Alcohol is readily absorbed from the stomach and the intestine, and the highest blood level occurs about 30 minutes from the time of the last drink. Alcohol absorption is slowed by the presence of food in the stomach; however, once it reaches the small intestine, alcohol absorption is rapid. Alcohol vapors can be inhaled and absorbed by the lungs and can be a significant occupational hazard where used industrially.

After consumption and absorption the majority of alcohol distributes into body water, and like most solvent and anesthetics some distributes into fat. It is excreted in the urine and breath, hence the utility of the taking breath samples to evaluate alcohol exposure. Your breath alcohol level is directly related to your blood alcohol level. The majority of alcohol in your body is metabolized in the liver. An enzyme, alcohol dehydrogenase (ADH), metabolizes alcohol to acetaldehyde. Acetaldehyde is toxic, with elevated levels causing flushing, headache, nausea, and vomiting. Acetaldehyde is in turn quickly metabolized to the less toxic metabolite acetate by another enzyme acetaldehyde dehydrogenase (ALDH) (Figure 3.1).

Figure 3.1 Metabolism of Alcohol

Humans have varying amounts and types of ALDH which affects their ability to metabolize the toxic metabolite acetaldehyde. For example of approximately 50% of people of Asian heritage have a single base change in a gene that encodes for ALDH resulting in an inactive form of ALDH, which makes alcohol consumption very unpleasant. Antabuse (disulfiram), a common drug prescribed to discourage alcohol consumption, blocks ALDH and causes blood levels of acetaldehyde to rise. The subsequent toxic side effects discourage continued alcohol consumption. Disulfiram was a chemical originally used in the rubber industry. Workers inadvertently exposed to disulfiram accidentally discovered its effects when they became sick after drinking alcoholic beverages.

The metabolism of most drugs or chemicals is proportional to the concentration of the compound in the blood. This allows us to calculate the rate of metabolism or a half-life. However, ethanol is different; its metabolism is relatively constant over time and the rate of metabolism does not increase with rising blood concentrations. We also know that metabolism is proportional to body weight; thus the bigger you are, the higher the rate of metabolism, but on average, ethanol is metabolized at a rate of 120 mg/kg per hour or about 1 oz (30 ml) in 3 hours.

Ethanol is easily measured in the blood and reported as milligrams per milliliter (mg/ml)of blood. Current laws regulating driving after drinking specify specific blood alcohol concentration (BAC) as unacceptable when operating a motor vehicle. Most states set 0.08 or 0.1, which is equivalent to 80 mg/100 ml or 80 milligrams per deciliter (mg/dL) of blood. Alcohol content of exhaled breath is about 0.05% of the BAC.

Another factor that influences blood alcohol concentrations and thus the effects of alcohol is gender. Drink for drink, a female will have a higher BAC than a male. First, women tend to be smaller, so by body weight they receive a higher dose of alcohol. Second, women metabolize less alcohol in the intestine than men, which results in great absorption of alcohol and a higher BAC. Finally, women usually have a greater proportion of body fat per body weight, which results in lower volume of fluid by weight. An average male of medium weight (160-180 pounds) must consume almost four drinks in an hour to reach an a BAC of 0.08, whereas an average female weighing 130 to 140 pounds requires on only 3 drinks within one hour to reach a BAC of 0.08. The exact number of drinks to reach a BAC of 0.08 of course depends on many variables, not the least of which is the percent alcohol in the drink.

How alcohol affects the central nervous system is still not completely clear. For some time, researchers thought that the depressant affects of alcohol, like other anesthetic agents, was caused by the dissolving of cells' lipid membranes and the disruption of the function of various proteins. More recently, researchers have focused on specific receptors such as glutamate (excitatory) and GABA (inhibitory). Despite intensive research, the mechanism by which alcohol affects the developing fetus is still unknown.

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