Nicotine is the chemical in tobacco smoke that speeds up the activities of the nervous system, heart, and other organs. It acts as a stimulant and increases the release of certain neurotransmitters, such as dopamine, which can lead to increased heart rate, elevated blood pressure, and heightened alertness. Nicotine addiction is a major reason why smoking is so difficult to quit, as the body becomes dependent on the stimulating effects of nicotine. Tar, carbon monoxide, and caffeine are also present in tobacco smoke, but they do not have the same direct effect on the nervous system and organs as nicotine does.