Alcohol and the Human Body: Absorption, BAC, and Physiological Effects
From glass to bloodstream
Ethanol is absorbed unchanged, partly through the stomach lining (roughly 20 percent) and mostly through the small intestine (roughly 80 percent). Anything that slows gastric emptying slows absorption and flattens the peak blood alcohol concentration (BAC); food, especially a fatty or protein-rich meal, does this. Carbonation and diet mixers speed gastric emptying and can raise the peak. Very strong and very dilute drinks are both absorbed more slowly than a typical mixed drink.
What actually determines BAC
BAC is the dose of alcohol divided by the body water it dissolves in. The variables that matter are: number of standard drinks, time over which they were consumed, body weight, body composition (fat holds little water, so a person with more body fat reaches a higher BAC at the same dose), sex (women generally have proportionally more body fat and less body water, and less gastric enzyme activity), age, liver health, food, and some medications. In the US a standard drink contains 0.6 fl oz of pure alcohol: 12 oz of 5 percent beer, 5 oz of 12 percent wine, or 1.5 oz of 40 percent spirits.
Elimination and the persistent myths
The liver metabolizes about 90 to 95 percent of alcohol at a fairly constant rate of roughly one standard drink per hour (about 0.015 BAC per hour); the rest leaves in breath, sweat, and urine. Because that rate is fixed, elapsed time is the only thing that lowers BAC. Coffee, cold showers, food, water, exercise, and fresh air do not sober anyone up. Tolerance changes how a person behaves at a given BAC; it does not change the BAC.
Physiological effects
At low BAC: relaxation, disinhibition, and measurable loss of judgment, reaction time, and divided-attention skill before anything visible appears. Higher up: slurred speech, poor coordination, mood swings, nausea, blackouts, and at very high levels respiratory depression and death. Alcohol combined with sedatives, opioids, or antihistamines is synergistic and can be lethal at doses that would be safe alone.
Worked rule of thumb: count standard drinks per person, not glasses, and assume each one takes about an hour to clear.