Strike a match. Light that cigarette and take a drag.

Inside the glowing tip, the temperature soars to 1,800 degrees Fahrenheit, setting off reactions that produce from the tobacco and paper an estimated 4,000 chemicals. Every class of carcinogen known to man spews forth, along with poisons, mutagens and mind-altering drugs.

Within seconds, these chemicals invade the blood and circulate through the body. They cut down the flow of oxygen to the heart even while they force it to pump harder. They sear the lungs and settle in the kidneys and bladder, causing cellular changes that can eventually become cancer. They also head straight for the brain, immediately producing calm, alertness and increased concentration.

"When you light up, it's like exposing your whole body to a miniature chemical factory," says Dr. John Holbrook of the University of Utah Medical Center. The best-known product of that factory, of course, is nicotine, which causes or contributes to most of smoking's physical and mental impact.

But the burning tobacco produces a host of less familiar things, including a large number of substances called oxidants, which scientists believe are responsible for most of the effects of aging on the body. One theory gaining credence is that much of the damage caused by cigarette smoke amounts to premature aging.

Oxidants are highly unstable because they have electrons missing. So they take electrons from most of life's fundamental compounds, breaking the chemical bonds that hold together the body's molecular machinery. They cause the same kind of damage that radiation does, although through a slightly different mechanism, says Bruce Ames, chairman of the biochemistry department at the University of California at Berkeley.

In the lungs, the arteries, or anywhere else for that matter, the action of oxidants adds up to wear and tear. And since these oxidants can damage DNA, the chemical repository of genetic information found within nearly every cell, they can be thought of as carcinogens, or cancer-causing chemicals. The oxidants themselves are carcinogenic; they cause cancer by scrambling the cell's genetic instructions.

Because oxidants are all around us, even in fruits and vegetables, the body has evolved both detoxifying agents and DNA repair mechanisms. But nothing in nature is as bad as the level or the diversity of oxidants in cigarette smoke. A day's worth of smoking is roughly equivalent to a year's Los Angeles smog, says Ames.

"We have come to view cigarette smoke as a super-toxic smog," says William A. Pryor, director of the program in free radical biology at Louisiana State University. (Free radicals are particularly reactive oxidants.) The level of oxides of nitrogen in cigarette smoke, he says, "would kill a small mammal in a few hours" of continuous exposure.

Dietrich Hoffmann of the American Health Foundation divides the carcinogens into so-called "contact carcinogens," those that can cause cancer wherever they touch body's tissues, and "organ specific carcinogens" which, he says, become activated by an organ's own unique metabolism.

Betanaphthylamine, for example, a known bladder carcinogen found in cigarette smoke, may be responsible for increased cancers of that organ in smokers, according to the surgeon general.

Smoke not only overwhelms the body with oxidants, it also damages the cells' ability to repair DNA, recent experiments by Richard Setlow at Brookhaven National Laboratory suggest.

Setlow deliberately damaged DNA in cells taken from lungs of both smokers and nonsmokers. The cells from nonsmokers repaired their DNA, but the cells from smokers did a much less thorough job of repair.

Tar is another product of the chemical processes in a burning cigarette. The body's reaction to tar in the lungs -- along with the direct effects of oxides of nitrogen -- apparently cause emphysema, smoking's most painful affliction.

"You feel as though you are close to suffocation. Eventually you get to the point of talking less because of the need to breathe," says Edward Brown, an emphysema sufferer who is a former board member of the D.C. Lung Association.

Emphysema may be caused by the body's own defenses, which are thought to destroy the lung while trying to clear it of the irritating tars.

A healthy lung looks like a sponge. With each breath, millions of microscopic balloon-like sacs called alveoli fill with air; the oxygen enters the blood through capillaries in the alveolar membranes. Emphysema results when these alveoli are destroyed, leaving the lung pocked with craters that in extreme cases measure several inches.

The presence of the tars in the alveoli mobilizes specialized defense cells that carry an enzyme that could be called the body's own Clorox, says State University of New York pathologist Aaron Janoff.

This enzyme, called elastase, is just about the only substance that can digest elastin, the extremely tough protein woven through the walls of the alveoli to give them, and the lungs themselves, their elasticity.

Cigarette smoke not only mobilizes elastase in the lungs, but it also shuts down the action of another body substance, alpha-1-anti-tripsin, a chemical that normally works to neutralize elastase. In the laboratory, bubbling fresh cigarette smoke through a buffer solution containing alpha-1-anti-tripsin destroys that protective molecule "in a matter of seconds," says Pryor.

That this happens in the body as in the test tube has yet to be proved, Janoff cautions. However, scientists have noted the similarity of emphysema to a rare genetic disorder in which the lungs degenerate. Its sufferers lack alpha-1-anti-tripsin.

Chronic bronchitis may add to the breathing difficulties of emphysema sufferers. Smoke causes the mucous glands of the lung to work overtime. At the same time, hydrogen cyanide in the smoke kills the cilia, tiny cells of the lung's surface whose function is to clear the lung.

A combination of smoke-borne chemicals causes coronary heart disease. Nicotine contributes in a roundabout way, mimicking the chemicals that nerve cells use to communicate with one another. Smoke causes false signals to go through the nervous system, telling the heart and lungs to work harder. At the same time, it tells the arteries to contract, constricting blood supply. This combination gives smokers a burst of high blood pressure.

While nicotine tampers with the nervous system, another chemical in the smoke, carbon monoxide, poisons the blood. Think of the circulatory system as a subway, each blood hemoglo-bin molecule a tiny subway car with four seats, or binding sites for oxygen. A delicately balanced, weak attraction between oxygen and the binding sites pulls oxygen into the blood from the oxygen-rich lungs, but releases it to oxygen-poor tissues as the blood travels through the body.

Carbon monoxide steals the oxygen-binding sites and hangs onto them with more than 200 times the strength of the oxygen-hemoglobin bond. "Heavy smokers have up to 12-14 percent of their hemoglobin tied up with carbon monoxide," says Dietrich Hoffmann of the American Health Foundation. The body responds by increasing the number of red blood cells, which thickens the blood and puts further stress on the heart.

Carbon monoxide also contributes to hardening of the arteries in smokers. The inner lining of normal arteries is a "smooth, shiny, glistening surface," says Utah's Holbrook. In contrast, the severely hardened artery becomes "a series of hills and craters like the surface of the moon." The plaque that accumulates inside the blood vessel walls consists of cholesterol and other fats, calcium, smooth muscle cells and fibrous tissue.

Smoking may contribute to the build-up of plaque by creating "leakage" through the cells that line the blood vessel into the blood vessel wall itself, says Holbrook. Plaque then can build up between the lining cells and the blood vessel wall. This could account for the fact that smokers with high cholesterol stand a much greater chance of dying from coronary heart disease than people who have high cholesterol but don't smoke.

Animal studies have shown that carbon monoxide can cause this leakage. One speculation as to how it might occur involves the fact that the life of every cell is one of constantly bailing to maintain the proper concentrations of various chemicals. The bailing takes energy, and carbon monoxide, by poisoning the blood, is robbing the lining cells of energy.

Hydrogen cyanide, the gas chamber gas, is also produced when a cigarette burns. Holbrook speculates that it may contribute to cells' energy problems, as it is known to poison cellular metabolism. Oxidants are thought to contribute to the leakage by damaging the cells' bailing mechanisms. Additionally, nicotine is known to kill the cells that line the blood vessels, says Jack Strong, chairman of pathology at Louisiana State University.

The narrowing of the arteries sets the stage for thrombosis, the complete or partial blocking of the vessel by blood clots, says Holbrook.

Smoking also makes the blood more prone to clotting. Clotting occurs when a protein called fibrinogen forms net-like structures, trapping small, circular cell-like platelets. Smoking increases the blood's fibrinogen, and makes the platelets more sticky, says Holbrook. Besides promoting clotting, this thickens the blood, putting extra stress on the heart. Along with all the other things it does, the nicotine released by those burning cigarettes helps smokers cope with the stresses of modern life.

"Nicotine has all of the key addictive properties of cocaine, morphine and alcohol, which are our reference standards," says Dr. Jack Henningsfield of the National Institute on Drug Abuse Addiction Research Center.

"If you've never smoked, it's hard to imagine the pleasure of it. It's like trying to describe the taste of wine to a nondrinker," says one smoker who tries to quit periodically.

The low doses of nicotine from shallow drags stimulate secretions from the adrenal gland, increasing alertness, while the higher doses from deep drags also releases beta-endorphins, "the body's natural morphine," which produces calm, says Dr. Ovide Pomerleau of the University of Connecticut Medical School.

The kicker is that it acts so quickly, he adds. "Twenty-five percent of the nicotine in inhaled smoke goes to the brain within seven seconds. It's well known from psychological research that the immediacy of delivery is very powerful in controlling behavior."

Tests conducted in Pomerleau's laboratory measured smokers' performance at mental arithmetic and showed that smoking helped them concentrate harder. There is also evidence that nicotine improves long-term memory. "Here's a drug that's compatible with the Puritan ethic," says Pomerleau.

Pomerleau has found that "smokers under stress will smoke more," presumably to get the calming influence of the beta-endorphins. Beta-endorphins flow naturally during "fight or flight" response to sudden threats or stress. The beta-endorphins help relieve the stress that caused this behaviorial response in the first place, says Pomerleau. "Unfortunately, modern life does not give us these outlets. In a highly stressful situation, smoking may reduce some of the anxiety and make a person feel more composed. We have demonstrated nicotine's ability to relieve both pain and anxiety."

At the same time, carbon monoxide in the blood may reduce mental performance and coordination, according to the surgeon general's 1983 report on smoking. But that doesn't change the smoker's ability to summon pep or pleasure at will. "Clearly," said Pomerleau, "smokers who are quitting are going to have to recognize that they will need to learn to respond to demands by non-pharmacological methods."