What is the primary action of nitroglycerin?

Nitroglycerin primarily works by dilating the coronary vessels, which is crucial for improving blood flow to the heart muscle. This vasodilation helps relieve angina (chest pain) by decreasing the workload on the heart and increasing the supply of oxygen-rich blood to areas of the heart that may be deprived due to narrowed arteries.

By relaxing the smooth muscles in the walls of blood vessels, nitroglycerin can help to lower the risk of myocardial infarction by ensuring that the heart receives adequate oxygen. This mechanism is especially vital during episodes of angina, where oxygen demand exceeds supply.

The other options are less aligned with the primary action of nitroglycerin. For instance, while nitroglycerin can have secondary effects on blood pressure due to its vasodilatory properties, its main therapeutic role is in the dilation of coronary arteries. Additionally, contracting the heart muscles or slowing the heart rate are not direct actions of nitroglycerin; rather, those actions are more characteristic of other medications that specifically target heart muscle contraction or the heart's pacing mechanisms.