Alcohol and the Body

By Gerry Kupferschmidt

Criminal lawyers who defend drunk driving charges must understand how the body absorbs, distributes and eliminates alcohol.

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 absorption

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.

Calculations using blood alcohol curve

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:

  • sex
  • age
  • height
  • weight
  • 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.

To approximate your blood alcohol level at a particular point in time use the Calculator.

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.

Mg%: Milligrams of ethyl alcohol in 100 millilitres of blood.

Serum/Plasma versus Whole Blood

Hospital analysis of blood samples for ethyl alcohol content is often based upon serum or plasma as the sample matrix.

Plasma is the liquid portion of the circulating blood (whole blood).

Serum is the liquid remaining after the red blood cells are removed by some mechanical means, such as centrifugation. Serum contains slightly more water than whole blood and hence will have a slightly higher alcohol level than whole blood.

Scientific studies have shown that serum will contain more alcohol than whole blood by a factor of 1.08:1 to 1.18:1, or on average, 1.12:1. As a result, a serum alcohol level of 108-118 mg% would be equivalent to an alcohol level of 100 mg% in whole blood.

What alcohol does to the body

Central nervous system depressant

Drinkers often perceive alcohol to be stimulating. This perception, which usually occurs at lower levels of alcohol intake, results from a depression of inhibitory control mechanisms in the brain. Alcohol, like other general anesthetics, is a primary and continuous depressant of the central nervous system (CNS). First it destroys the integrating control of the brain. Thought processes may be jumbled and disorganized; the drinker may become confused. In addition, motor functions may be disrupted.

Uncontrolled mood swings

The first mental processes to be affected are those that depend on training and experience. The finer grades of discrimination, memory, judgment, concentration and insight will be dulled, then lost.

Confidence may abound. Personality changes may occur as well as uncontrolled mood swings. Emotional outbursts may become frequent. The subject may suffer sensory and motor disturbances. As intoxication progresses, general impairment of nervous function and a condition of general anesthesia prevail.

Factors governing effect of alcohol

The effect of a given amount of alcohol on a person is a function, among other things, of the rate at which the alcohol is consumed, the BAC achieved, the subject’s tolerance to alcohol, and the setting (e.g., party atmosphere versus a more sombre setting).

The degree of impairment is dose related. However, it is not identical or linear for all behaviors. It is clear that behavioral skills requiring cognitive functioning suffer the greatest impairment. Put another way, impairment of the cognitive functions begins at lower levels than for simple tasks.
Alcohol tolerance

Tolerance will develop in regular drinkers, but not necessarily uniformly for all behavioral skills. Motor co-ordination shows the most tolerance. Whether tolerance develops with respect to complex skills and cognitive functioning is unclear.

Impairment of divided attention skills (performance of two or more tasks) shows little evidence of tolerance, whereas some short-term memory studies suggest that it may develop for complex tasks, as well as simple ones.

More alcohol needed to achieve same effect

Tolerance to many effects of alcohol is easily developed.

Alcohol is metabolized by the liver. A person who uses alcohol wants the desired effect to last as long as possible. Alcohol metabolism or transformation limits its duration of action.

Repeated exposure of the metabolizing system (mainly the liver) to alcohol increases the system’s capability and efficiency. As a result, the alcohol is metabolized more quickly and the duration and intensity of the desired effect are considerably reduced. This is called metabolic tolerance.

To regain the desired effect of the alcohol, the individual must increase the dosage and/or frequency of consumption.
Central nervous system tolerance

CNS tolerance occurs when cells adapt to the presence of alcohol so as to diminish the effect on them of a given level of alcohol. This tolerance does not develop at the same rate for all effects and may not develop at all for others. This is called functional tolerance. As with metabolic tolerance, the user increases the dose and/or frequency of administration to overcome this tolerance, reinstating or enhancing the desired effect.

Loss of tolerance

As with drugs, tolerance to alcohol, once developed, will lessen with time if alcohol is no longer taken regularly. Generally, if drinking stops, the person’s body will revert to the tolerance level in existence when alcohol was consumed for the first time. If, after a long period of abstinence, alcohol consumption again becomes regular, there is considerable evidence to suggest the former tolerance is acquired more quickly.
Impairment versus intoxication

It should be noted that individuals can be impaired by alcohol without displaying any outward signs. Impairment is not simply the appearance of gross physical symptoms but it is also a deterioration of judgment, attention, loss of fine co-ordination and control with a possible increase in reaction time and a diminishing of sensory perception.

Intoxication is an advanced state of impairment in which the gross physical symptoms of the effects of alcohol are apparent. The point at which “impairment” becomes “intoxication” is unique to the subject and depends on tolerance.
Impairment and rising or falling BAC

Studies have shown that impairment is greater at a given blood alcohol level when BAC is increasing than for the same BAC when the blood alcohol level is falling. This is called the Mellanby effect.

The manner of consumption also can affect impairment. If alcohol is consumed at a slow and steady pace, it is likely that there will be a slow and steady increase in impairment. If the alcohol is consumed more quickly, the rate of increase in impairment may also be more rapid and appear at lower BACs.

Bolus drinking

If alcohol is consumed quickly (bolus drinking), the rate of performance deficit may be further accelerated because the alcohol is absorbed into the blood stream more rapidly. The increasing impairment is generally obvious to the observer due to the greater than expected rate of deterioration in abilities and performance.

Tolerance developed to a given BAC achieved in social drinking, may not help to moderate the effects of alcohol when the same BAC is achieved by bolus consumption.

Alcohol and driving

The link between driving ability and alcohol impairment is probably the most intensely studied area with respect to the effect of alcohol on cognitive functions.

Driving is a complex task involving the integration and coordination of many skills and abilities. It involves dynamic and continuous interaction between the driver and his or her vehicle and the environment. It requires swift and accurate transfer of data from the environment to the driver, the processing of information, decisions, if any, on how to respond and the translation of any decisions into physical action.

Impairment at low BAC levels

The scientific community is unable to replicate the real-world driving task (an over-learned task).

To assess the effects of alcohol on the ability to drive, researchers have disassembled the driving function into theoretical parts for study. Although there is some evidence that impairment in some individuals may begin at low BAC levels, this data must be treated with some caution. What is clear, however, is that most persons with a BAC of 100 mg% would suffer some impairment.

Gerry Kupferschmidt is a highly regarded forensic toxicologist. He can be reached at