Which one of the following is synthesised in human body that dilates blood vessels and increases blood flow?

The correct answer is Nitric oxide (NO).

Nitric oxide is a crucial signaling molecule in the human body, playing a vital role in various physiological processes. One of its most important functions is its role as a vasodilator. It relaxes the smooth muscles in the walls of blood vessels, causing them to widen (dilate). This vasodilation leads to increased blood flow, which is essential for delivering oxygen and nutrients to tissues and organs, and for regulating blood pressure. Nitric oxide is synthesized in the body by the enzyme nitric oxide synthase (NOS).

• Nitrous oxide (N₂O): Known as 'laughing gas', used as an anesthetic, but not primarily for vasodilation in normal physiological processes.
• Nitrogen dioxide (NO₂): A toxic gas, primarily an air pollutant.
• Nitrogen pentoxide (N₂O₅): A chemical compound, not synthesized in the human body for physiological functions.

Therefore, Nitric oxide is the correct answer.