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.