries called arterioles act as the main gatekeepers of peripheral resistance because they can finely tune their radius. Their walls contain a substantial amount of smooth muscle for their tiny size, so they can constrict or dilate actively in response to nerve signals and local chemical signals. Because resistance to flow in a tube increases dramatically as radius decreases (roughly with the inverse fourth power of radius), even small changes in arteriole radius produce large changes in resistance. That makes arterioles the dominant determinant of total peripheral resistance and thus crucial for controlling blood pressure and directing blood flow to specific tissues. The other statements don’t fit: arterioles aren’t the closest vessels to the heart in terms of resistance control, they do have smooth muscle, and their walls are not the thinnest—that distinction belongs to capillaries.