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

1. A

   Nitric oxide

   Nitric oxide. Endothelium-derived relaxing factor; dilates vessels.

2. B

   Nitrous oxide

   Nitrous oxide. Anaesthetic (“laughing gas”), not the body’s dilator molecule.

3. C

   Nitrogen dioxide

   Nitrogen dioxide. Pollutant, not a vasodilator hormone.

4. D

   Nitrogen pentoxide

   Nitrogen pentoxide. Not a human signalling molecule.

Summary. Official key is (a) nitric oxide (NO). Vascular endothelium makes NO, which relaxes smooth muscle and increases blood flow. Nitrous oxide (N2O) is an anaesthetic. NO2 and N2O5 are not the endogenous dilator.