Which one of the following is synthesised in human body that dilates blood vessels and increases blood flow ?
- (a) Nitric oxide ✓ UPSC's answer
- (b) Nitrous oxide
- (c) Nitrogen dioxide
- (d) Nitrogen pentoxide
Why the answer is (a)
• Nitric oxide (NO) is produced in the body from the amino acid arginine by nitric oxide synthase in the lining of blood vessels.
• It relaxes the smooth muscle of vessel walls (vasodilation), lowering blood pressure and increasing blood flow; the 1998 Nobel Prize in Medicine recognised its role as a signalling molecule, and drugs like nitroglycerine and sildenafil act through this pathway — option (a).
• Nitrous oxide (N₂O) is 'laughing gas', an anaesthetic and greenhouse gas, not made in the body for this purpose.
• Nitrogen dioxide (NO₂) is a toxic air pollutant, and nitrogen pentoxide (N₂O₅) is an unstable oxide used in chemistry — neither is a physiological vasodilator.
• Hence option (a).
Why the other options are wrong
- (b) Nitrous oxide
- Nitrous oxide is an anaesthetic gas, not a body-made vasodilator.
- (c) Nitrogen dioxide
- Nitrogen dioxide is a pollutant, not a signalling molecule.
- (d) Nitrogen pentoxide
- Nitrogen pentoxide is a laboratory reagent, not a physiological compound.
Asked in the GS Paper I of the UPSC Civil Services Preliminary Examination 2024, held on 16 June 2024. Question and answer key: Union Public Service Commission. Explanation: UPSC Answer Check.