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

(a) Nitric oxide
(b) Nitrous oxide
(c) Nitrogen dioxide
(d) Nitrogen pentoxide

Correct Answer: (a) Nitric oxide


  • Nitric oxide (NO) is synthesized in the human body and acts as a potent vasodilator. It is produced by endothelial cells lining the blood vessels and plays a crucial role in regulating vascular tone and blood flow by relaxing the smooth muscle cells in the vessel walls, leading to vasodilation and increased blood flow.
  • Nitrous oxide (N2O), commonly known as laughing gas, is an anesthetic used in medical settings. It does not have a significant role in vasodilation or blood flow regulation in the human body.
  • Nitrogen dioxide (NO2) is a toxic gas that can cause respiratory issues. It is not involved in the physiological process of vasodilation or blood flow regulation.
  • Nitrogen pentoxide (N2O5) is a chemical compound used in industrial processes and is not synthesized in the human body. It does not play any role in vasodilation or blood flow regulation.

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  • Vasodilation is the process of widening blood vessels due to the relaxation of smooth muscle cells in the vessel walls. This process increases blood flow and decreases blood pressure.
  • Mechanism of Action:
    • Nitric oxide (NO) is produced from the amino acid L-arginine by the enzyme endothelial nitric oxide synthase (eNOS).
    • NO diffuses into the smooth muscle cells of blood vessels and activates soluble guanylate cyclase (sGC).
    • This activation increases the production of cyclic guanosine monophosphate (cGMP), which leads to the relaxation of smooth muscle cells and vasodilation.
  • Sources of NO:
    • NO is produced endogenously by endothelial cells.
    • It can also be introduced into the body through certain foods and supplements, such as those high in nitrates (e.g., beetroot).
  • Physiological Roles:
    • Regulates blood pressure: By inducing vasodilation, NO helps maintain proper blood pressure levels.
    • Enhances blood flow: Increased blood flow ensures efficient delivery of oxygen and nutrients to tissues and organs.
    • Supports cardiovascular health: Adequate NO levels are crucial for preventing hypertension and other cardiovascular diseases.
  • Factors Influencing NO Production:
    • Exercise: Physical activity stimulates NO production, improving endothelial function and vascular health.
    • Diet: Consuming nitrate-rich vegetables can boost NO levels in the body.
    • Age and health conditions: Reduced NO production is associated with aging and certain health conditions like diabetes and heart disease.
  • Clinical Applications:
    • Medications: Drugs like nitroglycerin and sildenafil (Viagra) work by enhancing NO pathways to treat conditions like angina and erectile dysfunction.
    • Therapeutic interventions: NO donors and vasodilators are used to manage hypertension and other cardiovascular conditions.

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