Alcohol and The Body
Criminal lawyers who defend drunk driving charges must understand how the body absorbs, distributes and eliminates alcohol. In the pages that follow, forensic toxicologist Gerald Kupferschmidt discusses these and other alcohol-related issues.
What the body does to alcohol.
Alcohol absorption, distribution and elimination occur simultaneously and start with consumption. Absorption is the passage of alcohol into the blood. Distribution is the temporary placement of alcohol into body tissue. Elimination is the removal of alcohol from the body. Diffusion is the method of passage of alcohol through cell membranes, and is governed by concentration differences on either side of the cell wall.
Blood alcohol concentration.
A blood alcohol concentration (BAC) or blood alcohol level (BAL) reflects the amount of alcohol in the body. Food, kind and quantity of beverage, weight, sex, and rate of elimination determine BAC after the consumption of alcohol. BAC is a measure of the difference between the rates of absorption and elimination. The change in BAC with time may be described graphically as a blood alcohol curve, where the absorption phase is represented by a rising limb and the elimination phase by a falling limb.
Alcohol is absorbed from the stomach and small intestine by diffusion. Most absorption occurs from the small intestine due to its larger surface area and rich blood supply. The rate of absorption varies with the emptying time of the stomach. Generally, the higher the alcohol concentration of the beverage, the faster the absorption. However, above a certain concentration, the rate of absorption may decrease due to delayed passage of alcohol from the stomach into the small intestine.
The maximum absorption rate is obtained with the consumption of an alcoholic beverage containing approximately 20-25% (by volume or v/v) alcohol on an empty stomach. The absorption may be slower when alcohol is consumed with food or when 40% (v/v) alcohol is consumed on an empty stomach. The rate may also slow when high fluid volume/low alcohol content beverages, such as beer, are consumed.
Normal social drinking
For normal social drinking, the highest BAC is usually achieved within 30 minutes of completion of consumption, though it could take up to 60 minutes. When large amounts of alcohol are consumed over a short time, or when a large quantity of food is eaten with the alcohol, the absorption may continue for up to two hours after last consumption.
Two-hour BAC plateau
In other situations, a person may develop a plateau where the blood alcohol level does not change for up to two hours. This occurs because the rate of absorption is equal to the rate of elimination. After two hours, the rate of elimination will exceed the rate of absorption and the blood alcohol level will begin to decrease.
Once in the blood, alcohol is carried throughout the body. The alcohol diffuses into tissues and fluids according to their water content (the more the water, the higher the alcohol level). During absorption, the BAC of arterial blood is greater than the BAC of venous blood. Arteries carry blood to a tissue, veins remove it. At equilibrium, when body fluids and tissue have absorbed a proportionate quantity of alcohol, the BAC of arterial blood is equal to the BAC of venous blood.
Weight and sex affect BAC
A person's weight and sex determine the total volume of body water and consequently the BAC obtained upon consumption of a particular quantity of alcohol. Generally, the more a person weighs, the larger the volume of body water and the lower the BAC obtained from the consumption of a given amount of alcohol.
A female may have more fat tissue than a male of the same weight and therefore a smaller volume of body water. As a result, a female may obtain a slightly higher BAC upon consumption of the same quantity of alcohol as a male, all other factors being equal.
As BAC decreases, alcohol diffuses from the tissues back into the blood.
Elimination of alcohol
Alcohol is eliminated from the body by excretion and metabolism. Most alcohol is metabolized, or burned, in a manner similar to food, yielding carbon dioxide and water. A small portion of alcohol is excreted, such as through the breath, leaving the body unchanged as alcohol. It is excretion which allows for breath alcohol testing.
Average rate of elimination
Elimination occurs at a constant rate for a given individual. The median rate of decrease in BAC is considered to be 15 milligrams per cent (mg%) per hour. In a normal population, the range of decrease in BAC is 10-20 mg% per hour. Most people eliminate between 13 and 18 mg% per hour. Of this group, most eliminate at the higher end. Few people eliminate at a rate of 10 mg% per hour.
Using a blood alcohol curve it is possible to estimate the following:
- blood alcohol level at a given time based upon an indicated consumption scenario
- quantity of alcohol required to produce a known blood alcohol level at a given time
- blood alcohol concentration at a time previous to sample collection (retrograde extrapolation), or at a time subsequent to sample collection (anterograde extrapolation)
To accurately estimate each of the above, knowledge of certain factors is required. These may include:
- consumption start time and stop time
- pattern of drinking
- type of alcohol consumed including number of drinks, size and alcohol content
- time for which BAC is being calculated (or BAC value(s) detected if a retrograde or anterograde extrapolation is required)
- times meals eaten
- disease states
- any medications that were taken
This data combined with the empirical factors derived from scientific studies, such as average rates for alcohol absorption, distribution and elimination provides the basis for the estimates noted above.
BAC Reporting Conventions
The Criminal Code of Canada reports the legal limit for alcohol as 80 milligrams of ethyl alcohol per 100 millilitres of blood (80 mg%). This is also often expressed as 0.08 grams of ethyl alcohol per 100 millilitres of blood.
In the clinical world, concentrations of substances are reported using the Systeme Internationale system of measurement. Hence, millimoles per litre. One millimole of ethyl alcohol per litre of blood is equivalent to 4.61 milligrams of ethyl alcohol per 100 millilitres of blood. As a result, 80 mg% is equivalent to 17.3 millimoles of ethyl alcohol per litre of blood.
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