Just 10 seconds after a cigarette smoker inhales, nicotine is absorbed through the skin and the mucosal linings in the nose, mouth and lungs, and travels through the bloodstream to the brain. It stimulates adrenal glands to produce epinephrine, a hormone and neurotransmitter you also know as adrenaline. This increases heart rate and blood pressure while constricting blood vessels; it also stimulates the production of dopamine, a neurotransmitter that controls the brain's pleasure center. Inhaling nicotine gives the most immediate effects, and that's not a coincidence; it's because your lungs are lined with millions of tiny air sacs called alveoli. Alveoli provide an enormous surface area -- more than 40 times the surface area of your skin -- making it the most efficient way to get nicotine into the bloodstream [source: Richardson]. Nicotine only stays in the human body for a few hours; it has a half-life of about an hour or two, meaning that six hours after smoking a cigarette, only about 0.031 milligram of the 1 milligram of nicotine you inhaled remains [source: Hukkanen, Benowitz].
Nicotine can also be absorbed through your gastrointestinal tract and your skin -- this is how smokeless tobacco products such as chewing tobacco, skin patches and gum deliver their nicotine fix.
Once absorbed by the body, enzymes in the liver break down most of the nicotine -- about 80 percent; here it becomes the metabolite cotinine. Nicotine is also metabolized into cotinine and nicotine oxide by the lungs. Cotinine and other metabolites are excreted in urine, and they're also found in saliva and hair. Cotinine has about a 16-hour half-life, which means if you've smoked in the past day, using this metabolite as a biomarker will give your secret away in a urine screening test [source: CDC].