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Alcohol is a central nervous system depressant. The degree to which the central nervous system function is impaired is directly proportional to the concentration of alcohol in the blood. There are 3 stages: absorption, distribution, and elimination.

Absorption

Absorption is the process of moving alcohol from outside the body into the bloodstream where it can be distributed throughout the body. Alcohol typically enters the body through the oral ingestion of a beverage containing ethyl alcohol. Unlike most other ingested substances, alcohol is not digested and can be absorbed unchanged directly through the stomach lining. The small intestine is by far the most efficient region of the gastrointestinal tract for alcohol absorption because of its large surface area, unlike the stomach which has a small surface area and absorbs proportionally less alcohol.

Blood alcohol content rises as alcohol is absorbed until it reaches a peak concentration, and then it gradually tapers off. It generally takes 30-60 minutes to reach peak alcohol levels after drinking has stopped.

Distribution

Alcohol enters the bloodstream through the stomach and the small intestine where it is rapidly absorbed and distributed by the blood throughout all of the water-containing components of the body. Because it is distributed so quickly and thoroughly, alcohol can affect the central nervous system even in small concentrations.

The bloodstream transports the alcohol to the bodily tissues. Veins carry the blood to and through the lungs where the blood becomes oxygenated. Arteries then carry the oxygen-rich blood to the brain and the rest of the body.

Because alcohol it completely soluble in water, the alcohol content in the whole body is directly proportional to total body water content. Water content varies from person to person.
 In general, the less you weigh the more you will be affected by a given amount of alcohol. This is because there is less water in a smaller person than in a larger person.
 For people of the same weight, a well-muscled individual will be less affected than someone with a higher percentage of fat since fatty tissue does not contain very much water and will not absorb very much alcohol.
 Women are more affected by alcohol consumption than men, because in general women have more body fat than men, which is directly proportional to water weight.
o Men: about 68% of an average man's body weight is due to body water
o Women: about 55% of an average woman's body weight is due to body water




Elimination

The body eliminates alcohol through metabolism, excretion, and evaporation.

Metabolism
The chemical and physiological processes by which the body builds and maintains itself and by which it breaks down food and nutrients to produce energy. Metabolism accounts for approximately 95% of alcohol elimination. The liver is responsible for metabolism. As a rule of thumb, a person metabolizes one average drink (.5 oz) of alcohol per hour.
 Healthy people metabolize alcohol more efficiently than unhealthy people.
 Chronic alcoholics metabolize alcohol quicker than the average person, so long as the liver is healthy.
 Younger people metabolize alcohol quicker than older people.

Excretion
Alcohol is excreted unchanged through urine, tears, sweat, semen, and saliva. Excretion may account for a person "smelling" of alcohol.

Evaporation
Alcohol evaporates from the blood into the lungs and is excreted in breath, allowing it to be measured in a breath sample.

Note: Not all evaporated air from the lungs is equal in alcohol concentration. The highest alcohol concentration in the lungs comes from the end of a long exhalation of breath, where the air was in closest proximity to the blood. This is the reason that police officers will make a person blow very hard and very long into the breath machine, because the deep lung air will have the highest concentrations of alcohol, making the test results higher.
The Effect of Food on Blood Alcohol Content

Eating immediately before drinking alcohol or eating and drinking at the same time affects the levels of alcohol in the blood. As a rule of thumb, the greater the amount of food in the stomach, the less blood alcohol concentration in the body. This is because having food in the stomach affects the absorption of alcohol by the small intestine. The pyloric valve at the bottom of the stomach will close in order to hold food in the stomach for digestion, thus keeping the alcohol from reaching the small intestine. The alcohol in the stomach is absorbed at a slower rate, which affects the distribution into the bloodstream, and ultimately the rate of elimination.

Alcohol elimination rates are inversely proportional to alcohol concentration in the blood. This means that the higher the blood alcohol levels, the slower the rate of elimination. And the lower the blood alcohol levels, the higher the rate of elimination.