Alcohol is absorbed into the bloodstream all throughout the gastrointestinal tract.  A small amount of absorption can take place in the mouth, although this amount is generally insignificant because residual alcohol leaves the mouth quickly.  After alcohol leaves the mouth, it flows down the esophagus and into the stomach. 

Although some absorption, perhaps as much as 20%, takes place in the stomach itself, the overwhelming majority of absorption takes place in the small intestine.  The amount of absorption which occurs in the stomach is controlled in part by the pyloric valve - a valve at the base of the stomach which opens to empty the stomach.  If the stomach is empty when alcohol is consumed, the alcohol can pass quickly through the stomach and pyloric valve into the small intestine.  However, if there is food being consumed while the person is drinking alcohol, the alcohol will be trapped with the food in the stomach, where absorption is not very efficient, while the food is being digested.  The alcohol won't be able to pass through the pyloric valve and into the small intestine, where absorption is far more efficient, until the food has been sufficiently digested and the pyloric valve opens.  Since alcohol is trapped in the stomach, the absorption process is slowed and the person will become impaired more slowly and will reach a lower peak alcohol concentration.  This is why it is a good idea to eat a nice big meal while you are drinking.  However, the best way to avoid ever being charged with a DUI in Virginia is to never drive after drinking alcoholic beverages.

The overwhelming majority of absorption takes place in the small intestine because of the extremely large surface area of the small intestine. 

It is generally agreed that, on an empty stomach, up to 25% of absorption may occur in the stomach and at least 75% of absorption occurs in the small intestine.  Therefore, peak blood alcohol concentrations are achieved in fasting people within 0.5 to 2.0 hours, while non-fasting people reach peak alcohol concentrations within 1.0, and in extreme cases up to as much as 6.0 hours.