The concentration of alcohol in a person's blood is known as their Blood Alcohol Concentration (BAC), or Blood Alcohol Level (BAL). These terms are used interchangably, and both use the same scale of measurement.
The effect that alcohol has on a person's body is dependent on the amount of alcohol that their organs -- the brain in particular -- are exposed to. Since the alcohol gets to the brain by diffusing itself throughout the blood, a convenient gauge of intoxication is the amount of alcohol in a person's blood. But, a 300 pound football player certainly has more blood in his body than a 100 pound ballerina. The same amount of alcohol is going to affect the ballerina much more than it will affect the football player. But since different people have different amounts of blood, an even more convenient measure is the concentration of alcohol in the blood. This is what the BAC measures.
The units that the BAC is measured in are a more than a bit confusing. For the curious, some interesting footnotes have been included. In most cases, the BAC is expressed without units in a form like the following: "the driver has a BAC of .18 at the time of the accident." This really means that the driver had .18 grams of ethanol per 100 milliliters of blood. Remember that this is grams of the actual chemical ethyl alcohol, not grams of actual beverage.
An interesting and useful property of ethanol is that the amount of ethanol in one's breath is directly proportional to the amount in one's blood. Police use special devices called "breathalyzers" to take an accurate measurement of a person's BAC and determine whether it is over the legal limit. The breathalyzer can tell exactly what BAC a person has simply by analyzing a breath of air from that person. While BAC for blood is measured in weight of ethanol per 100 mL of blood, it can be measured as weight of ethanol per 210 L (liters, not milliliters) of breath, and produce an equal result.
The liver is a very important organ. All the blood in your body passes through your liver every three minutes. It processes blood and tries to remove harmful or foriegn substances. Upon entering the bloodstream, the alcohol is immediately recognized as a foriegn substance. Enzymes in the liver metabolize the ethanol into highly toxic acetaldehyde, then into acetate, which is a useful chemical for the body, and is either converted into energy or other useful substances. The liver processes alcohol at a constant rate. However, the rate at which alcohol is metabolized differs by individual. The average rate of metabolism reduces a person's BAC by .017 g/dL per hour, but depending on how heavily the individual drinks, can go as low as .01 g/dL/hr or as high as .04 g/dL/hr. 90% of the alcohol in the body is metabolized in this manner. 2% exits the body through sweat, and 8% of the alcohol exits through breath. This not only shows how once in the bloodstream, alcohol is spread all throughout the body, but also explains how the breathalyzer can detect alcohol concentrations.
So every time a drink is ingested, the ethanol in that drink is absorbed into the blood, thus boosting the blood alcohol concentration. However, the process is really not that simple. The BAC depends on many, many factors. Probably the most important of these is time. It does take some time for the alcohol the be absorbed by the stomach and small intestine. In addition, drinking on an empty stomach has much more of an effect than drinking while full. When drinking on a full stomach, it takes much longer for the alcohol to reach the small intestine, where most of it is absorbed. All the alcohol is eventually absorbed, but the full stomach produces a time-release effect. On an empty stomach, the alcohol immediately enters the small intestine and is absorbed almost instantaneously. Since the liver processes alcohol at a constant rate, drinking on a full stomach produces a low peak BAC, but sustained for a long time, as the stomach gradually releases alcohol to the intestine for absorbing, and the liver gradually eliminates it. Drinking on an empty stomach, two or three drinks in rapid succession can produce a very high BAC, since the liver is given no time to process the alcohol, and it is all released into the blood very rapidly. Other factors that determine BAC include body weight, percent of body that is blood, percent of body that is fat, etcetera. But the factors which influence BAC most are rapidity of consumption and body weight.
In the next few chapters, BAC will be used extensively to gauge effects of alcohol on the nervous system.