Vasodilation -> _____ SVR

• A. Low
• B. High
• C. No change
• D. Variable

Answer: (A)

Vasodilation lowers systemic vascular resistance by relaxing the smooth muscle in systemic arterioles and arteries, widening the vessel radius. Resistance to flow in small vessels drops dramatically when radius increases (Poiseuille’s principle: resistance ∝ 1/r^4). Since systemic vascular resistance is the pressure the heart has to overcome to push blood through the circulation (essentially MAP minus right atrial pressure divided by cardiac output), dilating vessels reduces that afterload. If cardiac output doesn’t change, MAP tends to fall; if CO rises to compensate, MAP may stay similar. The key point is that dilation reduces resistance, so SVR becomes low.