Final answer:
Nitroglycerin is the drug that mainly acts on the venous system to dilate blood vessels by releasing nitric oxide, causing vasodilation and improving blood flow.
Step-by-step explanation:
The drug that mainly acts on the venous system to dilate blood vessels is nitroglycerin. Nitroglycerin works by releasing nitric oxide (NO), which diffuses across the plasma membrane and induces relaxation in smooth muscle, resulting in vasodilation. This process not only restores blood flow to the heart in the case of heart disease but also affects areas like erectile tissue where blood vessels dilate in response to parasympathetic input, leading to engorgement.