The cigarette is a very efficient and highly engineered drug delivery system.
By inhaling tobacco smoke, the average smoker takes in 1 to 2 mg of nicotine per cigarette. When tobacco is smoked, nicotine rapidly reaches peak levels in the bloodstream and enters the brain in a few seconds. A typical smoker will take 10 puffs on a cigarette over a period of 5 minutes that the cigarette is lit. Thus, a person who smokes about 1-1/2 packs (30 cigarettes) daily gets 300 “hits” of nicotine to the brain each day.
In those who typically do not inhale the smoke—such as cigar and pipe smokers and smokeless tobacco users––nicotine is absorbed through the mucosal membranes and reaches peak blood levels and the brain more slowly.
Here is a video of what happens to your body when you quit smoking cigarettes.
http://quitsmoking.about.com/cs/afterquitting/a/after_quitting.htm