Here’s what can happen in a “power hour” — that first hour of a 21st birthday celebration in which young adults try to consume 21 drinks between midnight and 1 a.m.

Ethanol, the chemical in alcoholic drinks, depresses parts of the brain, one after the other. First go those pesky emotional things like inhibitions that make you cautious and tame. Then it hits your speech centers, making you slur, and the neurons that tell your muscles how to walk, making you stumble.

Finally, when you’ve drunk enough fast enough, it shuts down the deep part of the brain that controls your lungs and your heart.

“If they didn’t vomit, it would probably kill them,” said James Rothenberger, a professor of epidemiology at the University of Minnesota. “That’s a strange way to celebrate their 21st birthday.”

To members of Alcoholics Anonymous, who first coined the term, a drinking binge is a big chunk of time — like a week or weekend — lost to the state of drunkenness. But researchers define a binge as when someone has five or more drinks at a sitting. It could be an hour, or it could be eight hours. That troubles Rothenberger a bit. Someone who has five drinks in, say, eight hours is not likely to be dangerously drunk, he said.

By whatever definition, high-risk drinking is a problem. The university recently reported that in 2005 about 45 percent of students age 18 to 24 reported at least one binge-drinking episode in the previous two weeks. That’s up from 40 percent in 2004.

Rothenberger teaches an online course at the university called Freshman Survival Skills, which, not surprisingly, has several sections devoted to alcohol and binge drinking.

He said a lot of young adults between 18 and 24 — the peak age for excessive drinking — don’t believe alcohol can kill. They’ve seen too many people pass out from drinking and then wake up with just a bad hangover. Those news stories about kids who died from alcohol poisoning? That happens to other people.

So his online course, which is taken by about 10 percent of freshmen, provides a detailed explanation of what alcohol does to the body.

Alcohol works fast. A little is absorbed almost immediately through the small capillaries in your mouth. Twenty percent of the alcohol you drink is absorbed in the stomach. At the same time, ethanol increases the amount of hydrochloric acid released into the stomach, which may make you feel pretty lousy. Food — especially milk, protein and fatty foods — reduces the rate of absorption by about half.

When you drink too much too fast, you throw up. That’s because an excessive amount of alcohol in your stomach makes the valve that connects to the small intestine get stuck. The irritation and distress gets worse and worse until, uh-oh, it’s time to kneel before the porcelain god.

The rest of the alcohol gets into your bloodstream through the small intestine. If you drink wine coolers or other fizzy drinks with carbonation, that absorption happens faster and you get drunk faster.

The liver is the body’s garbage collector, and it’s put to the test when you drink. It removes 95 percent of the alcohol from your bloodstream, breaking it down into carbon dioxide and water at a rate of about one drink per hour — unless you’re female or Asian. Women metabolize alcohol more slowly, and Asians often carry a gene that makes it harder to metabolize alcohol.

But people have different tolerance levels for alcohol, Rothenberger said. The number of drinks needed to reach the legal limit for driving can vary among people of the same size and gender.

That feeling of being drunk is a result of ethanol’s depressive effects on the brain. It slows down the central nervous system that includes areas of the brain that control heart rate and breathing, as well as motor control.

If you had between one and two drinks in an hour you would start to feel relaxed and loosen up. This is when inhibitions decide to go on vacation.

But after drinking two to four drinks in an hour, deeper levels of the brain are affected. Speech becomes slurred. Walking becomes difficult. Here is where you start to lose your judgment. This is the danger zone for driving. And whether you know it or not, you could start getting obnoxious. At five drinks per hour your speech slurs, and dancing is out of the question. At 10 drinks in one hour — if you’re still talking — emotions become erratic. You can go from happy to an emotional wreck in moments.

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