How alcohol moves through your body
Ethanol is a small, water-soluble molecule that diffuses across biological membranes without needing a carrier, which is why it reaches the bloodstream faster than almost any food-borne nutrient. Absorption begins in the mouth and esophagus for a small fraction of the dose (roughly 20%), but most of it — about 80% — crosses the wall of the small intestine, especially the duodenum and jejunum. On an empty stomach, peak blood alcohol concentration (BAC) typically arrives 30-45 minutes after the last sip; with food, the pyloric sphincter stays closed longer and the peak can slide to 60-90 minutes or beyond.
Once absorbed, ethanol distributes through total body water, not total body mass. This is why lean body composition matters more than weight alone. The Widmark distribution ratio (r) captures this: in adult men, r averages around 0.68 litres of body water per kilogram of body mass; in adult women, it averages roughly 0.55 because women carry a higher proportion of adipose tissue, which holds almost no ethanol. Older adults and people with higher body-fat percentages have lower r-values and therefore reach higher peak BAC from the same number of drinks.
The liver performs roughly 90% of ethanol clearance. Alcohol dehydrogenase (ADH) oxidises ethanol to acetaldehyde, which aldehyde dehydrogenase (ALDH) then converts to acetate — the same acetate your body produces from carbohydrates. What makes alcohol unusual is that this pathway is saturated at very low concentrations. The enzymes work at a constant maximum rate regardless of how much ethanol is still in the blood, which is why elimination follows near zero-order kinetics at roughly 0.015% BAC per hour (about 0.015 g/dL/h, or ~0.15 g/L/h). The remaining ~10% leaves through breath, urine, and sweat — which is exactly what breathalysers measure.