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Nitroglycerin primarily acts as a vasodilator, meaning it relaxes smooth muscle tissue in blood vessels. When nitroglycerin is administered, it is converted to nitric oxide (NO) in the body, which initiates a cascade of events leading to the relaxation and dilation of the smooth muscles lining the blood vessels. This action helps to reduce the workload on the heart by decreasing the amount of oxygen required by the heart muscle, making it especially effective in the treatment of angina pectoris.

The relaxation of smooth muscle tissue leads to the dilation of both arteries and veins. This results in a decrease in venous return to the heart and lowered blood pressure, which alleviates chest pain. The primary therapeutic effect of nitroglycerin is its ability to improve blood flow and reduce myocardial oxygen demand through this mechanism of smooth muscle relaxation.

In contrast, the other options involve actions that are not characteristic of nitroglycerin. For example, increasing heart contractility is more associated with medications like inotropes, while increasing blood viscosity and decreasing respiratory rate do not relate to the pharmacological effects of nitroglycerin. Thus, the ability of nitroglycerin to relax smooth muscle tissue is what makes it an effective treatment