For thousands of years, people have smoked or chewed the leaves of the tobacco plant, Nicotiana tabacum. Tobacco was first found and cultivated in the Americas, perhaps as early as 6000 B.C. Following the discovery and colonization of North and South America, the tobacco plant was exported widely, to continental Europe and the rest of the civilized world. Even in its early days, tobacco use was controversial. Some hailed its medicinal properties. For example, tobacco was supposed to be protective against the ravages of the Plague! As early as the 1600s, people speculated that there might be a link between diseases, like cancer, and tobacco use. Since then, modern research methods have provided evidence of this link, and public service announcements that warn of tobacco's health risks and addictive nature are seen regularly on TV.
Nicotine comes from the leaves of the tobacco plant.
What is it about tobacco that makes people so compelled to use it despite all of the admonitions? Smoking or chewing tobacco makes people feel good, even mildly euphoric. While there are thousands of chemicals in the tobacco plant (not to mention those added by cigarette manufacturers), one, nicotine, produces all the good feelings that draw people back for another cigarette or plug of tobacco. In this edition of HowStuffWorks, we'll examine nicotine and how it affects the human body.
What is Nicotine?
Nicotine (C10H14N2) is a naturally occurring liquid alkaloid. An alkaloid is an organic compound made out of carbon, hydrogen, nitrogen and sometimes oxygen. These chemicals have potent effects on the human body. For example, many people regularly enjoy the stimulating effects of another alkaloid, caffeine, as they quaff a cup or two of coffee in the morning.
The structure of nicotine
Nicotine normally makes up about 5 percent of a tobacco plant, by weight. Cigarettes contain 8 to 20 milligrams (mg) of nicotine (depending on the brand), but only approximately 1 mg is actually absorbed by your body when you smoke a cigarette.
Nicotine in the Body
As with most addictive substances, humans have devised a number of ways of delivering nicotine to their bodies. Nicotine readily diffuses through:
- Mucous membranes (such as the lining of your nose or your gums)
Nicotine moves right into the small blood vessels that line the tissues listed above. From there, nicotine travels through your bloodstream to the brain, and then is delivered to the rest of your body.
The most common (and the most expedient way) to get nicotine and other drugs into your bloodstream is through inhalation -- by smoking it. Your lungs are lined by millions of alveoli, the tiny air sacs where gas exchange occurs. These alveoli provide an enormous surface area -- 90 times greater than that of your skin -- and thus provide ample access for nicotine and other compounds. Once in your bloodstream, nicotine flows almost immediately to your brain. Although nicotine takes a lot of different actions throughout your body, what it does in the brain is responsible for both the good feelings you get from smoking, as well as the irritability you feel if you try to quit (see the section on addiction and withdrawal for details). Within 10 to 15 seconds of inhaling, most smokers are in the throes of nicotine's effects.
Nicotine doesn't stick around your body for too long. It has a half-life of about 60 minutes, meaning that six hours after a cigarette, only about 0.031 mg of the 1 mg of nicotine you inhaled remains in your body.
How does your body get rid of nicotine? Here's the process:
- About 80 percent of nicotine is broken down to cotinine by enzymes in your liver.
- Nicotine is also metabolized in your lungs to cotinine and nicotine oxide.
- Cotinine and other metabolites are excreted in your urine. Cotinine has a 24-hour half-life, so you can test whether or not someone has been smoking in the past day or two by screening his or her urine for cotinine.
- The remaining nicotine is filtered from the blood by your kidneys and excreted in the urine.
Different people metabolize nicotine at different rates. Some people even have a genetic defect in the enzymes in their liver that break down nicotine, whereby the mutant enzyme is much less effective at metabolizing nicotine than the normal variant. If a person has this gene, their blood and brain nicotine levels stay higher for longer after smoking a cigarette. Normally, people keep smoking cigarettes throughout the day to maintain a steady level of nicotine in their bodies. Smokers with this gene usually end up smoking many fewer cigarettes, because they don't constantly need more nicotine.
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.
Nicotine and the Brain
Your brain is the key player in nicotine's action. Like a computer, your brain processes, stores and uses information. In a computer, information travels in the form of electricity moving through wires; information transfer is a binary process, with switches being either "on" or "off." In your brain, neurons are the cells that transfer and integrate information. Each neuron has thousands of inputs from other neurons throughout the brain. Each of these signals is included in the calculation of whether or not the neuron will pass the signal it receives on to other neurons in the pathway.
A synapse is the site where two neurons come into contact. The presynaptic neuron releases a neurotransmitter, which binds to receptors on the postsynaptic cell. This allows signals to be transmitted from neuron to neuron in the brain.
While signals are conducted through individual neurons as electric current, communication between neurons is mediated by chemical messengers, called neurotransmitters. Neurotransmitters traverse the physical space between two neurons and bind to special protein receptors on the postsynaptic cell. Once bound, these receptors set in motion physiological changes within the neuron that allow it to send the signal on down the line.
Each neurotransmitter has its own specific family of receptors. Nicotine works by docking to a subset of receptors that bind the neurotransmitter acetylcholine. Acetylcholine is the neurotransmitter that (depending on what region of the brain a neuron is in):
- Delivers signals from your brain to your muscles
- Controls basic functions like your energy level, the beating of your heart and how you breathe
- Acts as a "traffic cop" overseeing the flow of information in your brain
- Plays a role in learning and memory
Acetylcholine is released from one neuron and binds to receptors on adjacent neurons.
Like acetylcholine, nicotine leads to a burst of receptor activity. However, unlike acetylcholine, nicotine is not regulated by your body. While neurons typically release small amounts of acetylcholine in a regulated manner, nicotine activates cholinergic neurons (which mainly use acetylcholine to communicate to other neurons) in many different regions throughout your brain simultaneously. This stimulation leads to:
- Increased release of acetylcholine from the neurons, leading to heightened activity in cholinergic pathways throughout your brain. This cholinergic activity calls your body and brain to action, and this is the wake-up call that many smokers use to re-energize themselves throughout the day. Through these pathways, nicotine improves your reaction time and your ability to pay attention, making you feel like you can work better.
- Stimulation of cholinergic neurons promotes the release of the neurotransmitter dopamine in the reward pathways of your brain - This neural circuitry is supposed to reinforce behaviors that are essential to your survival, like eating when you're hungry. Stimulating neurons in these areas of the brain brings on pleasant, happy feelings that encourage you to do these things again and again. When drugs like cocaine or nicotine activate the reward pathways, it reinforces your desire to use them again because you feel so at peace and happy afterwards.
- Release of glutamate, a neurotransmitter involved in learning and memory - Glutamate enhances the connections between sets of neurons. These stronger connections may be the physical basis of what we know as memory. When you use nicotine, glutamate may create a memory loop of the good feelings you get and further drive the desire to use nicotine.
Nicotine also increases the level of other neurotransmitters and chemicals that modulate how your brain works. For example, your brain makes more endorphins in response to nicotine. Endorphins are small proteins that are often called the body's natural pain killer. It turns out that the chemical structure of endorphins is very similar to that of heavy-duty synthetic painkillers like morphine. Endorphins can lead to feelings of euphoria also. If you're familiar with the runner's high that kicks in during a rigorous race, you've experienced the "endorphin rush." This outpouring of chemicals gives you a mental edge to finish the race while temporarily masking the nagging pains you might otherwise feel.
Nicotine's Dark Side
Some of nicotine's effects have spurred studies on whether it might be useful as a therapy for certain conditions. Some diseases that nicotine might improve include:
- Alzheimer's Disease - The first neurons lost to Alzheimers are cholinergic neurons in a specific region of the brain. Nicotine may improve the function of the neurons that are left and slow the onset of symptoms.
- Tourette's Syndrome - This disease produces tics (uncontrolled movements of the head, hands and other body parts) and violent urges in its sufferers. Nicotine patches that slowly deliver nicotine through the skin can reduce symptoms of people with Tourette's.
But, for the average person, the health problems associated with using nicotine-containing products are far worse than any benefits. These include:
- Heart disease
Many of these are actually caused by other chemicals in cigarette smoke or in smokeless tobacco products. The biggest problem with nicotine is how easily you become dependent on smoking or chewing tobacco.
Addiction and Withdrawal
Billions of dollars have been spent in the United States fighting over whether or not nicotine is addictive. The position of the medical and scientific communities is that nicotine is most definitely addictive. Nicotine meets both the psychological and physiological measures of addiction:
- Psychological - People who are addicted to something will use it compulsively, without regard for its negative effects on their health or their life. A good example would be someone who continues to smoke, even as they use an oxygen tank to breathe because of the damage smoking has done to their lungs.
- Physiological - Neuroscientists call anything that turns on the reward pathway in the brain addictive. Because stimulating this neural circuitry makes you feel so good, you will continue to do it again and again to get those feelings back.
Nicotine's effects are short-lived, lasting only 40 minutes to a couple of hours. This leads people to smoke or chew tobacco periodically throughout the day to dose themselves with nicotine. Add to this the fact that you can become tolerant to nicotine's effects -- you need to use more and more nicotine to reach the same degree of stimulation or relaxation -- and you can see how people would quickly move from smoking one cigarette to a pack a day habit.
What happens when smokers abruptly stop using nicotine? While you're using nicotine-containing products, your body adapts the way it works to compensate for the effects of the nicotine. For example, neurons in your brain might increase or decrease the number of receptors or the amount of different neurotransmitters affected by the presence of nicotine. When you no longer have nicotine in your body, these physiological adaptations for nicotine remain. The net result is that your body can't function the same way in the absence of the drug as it did before, at least in the short term. People trying to quit nicotine experience this as:
- Craving for nicotine
Over a period of about a month, these symptoms and the physiological changes subside. But for many smokers, even a day without nicotine is excruciating. Every year, millions of people try to break the nicotine habit; only 10 percent of them succeed. Most people throw in the towel after less than a week of trying, because the way that nicotine rewires the reward system in the brain makes nicotine's pull irresistible.
Anti-smoking advocates highlight the long-term health effects, like cancer and emphysema, that result from a lifetime of smoking or chewing tobacco -- but these maladies are the result of chemicals in cigarettes other than nicotine. Unfortunately, the fact that nicotine alone is an extremely toxic poison often goes unmentioned. Not many people realize that nicotine is also sold commercially in the form of a pesticide! And every year, many children go to the emergency room after eating cigarettes or cigarette butts. Sixty milligrams of nicotine (about the amount in three or four cigarettes if all of the nicotine were absorbed) will kill an adult, but consuming only one cigarette's worth of nicotine is enough to make a toddler severely ill!
What happens to people after ingesting nicotine? Nicotine poisoning causes vomiting and nausea, headaches, difficulty breathing, stomach pains and seizures. Each of these symptoms can be traced back to excessive stimulation of cholinergic neurons. People poisoned by organophosphate insecticides experience the exact same symptoms. With organophosphates, acetylcholine builds up at synapses and overstimulates the neurons. Because nicotine is so similar to acetylcholine, and binds to cholinergic receptors, nicotine in excess produces the same overstimulation and toxicity. The more nicotine binding to the nicotinic cholinergic receptors, the more acetylcholine is subsequently released and free to activate other subsets of cholinergic receptors.
The treatment for nicotine poisoning has two goals:
- Keep the victim breathing and keep the heart pumping until nicotine is broken down by the body.
- Prevent any more nicotine from reaching the bloodstream.
There are several options to get rid of nicotine that has been ingested:
- Syrup of Ipecac - induces vomiting
- Activated charcoal - binds nicotine in the stomach and keeps it from being taken up into your bloodstream
For more information on nicotine and related topics, check out the links on the next page.
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