HowStuffWorks "How Nicotine Works"
Effects of Nicotine
Nicotine changes how your brain and your body function. The net results are somewhat of a paradox: Nicotine can both invigorate and relax a smoker, depending on how much and how often they smoke. This biphasic effect is not uncommon. Although the actions of nicotine and ethanol in the body are quite different, you also see dose-dependent effects when you drink alcoholic beverages. Your first drink may loosen your inhibitions and fire you up, but after several drinks, you're usually pretty sedate.
Nicotine and the Body
Nicotine initially causes a rapid release of adrenaline, the "fight-or-flight" hormone. If you've ever jumped in fright at a scary movie or rushed around the office trying to finish a project by your deadline, you may be familiar with adrenaline's effects:
* Rapid heartbeat
* Increased blood pressure
* Rapid, shallow breathing
Adrenaline also tells your body to dump some of its glucose stores into your blood. This makes sense if you remind yourself that the "fight-or-flight" response is meant to help you either defend yourself from a hungry predator or hightail it out of a dangerous situation -- running or brawling both require plenty of energy to fuel your muscles. (For more information, see How Exercise Works.)
Nicotine itself may also block the release of the hormone insulin. Insulin tells your cells to take up excess glucose from your blood. This means that nicotine makes people somewhat hyperglycemic, having more sugar than usual in their blood. Some people think that nicotine also curbs their appetite so that they eat less. This hyperglycemia could be one explanation why: Their bodies and brain may see the excess sugar and down-regulate the hormones and other signals that are perceived as hunger.
Nicotine may also increase your basal metabolic rate slightly. This means that you burn more calories than you usually would when you are just sitting around. (For more information on metabolism, see How Calories Work.) However, losing weight by smoking doesn't give you any of the health benefits that you'd get if you were losing weight by exercising -- it actually does the opposite! Over the long haul, nicotine can increase the level of the "bad" cholesterol, LDL, that damages your arteries. This makes it more likely that you could have a heart attack or a stroke.