How does nitroglycerin cause a headache while reducing chest pain?

Nitroglycerin is a medication primarily used to relieve chest pain, or angina, by acting as a vasodilator, which means it relaxes and widens blood vessels. This dilation reduces the workload on the heart and improves blood flow, alleviating chest pain associated with conditions like coronary artery disease.

However, when nitroglycerin dilates blood vessels, it does not solely target the coronary arteries; it also affects vessels throughout the body, including those in the brain. The resultant decrease in vascular resistance can lead to an increase in cerebral blood volume, which may raise intracranial pressure. This change can trigger headaches as a side effect, given that the increased pressure and volume exert stress on the surrounding neural structures.

The other options do not accurately describe the mechanism involved. Increasing blood pressure is counterintuitive to nitroglycerin's function as a vasodilator. Decreasing blood flow to the brain would not typically cause a headache, and contracting blood vessels in the brain would likely lead to vasospasm rather than the headaches observed with nitroglycerin use. Thus, the correct answer highlights the relationship between vascular dilation and increased cerebral pressure, which explains both the reduction in chest pain and the side effect of headaches.