Final answer:
Nicotine is absorbed through the mucous membranes and is extensively metabolized in the liver. Tobacco smoke can impair lung immune defenses, increasing the risk of pneumonia. Nicotine's effect on the brain's reward system contributes to its addictive nature.
Step-by-step explanation:
Nicotine Absorption and Mucous Membranes:
Nicotine is the primary addictive substance in tobacco products. When a person uses tobacco via smoking or other means, nicotine is absorbed into the bloodstream through the mucous membranes, which line various body tracts including the respiratory, digestive, and reproductive systems. Mucous membranes provide a protective barrier that secretes mucus to trap pathogens and cilia to move these substances toward body openings for expulsion.
Smokers are at higher risk of developing respiratory infections like pneumonia because tobacco smoke components hinder the lungs' immune defenses by disrupting ciliated cells and inhibiting natural antimicrobial functions. This results in a dysfunction of the mucociliary escalator effect, allowing pathogens to remain in the lungs and cause infection.
Upon absorption, nicotine is extensively metabolized in the liver. The most common metabolite is cotinine, among various others that are produced. Nicotine acts as an agonist of the nicotinic acetylcholine receptor, influencing the midbrain's reward system and affecting the expression of monoamine oxidase in the brain. It is this process that contributes to the addictive properties of nicotine.