Nitroglycerin relaxes which vascular beds at low and high doses, respectively?

• A. Relaxes the venous system at low doses and arterioles at high doses.
• B. Relaxes arterioles at low doses and veins at high doses.
• C. Causes vasoconstriction of both venous and arterial beds.
• D. Has no vascular effect.

Answer: (A)

Nitroglycerin works by releasing nitric oxide in vascular smooth muscle, which increases cGMP and causes relaxation. At low doses, venous capacitance vessels dilate more than arteries, so preload drops as venous return to the heart decreases. As the dose rises, arterial dilation becomes significant, lowering systemic vascular resistance or afterload. This dose-dependent shift—from reducing preload to also reducing afterload—explains why nitroglycerin relaxes the venous system at low doses and arterioles at high doses. This pattern best fits the pharmacology and the hemodynamic effects you’d expect. The other options describe patterns that don’t match this mechanism (no effect, vasoconstriction, or the opposite bed effects).