Nicotine in the Body

As with most addictive substances, humans have devised a number of ways of delivering nicotine to their bodies. Nicotine readily diffuses through:

• Skin
• Lungs
• Mucous membranes (such as the lining of your nose or your gums)

Nicotine moves right into the small blood vessels that line the tissues listed above. From there, nicotine travels through your bloodstream to the brain, and then is delivered to the rest of your body.

The most common (and the most expedient way) to get nicotine and other drugs into your bloodstream is through inhalation -- by smoking it. Your lungs are lined by millions of alveoli, the tiny air sacs where gas exchange occurs. These alveoli provide an enormous surface area -- 90 times greater than that of your skin -- and thus provide ample access for nicotine and other compounds. Once in your bloodstream, nicotine flows almost immediately to your brain. Although nicotine takes a lot of different actions throughout your body, what it does in the brain is responsible for both the good feelings you get from smoking, as well as the irritability you feel if you try to quit (see the section on addiction and withdrawal for details). Within 10 to 15 seconds of inhaling, most smokers are in the throes of nicotine's effects.

Nicotine doesn't stick around your body for too long. It has a half-life of about 60 minutes, meaning that six hours after a cigarette, only about 0.031 mg of the 1 mg of nicotine you inhaled remains in your body.

How does your body get rid of nicotine? Here's the process:

• About 80 percent of nicotine is broken down to cotinine by enzymes in your liver.

• Nicotine is also metabolized in your lungs to cotinine and nicotine oxide.

• Cotinine and other metabolites are excreted in your urine. Cotinine has a 24-hour half-life, so you can test whether or not someone has been smoking in the past day or two by screening his or her urine for cotinine.

• The remaining nicotine is filtered from the blood by your kidneys and excreted in the urine.

Different people metabolize nicotine at different rates. Some people even have a genetic defect in the enzymes in their liver that break down nicotine, whereby the mutant enzyme is much less effective at metabolizing nicotine than the normal variant. If a person has this gene, their blood and brain nicotine levels stay higher for longer after smoking a cigarette. Normally, people keep smoking cigarettes throughout the day to maintain a steady level of nicotine in their bodies. Smokers with this gene usually end up smoking many fewer cigarettes, because they don't constantly need more nicotine.