Nicotine is a substance largely responsible for both the pleasurable and the addictive effects of tobacco use. It and other ingredients in tobacco and cigarette smoke are responsible for a wide variety of health problems. Nicotine is found in small amounts in the leaves, roots, and seeds of the tobacco plant. In nature, it acts as a poison that protects the plant from being eaten by certain insects. Nicotine can also be made synthetically. Chemists classify nicotine as an alkaloid (see Alkaloid ).
A smoker takes in about 0.05 milligram to 2 milligrams of nicotine per cigarette. Nicotine is quickly absorbed into the bloodstream from the lungs and rapidly distributed throughout the body. Blood saturated with nicotine reaches the brain within seconds and accounts for the feeling of euphoria or “rush” felt from smoking. In pregnant women, nicotine can harm the developing fetus. The chemical easily crosses the placenta, the organ that supplies the fetus with food and oxygen. Smokers have nicotine in all their bodily fluids, including breast milk.
Nicotine stimulates certain receptors in the brain, increasing levels of the chemical dopamine. Dopamine plays a key role in motivation, pleasure, and addiction. Nicotine improves memory and relaxes the muscles of the body. It also increases heart rate, blood pressure, and the force of heart muscle contractions, putting added stress on the heart.
The liver breaks down most nicotine in the body, and the kidneys excrete the remainder through urine. Frequent smokers suffer withdrawal symptoms when they have low levels of nicotine in their blood. They may feel irritable and anxious and crave cigarettes. Many smokers smoke throughout the day to maintain the accustomed nicotine levels in their blood.
