How do nitrates function to relieve angina?

Nitrates are primarily used to relieve angina through their ability to dilate blood vessels. By relaxing the smooth muscle in the walls of blood vessels, nitrates reduce vascular resistance and increase blood flow, particularly to the heart muscle. This enhanced blood flow helps to alleviate the chest pain associated with angina, which often occurs due to insufficient oxygen supply to the heart under stress or exertion.

Moreover, the dilation of veins (venous dilation) leads to a reduction in venous return to the heart, decreasing the workload on the heart and lowering myocardial oxygen demand. This combination of increased blood supply and decreased workload effectively addresses the imbalance in oxygen supply and demand that characterizes angina.

Other options are focused on different physiological mechanisms that do not align with the primary action of nitrates. For instance, increasing heart contractility, stimulating the heart rate, or reducing blood volume may affect cardiac function but do not specifically relate to the mechanism through which nitrates provide relief from angina. The key effect of nitrates is their vasodilatory action, making blood vessel dilation the correct choice for how they function in the context of angina relief.