Nicotine is an alkaloid derived from the tobacco plant "Nicotiana tabacum". It can be absorbed into the blood stream via mucosal membranes in the mouth, nose and airways.
Within the body, it is absorbed into various organs until it is eventually detoxified by the liver. The liver contains a microsomal enzyme system called the cytochrome P450 system which detoxifies and metabolizes most drugs, medicines and chemical pollutants. Such toxins are then eliminated by the kidneys, preventing the accumulation of harmful substances in the body.
When smoking tobacco, nicotine enters the blood stream. It may also be absorbed into the blood stream from the mouth when chewed, or through the skin when a nicotine patch is worn. Once in the bloodstream, nicotine travels through the body to the brain where it binds to and activates cholinergic receptors.
These receptors are usually bound by a transmitter called acetylcholine which helps maintain heart rate, alertness and movement. Nicotine binding to the receptors can also stimulate muscle movement and may be responsible for the muscle twitching sometimes associated with smoking.
Nicotine intake causes an increase in the number of cholinergic receptors, which is thought to be responsible for the tolerance that develops with smoking. Once tolerance has increased, a smoker needs to supply adequate nicotine to the brain and the smoking habit becomes addictive.
Nicotine also triggers an increase in levels of free radicals or reactive oxygen species (ROS) which causes oxidative stress, damages cell membranes and causes tissue damage. Examples of ROS include nitric oxide (NO), and peroxide (H2O2). Nicotine also activates the nuclear transcription factor kappaB (NF-kappaB), which is involved in various biological processes including inflammation and cell death.
Eventually, smoking can cause respiratory problems, heart problems, lung cancer, emphysema, and vascular disease. Smokers are also at an increased risk of developing breast cancer, Kaposi's sarcoma, asthma, and high blood pressure. Studies have shown that it may be the immuno-modulating effects of nicotine that raises the risk of these disorders.
Further Reading
Are synthetic nicotine brands following FDA marketing guidelines?