Which of the following substances is known as a vasodilator?

Nitric oxide is a well-known vasodilator that plays a crucial role in regulating blood vessel tone. It is produced by endothelial cells lining the blood vessels and acts by relaxing the smooth muscle within the vessel walls. When nitric oxide is released, it diffuses into the adjacent smooth muscle cells and stimulates the production of cyclic guanosine monophosphate (cGMP), leading to muscle relaxation and, subsequently, vasodilation. This process increases blood flow and decreases blood pressure, highlighting the importance of nitric oxide in cardiovascular health.

In contrast, the other substances listed have different roles. Endothelin is a potent vasoconstrictor, meaning it constricts blood vessels and increases blood pressure. Angiotensin II is also primarily a vasoconstrictor that plays a significant role in raising blood pressure and stimulating thirst mechanisms through the renin-angiotensin-aldosterone system. Serotonin can have varying effects depending on the type of receptors activated but is generally associated with vasoconstriction in most vascular beds. Therefore, nitric oxide stands out for its function as a vasodilator, making it the correct choice in this context.