Which factor contributes to the overall resistance to blood flow in the systemic circulation?

• A. Peripheral resistance
• B. Vasodilation
• C. Hypotension
• D. Vasoconstriction

Answer: (A)

The main concept here is peripheral (systemic vascular) resistance—the total friction the heart must overcome as blood moves through the systemic vessels. Most of this resistance comes from the small arteries and arterioles, whose smooth muscle tone can tightly control their radius. Because resistance increases very steeply as radius decreases (a small drop in radius causes a large rise in resistance), even modest vasoconstriction can markedly raise overall resistance, while vasodilation lowers it. Hypotension describes low blood pressure and isn’t itself a factor that adds resistance. So the factor that contributes to the overall resistance to blood flow in the systemic circulation is peripheral resistance—the cumulative resistance of the systemic vessels.