Your blood alcohol level is simply the concentration of alcohol in your blood.

As alcohol is consumed, it enters into your bloodstream. On average, one beer can be processes completely in about one hour. Anymore alcohol than this amount will lead to more alcohol in your blood which in response raises your blood alcohol level, or BAL, as it is coming in much faster than the body can process the alcohol.
Your body can remove alcohol already absorbed in your bloodstream in a few different ways. These are metabolism, evaporation through your breath and excretion through urinating, sweating, tears and so on. But there are also many people who can actually begin breaking down the alcohol while in their stomachs, before it is absorbed into the bloodstream through higher levels of the enzymes called alcohol dehydrogenase which convert the alcohol into a toxic acetaldehyde, then the next set of enzymes called acetaldehyde dehydrogenase break it all down into the non-toxic acetic acid. It is here where many individuals notice the differences in their ‘tolerances’ as compared to those who can consume much more alcohol and/or sober up much quicker than others.

Research has shown that there are different types of people who can process a lot more quickly such as males with larger body mass. However, in contrast, there are several types of people who seemingly do no process alcohol as quickly.

Factors that can affect one’s ability to process alcohol more rapidly can be attributed to age, sex, ethnicity, usage of birth control pills, heightened emotional state, mixing alcohol with other drugs, physical illness such as a cold, and, of course, those who have physical issues with their body such as liver problems.

Some contributing factors to a slower processing of alcohol can also be attributed to types of pain killers such as Tylenol or aspirin which can not only hinder the speed of metabolism, but can also cause serious damage to your liver due to the fact that two toxins (acetaminophen and acetaldehyde) would be affecting your liver and not just the one from alcohol metabolism, or acetaldehyde.
If you find yourself with a slower alcohol metabolism than others, try diluting the alcohol in your stomach by eating before drinking. This will slow down the absorption process as well as help prevent the ugliness of a hangover.
Currently, there are no drugs which can be taken in order to speed up one’s metabolism of alcohol; however, carbonated beverages will speed up metabolism in general, but this is a different type of speeding up which can cause very nasty hangovers if carbonated drinks are mixed with non-carbonated alcoholic beverages as the body’s metabolism has sped up but it does not realize so quickly that it desperately needs to slow down in order to process the harder and/or stronger levels of alcohol being consumed.

Remember, your blood alcohol level, or BAL, can continue to increase even after you have finished drinking due to the body still absorbing alcohol into the bloodstream.
A link is provided below for a blood alcohol chart which shows the relation between blood alcohol level, or BAL, and the amount of alcohol being consumed.

Blood Alcohol Chart http://www.alcohol.vt.edu/Students/alcoholEffects/estimatingBAC/images/drinksLarge.gif