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Alcohol is commonly ingested orally and passes from the mouth through the esophagus, into the stomach, and then into the small intestine via the pyloric sphincter. A majority of the alcohol consumed is absorbed into the blood as it passes through the small intestine; however, some absorption occurs in the mouth and throat lining and stomach. The alcohol absorbed into the blood is then distributed throughout the body by blood flow; it passes through the liver, which is primarily responsible for its metabolism, then to the heart, then to the lungs, then back to the heart where it is distributed to the rest of the body simultaneously. Tortora, Principles of Human Anatomy, chap 12 (13th ed 2014).

Because alcohol is completely soluble (miscible) in water, it is eventually distributed uniformly in body water and in all the soft tissues of the body. Urine, being mainly water, will contain a given ratio of alcohol relative to the blood, which makes the urine a potential fluid for determining blood alcohol level. Likewise, due to the water solubility and the volatility of alcohol, the breath contains a given ratio of the alcohol relative to the blood, therefore enabling the breath to be another suitable means of measuring blood alcohol level. See Saferstein, Criminalistics: An Introduction to Forensic Science, chap 13 (12th ed 2017).

However, during the absorptive phase, there are significant differences between alcohol concentrations in arterial blood, venous blood, breath, and urine, with arterial blood having a higher concentration of alcohol than the venous blood. The alcohol concentration would also be higher in those areas closer to the gut (the source of absorption during this phase). So the lungs would have a higher concentration than those areas in the extremities (arms, legs, brain). It is believed that during the absorptive phase, breath alcohol concentration (BrAC) is closer to the concentration of alcohol in arterial blood than it is to the concentration of venous blood. The question of whether BrAC more accurately reflects the concentration of alcohol in the brain, where the depressant effects are manifest, is still unresolved. For more information see Saferstein, Criminalistics, chap 13. See also Simpson, Accuracy and Precision of Breath Alcohol Measurements for Subjects in the Absorptive State, 33 Clinical Chemistry 753, 755 (June 1987), available at

Fatty tissues have lower water concentrations than other tissues such as muscle. Thus, a lean person who consumes a given amount of alcohol potentially attains a lower blood alcohol concentration (BAC) than a person of the same weight with a higher body fat content. Because women normally have more subcutaneous fat and less total body water than men do, a woman who consumes the same amount of alcohol as a man of the same weight potentially attains a higher BAC, although some studies have suggested that women tend to have faster elimination rates. Mumenthaler, Taylor, O’Hara & Yesavage, Gender Differences in Moderate Drinking Effects, 23 Alcohol Res & Health 55, 57 (Winter 1999), available at See also National Institute on Alcohol Abuse and Alcoholism (NIAAA), Women and Alcohol (last modified Dec. 2015), available at