What does nitroglycerin primarily decrease in the cardiovascular system?

Nitroglycerin primarily decreases pulmonary vascular resistance by causing vasodilation. It is often used in the management of conditions like angina pectoris and heart failure because it dilates blood vessels, particularly veins, which leads to a decrease in venous return (preload) to the heart. This reduction in preload can lower the pressure in the pulmonary circulation, thus alleviating symptoms associated with heart failure and pulmonary congestion.

While it may have effects on heart rate and other cardiovascular parameters, its primary role is in vasodilation, making pulmonary vascular resistance the most relevant option here. The other choices may not be closely related to the primary pharmacological action of nitroglycerin. Therefore, the significant decrease in pulmonary vascular resistance is what makes this answer correct within the context of nitroglycerin's pharmacological action.