We've all been there: One drink leads to more - and then to a horror hangover. How does it happen? Science reporter Jamie Morton explores alcohol's journey through the body.
It's late afternoon on a Friday, and you're rushing to the pub to join workmates who are already settled into their second round when you get there.
Given it's been a busy day, after a busy week, you've barely eaten when you dive into that first pint of pale ale.
As you chat with your colleagues, that first gulp of beer is following the standard path food takes on its way to digestion: from your mouth, through your oesophagus, and into your stomach.
Here, about 20 per cent of alcohol is absorbed through the stomach lining into the bloodstream - which means it reaches the bloodstream very quickly.
From the stomach, the remaining alcohol travels to the small intestine.
Because you skipped lunch and are now drinking on an empty stomach, that alcohol has managed to reach the small intestine much faster than it otherwise would have.
Eventually, all alcohol - or ethanol, more precisely - absorbed into the bloodstream travels to the liver, where it's oxidised by an enzyme called alcohol dehydrogenase, or ADH, which is also present in the stomach lining.
The ADH enzyme changes the chemical structure of the ethanol, turning it into something called acetaldehyde, believed to be damaging to the body.
Acetaldehyde is then metabolised to acetic acid, which is less harmful to us, and can be broken down into carbon dioxide and water.
When alcohol is present, the liver will work on metabolising it first. So, fatty acids can accumulate, which is why so many heavy drinkers develop fatty livers.
It's estimated the liver can eliminate about one beer, like the one you're having now, or one glass of wine or shot, per hour, with upper limits.
The heart then pumps the alcohol rich blood to the lungs, from where some it is exhaled - think booze-breath.
Your lungs also send the alcohol-containing blood back to the heart, where it's pumped to all parts of the body - including the brain.
That first drink has hit you. What just happened?
Once the alcohol entered your brain, it slowed down the nerve cells that control your ability to move and think.
By blocking the chemical signals between neurons, it can trigger those immediate symptoms of intoxication, like impulsive behaviour, poor memory and slowed reflexes.
You're beginning to feel all of these - but you've decided to stay on for some more rounds, and the night is young. Another tray of beers arrive at your leaner.
Factors like our gender, size, physical conditions and how much we've had to eat all play a part in how alcohol affects your body.
But even at under 50mg of alcohol per litre of blood (BAC) - the current drink-driving limit for people over 20 years old in New Zealand - many men can experience symptoms like relaxation and talkativeness.
At 50mg to 150mg - that can be roughly between two and eight standard drinks for the average 90kg Kiwi man, or between two and five for a typical 75kg woman - you might feel happier, or unhappier; more friendly, shy or argumentative; find it harder to concentrate, or have fewer sexual inhibitions.
The effect of alcohol on our brain could particularly impair our judgment, and disrupt our movement, said Dr Andrea Braakhuis, a senior lecturer in nutrition and dietetics at the University of Auckland.
"Some people will begin to sweat, and may smell like alcohol. Alcohol also decreases the body's production of antidiuretic hormone," she said.
"The antidiuretic hormone helps your kidneys manage the amount of water in your body.
"A decrease in antidiuretic hormone causes the kidneys to limit reabsorption of water; instead, it is excreted as urine, possibly resulting in dehydration."
It's now well into the evening, and you've moved on to rounds of shots.
For men, moving into that blood-alcohol territory of 150mg to 250mg - think anything past seven or eight standard drinks - comes with those classic signs of intoxication.
Others at the bar have noticed your slurred speech and unsteady walking, while you're feeling nauseous, your heart rate has sped up, and you're starting to see double.
It's time to get a kebab and call a taxi home, as it's illegal for bars to allow you to become intoxicated - and any more drinking might lead to black-out.
"And massive alcohol consumption or binge drinking could lead to alcohol poisoning," Braakhuis said.
"This happens when there is a high concentration of alcohol in the bloodstream, resulting in coma, respiratory depression or possibly death."
You wake up at 11am - and feel like death.
Your head is pounding and the morning light streaming through the window, and your neighbour mowing his front lawn, is only making it worse.
On top of that, you're sweaty, your whole body is aching, and your mouth feels like a bird's been nesting on it.
You sit up in bed, and immediately experience vertigo - prompting a quick dash to the bathroom for some close communication with your toilet bowl.
"The exact causes of a hangover are not entirely understood, but several factors may contribute," explained Dr Rajshri Roy, a lecturer in nutrition in the University of Auckland's Faculty of Medical and Health Sciences.
One of them is likely acetaldehyde. The toxic, short-lived compound is known to contribute to inflammation in the liver, pancreas and brain, and may cause headaches associated with hangovers.
Another culprit is dehydration.
Because alcohol can suppress the release of vasopressin - a hormone produced by the brain that sends signals to the kidneys, causing them to retain fluid - it can cause us to urinate more, to the point we've dried ourselves out too much.
The mild dehydration that results likely contributes to thirst, fatigue, and a headache.
What about that killer nausea?
"Some immune cells produce substances called cytokines, which can contribute to nausea and vomiting," Roy said.
"Alcohol can also interfere with the liver's production of glucose, the main form of energy for cells, which could contribute to dizziness, disorientation, and lack of energy."
Does it matter what we drank? Yes, it can.
Congeners, for instance, are compounds that, other than ethyl alcohol, are produced during fermentation, and add to the taste and smell of certain beverages.
Darker spirits, such as bourbon, which tend to have higher levels of congeners than clear spirits, can certainly worsen hangover symptoms for some people.
Then there are sulphites, which are compounds added to wine as preservatives.
People who have a sensitivity to them might experience a headache, as anyone who's had too many chardonnays the night before would know.
And what about that chest full of acid you sometimes feel?
"Some alcoholic beverages increase the release of gastric acid in the stomach, and delay the emptying of the stomach contents, which could be the reason for stomach pain associated with hangovers," Roy said.
The psychological effects can also be painful.
When we drink, we might feel calmer, more relaxed, and even euphoric - but the brain quickly adjusts to those positive effects as it tries to maintain balance.
As a result, when the buzz wears off, we can feel more restless and anxious than we were before that first sip the night before.
The internet might be full of hangover "cures", but there's no scientifically-proven remedies to immediately end one, and only time can help.
We have to wait for our bodies to finish clearing the toxic byproducts of alcohol metabolism, to rehydrate, and to restore immune and brain activity back to normal.
Certain over-the-counter pain relievers, including aspirin and ibuprofen, can help.
Some people also turn to electrolyte-rich sports drinks - blue Powerade, anyone? - in an effort to treat electrolyte imbalance caused by increased urination and fluid loss.
Yet research has not found a correlation between the extent of electrolyte disruptions and the severity of hangovers - or the impact of added electrolytes on hangover severity.
In most people, the body will quickly restore electrolyte balance once the effects of alcohol subside.
As trite as it might sound, the best way to stop a hangover is to avoid getting one by not drinking too much.
"It's always about moderation," Braakhuis said.
"Limiting yourself to one or two drinks from time to time is probably a good strategy."
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