Which factor most increases peripheral resistance (PR)?

• A. Vasoconstriction mediated by sympathetic activity and hormones such as catecholamines and angiotensin II.
• B. Vasodilation from parasympathetic activity.
• C. Decreased blood viscosity.
• D. Increased heart rate independently.

Answer: (A)

The main idea here is that peripheral resistance is driven mainly by the diameter of small arteries and arterioles. Because resistance rises dramatically when vessels narrow, small changes in radius have a big effect. Vasoconstriction caused by sympathetic activity and hormones like catecholamines and angiotensin II decreases the radius of arterioles. Since resistance is roughly proportional to 1 divided by the radius to the fourth power, even modest constriction leads to a large increase in peripheral resistance. That’s why this factor most increases PR.

Vasodilation from parasympathetic activity would widen vessels and lower PR, not increase it. Decreased blood viscosity would reduce PR, but the effect is smaller than the impact of changing vessel radius. An increased heart rate by itself doesn’t directly raise peripheral resistance, though it can influence flow and pressure indirectly through cardiac output.