UPSC Prelims 2024 Q33: Nitrogen Oxides and Blood Flow — Explained
Which one of the following is synthesised in human body that dilates blood vessels and increases blood flow?
- Nitric oxide
- Nitrous oxide
- Nitrogen dioxide
- Nitrogen pentoxide
Answer: (a) Nitric oxide
Statement-wise explanation
- a: correct — Nitric oxide (NO) is made by the endothelium, the inner lining of blood vessels. Enzymes called nitric oxide synthases make it from the amino acid L-arginine. NO passes into the smooth muscle of the vessel wall, the muscle relaxes and the vessel widens. That lets more blood flow through. The 1998 Nobel Prize in Physiology or Medicine went to Furchgott, Ignarro and Murad for showing that NO acts as a signalling molecule in the cardiovascular system.
- b: incorrect — Nitrous oxide (N2O) is 'laughing gas'. It is used as an anaesthetic and is a long-lived greenhouse gas that also depletes ozone. The body does not make it as a signal to relax blood vessels. Do not confuse it with nitric oxide: the names differ by one word and the formulas by one nitrogen atom.
- c: incorrect — Nitrogen dioxide (NO2) is a reddish-brown, toxic gas that comes mainly from vehicle and power-plant combustion. It is an air pollutant listed under India's National Ambient Air Quality Standards, and it irritates the airways. It is not a signalling molecule that the body makes.
- d: incorrect — Nitrogen pentoxide, properly called dinitrogen pentoxide (N2O5), is an unstable white solid. It is the acid anhydride of nitric acid: it reacts with water to form HNO3. In the air it forms mainly at night and is one of the routes that turns NOx into nitric acid. It has no role in human physiology.
How to eliminate
Basic knowledge rules out two options. Nitrogen dioxide is a well-known toxic pollutant from vehicle exhaust, and nitrogen pentoxide is an unstable laboratory oxide that forms nitric acid. Neither is likely to be a molecule the body makes on purpose. That leaves nitric oxide and nitrous oxide, and this is the trap the paper sets with near-identical names. Nitrous oxide is what you know from anaesthesia ('laughing gas') and from climate lists of greenhouse gases. Nitric oxide (NO) is the one tied to blood vessels, through angina drugs like nitroglycerin that release NO, and through the 1998 Nobel Prize. To get the final pick you have to know this fact: nitric oxide = NO = the body's own vasodilator. Nitrous oxide = N2O = laughing gas and a greenhouse gas.
Know more
Nitric oxide: a gas that works as a body signal
Nitric oxide (NO) is a small, short-lived gas. Cells in the endothelium (the inner lining of blood vessels) make it from the amino acid L-arginine, using the enzyme nitric oxide synthase (NOS). NO spreads into the smooth muscle around the vessel. There it switches on the enzyme guanylyl cyclase, which raises the level of cGMP. The muscle relaxes, the vessel widens (vasodilation), blood flow rises and blood pressure falls.
Discovery: Robert Furchgott found an 'endothelium-derived relaxing factor' (about 1980). Louis Ignarro identified it as NO (1986). Ferid Murad showed that nitroglycerin works by releasing NO (1977). The three shared the 1998 Nobel Prize in Physiology or Medicine.
Uses in medicine:
- Nitroglycerin for angina: it releases NO and widens the coronary arteries.
- Sildenafil: it blocks the enzyme that breaks down cGMP, so the NO effect lasts longer.
- Inhaled NO: used in intensive care for pulmonary hypertension in newborns. It is now being tested against drug-resistant lung infection (immune cells such as macrophages use NO to kill microbes).
The nitrogen oxides compared
| Compound | Formula | Where it matters |
|---|---|---|
| Nitric oxide | NO | Vasodilator made in the body; also part of NOx from combustion |
| Nitrous oxide | N2O | Laughing gas (anaesthetic); greenhouse gas (IPCC AR6 puts its 100-year warming potential at about 273 times CO2) that also depletes ozone |
| Nitrogen dioxide | NO2 | Reddish-brown, toxic; vehicle pollutant listed under India's air-quality standards (NAAQS); helps cause acid rain and smog |
| Dinitrogen pentoxide | N2O5 | Anhydride of nitric acid; unstable; forms in the air at night |
UPSC often tests chemicals with confusable names, like NO vs N2O or ozone 'good' vs 'bad'. Learn each formula alongside its one defining role.
NCERT Concepts to Revise
- Nitrous oxide: One option is nitrous oxide; knowing it as a long-lived greenhouse gas helps you set it apart from its similarly named cousin.
- Acid rain: Two of the options are oxides that end up as nitric acid in the air, the chemistry behind acid rain.