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Alcohol Absorption, Distribution & Elimination

Alcohol absorption, distribution and elimination are simultaneous processes that commence upon consumption.

Absorption is the passage of alcohol into the blood. Distribution is the temporary placement of alcohol into various body tissues. 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.

Use the Blood Alcohol Calculator to estimate your blood alcohol level at a given time.

Blood alcohol concentration

A blood alcohol concentration (BAC) or blood alcohol level (BAL) reflects the amount of alcohol in the body. Food, type and quantity of beverage, weight, sex, and rate of elimination determine the BAC after the consumption of alcohol. The 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 line and the elimination phase by a falling line.

Alcohol absorption

Alcohol is absorbed from the stomach and small intestine by diffusion. Most absorption occurs from the small intestine due to its large 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 rate of absorption. However, above a certain concentration, the rate of absorption may decrease due to the 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 solution on an empty stomach. The absorption rate may be less when alcohol is consumed with food or when a 40% (v/v) alcohol solution is consumed on an empty stomach. The rate may also slow down when high fluid volume/low alcohol content beverages, such as beer, are consumed.

Normal social drinking

For normal social-type drinking, the highest BAC is usually achieved within 30 minutes after completion of consumption, though it could take as long as 60 minutes. When large amounts of alcohol are consumed over a short time interval, or when a large quantity of food is eaten with the alcohol, the absorption phase may not be complete for up to two (2) hours after last consumption.

Two-hour BAC plateau

In other situations, a subject may develop a plateau, where the blood alcohol level does not change for up to two hours. When this occurs the rate of absorption is equal to the rate of elimination and hence the blood alcohol concentration does not change. 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. During the absorption phase, the BAC of arterial blood is greater than the BAC of venous blood. Arteries carry blood to a tissue, and veins remove blood from the tissue. At equilibrium, where the tissue has 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 the 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 as alcohol, unchanged. It is this latter process that 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. The range of decrease in BAC is 10-20 mg% per hour. This range represents the extreme ends of the rate encountered in a normal population. Most people eliminate at a rate of between 13 and 18 mg% per hour. Of these, the majority eliminates at the higher end. Very few people eliminate at as low a rate as 10 mg% per hour.

Calculations using blood alcohol curve

Using a blood alcohol curve it is possible to estimate the following: 1) Blood alcohol level at a given time based upon an indicated consumption scenario; 2) Quantity of alcohol required to produce a known blood alcohol level at a given time; 3) Blood alcohol concentration for a given subject 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 factors may include sex, age, height, and weight of the subject; consumption start time and stop time, as well as pattern of drinking; type of alcohol consumed including number and size of drinks, their alcohol content; time for which BAC is being calculated; or BAC value(s) detected if a retrograde or anterograde extrapolation is required. Other factors that could affect these estimates include times when meals were eaten; disease states; and any medications that may be taken. This information, combined with the empirical factors for alcohol absorption, distribution and elimination, provides the basis for the estimates noted above.

See the Blood Alcohol 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 S.I. (International System of Units) 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 of 80 mg% is equivalent to 17.3 millimoles of ethyl alcohol per litre of blood.

Information Required for Alcohol Related Reports

Personal data

Sex; height; weight; medical conditions (if any); medication (if any) including the dosage regimen being followed and the period of time this medication was used prior to the incident in question (day, months or years).

Drinking data

Time consumption began; time consumption ceased; approximate time at which each drink was consumed (consumption pattern -- evenly spaced or more drinks at the beginning or at the end of the drinking time interval); meals eaten (times and description).

Each drink should be identified by beverage size and alcohol content. Beer should be described by brand, container type (cans, bottles, draft glasses etc.), and whether it was light, regular or extra-strength. Wine should be identified by brand and alcohol content. Wine glasses should be described by beverage volume. Liquor, liqueurs and shooters should be identified by "shot" size, brand and alcohol content whenever possible. Shooters with multiple ingredients should be identified according to the components used to formulate the mixture and the proportions used.

Offence data

The offences committed should be provided along with the date and location for trial. The Crown disclosure material complete with the Alcohol Influence Report should be forwarded if available. If not, please provide the time of the alleged offence and the times and values of each breath test. All instruments used should be identified according to manufacturer make and model number. All signs of impairment should be described.

Serum/Plasma versus whole blood

It has been observed that the hospital analyses of blood samples for ethyl alcohol content are often based upon serum or plasma as the sample matrix.

Plasma is the liquid portion of the circulating blood.

Serum is the liquid remaining after the red blood cells are removed by 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 between 1.08:1 and 1.18:1, or on average, a factor of 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.

Urine alcohol concentration (UAC) considerations

After alcohol is absorbed into the bloodstream of a given individual, it is distributed throughout all body fluids and tissues according to the water content of those fluids or tissues. At any given point in time the UAC will be considerably different from the BAC

After the cessation of drinking the BAC may rise for a period of time. At this point, the UAC will be less than the BAC because of absorption and distribution considerations. Thereafter, the BAC and UAC curves will cross. For some period of time the UAC will continue to rise, whereas the BAC will remain constant (plateau) or begin to decrease. In the fully post-absorptive state, the UAC will always exceed the BAC because of the higher water content for urine compared to that for blood. The peak BAC is generally reached within 30 to 90 minutes after the peak BAC is achieved.

Blood analysis is a direct method for the determination of a blood alcohol level and urine analysis is an indirect method. In other words, a blood sample analysis is the most accurate means to predict a blood alcohol level. To some extent a UAC can corroborate a BAC, but difficulties can arise with the use of a single urine sample because of pooled urine in the bladder. In living subjects, more than one urine sample should be obtained over a known time interval. Obviously in cases where death has occurred, the only sample that is available for analysis is the urine sample found in the bladder, which was formed prior to death.

Urine alcohol levels are far more reliable when two urine samples are collected about 0.5-1.0 hours apart and the bladder is completely emptied at the first void. This ensures that the urine sample collected at second void was formed within the period of time between the first and second void samples. The difference in UAC values between the first and second void provides information concerning the state of the UAC curve (rising or falling). In addition, the alcohol content of the second void represents the average alcohol concentration of the urine formed between the first and second void. When a urine sample is collected in this manner it is known that the UAC:BAC relationship is approximately 1.33:1. This means that the urine alcohol content will be 1.33 times greater than the blood alcohol content. Under these conditions a UAC of 133 mg% would equate to a BAC of 100 mg%. Because the urine is formed over a period of time, the predicted BAC based upon a UAC result refers to a blood alcohol level at some time prior to the collection of the urine sample.

When single urine samples are analyzed, a far greater range of values are reported in the scientific literature for the UAC:BAC ratio. One scientific study reported that the mean UAC:BAC ratio varied from 1.4-1.7:1, when the BAC of the subjects studied exceeded 50 mg%. When second specimens of urine were obtained approximately 60 minutes after an initial void, the mean UAC:BAC ratio was found to be 1.35:1. This study confirms the highly desirable feature of collecting two urine samples, when urine alcohol levels are used to predict a BAC level, as well as to assess the state of the blood alcohol curve (rising or falling) at a time interval of interest.

Alcohol Pharmacology

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 is classified as a general anesthetic, which produces a range of central nervous system (CNS) effects similar to those of other sedative/hypnotic drugs. First it destroys the integrating control of the brain which may cause thought processes to become disorganized and chaotic. The drinker may become confused and disoriented. In addition motor functions may become less fluid.

Uncontrolled mood swings

The first mental processes to be altered are those that depend on training and experience. The finer components of disrimination, memory, judgement, decision-making, concentration and insight are eroded and eventually lost as drinking continues.

The drinker may become very confident and exhibit personality changes with uncontrolled mood swings. Emotional outbursts may become frequent and the subject may suffer sensory and motor disturbances. As intoxication progresses, general impairment of nervous function and general anesthesia could result in respiratory depression, and ultimately death.

Factors governing effect of alcohol

The effect of a given amount of alcohol on a specific person is a function, among other things, of the rate at which the alcohol is consumed, the subject's tolerance to alcohol, and the circumstances related to drinking (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 of alcohol consumption 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

Central nervous system (CNS) tolerance occurs when cells adapt to the presence of alcohol in such a way as to diminish the effect of a given level of alcohol on them. This kind of tolerance is characterized by differential development for different effects. In other words, it does not develop at the same rate for all effects of a drug or it may not develop at all for some effects. This is called functional tolerance. As with metabolic tolerance, the user increases the dose or frequency of administration to overcome this tolerance, reinstating or enhancing the desired effect.

Loss of tolerance

Tolerance to a drug, such as alcohol, once developed, will be minimized with time, if the drug is no longer taken regularly. Generally, cessation of drug use will cause the body to revert to its original tolerance levels, when it first experienced the presence of the drug. If, after a long period of abstinence, the drug is used again regularly, there is considerable evidence to suggest the former tolerance is acquired more easily and quickly.

Impairment versus intoxication

It should be noted that individuals can be impaired by alcohol without manifesting any visible signs. Impairment is not simply the appearance of gross physical symptoms but a deterioration of judgment, attention, loss of fine co-ordination and control with a possible increase in reaction time and a diminishing of sensory perceptions. 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 the 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, which is achieved on the basis of a social-drinking pattern, may not help to moderate the effects of alcohol when the same BAC is achieved by bolus consumption.

Alcohol and Driving

The relationship between driving ability and alcohol impairment is particularly significant as it is probably the most intensely studied area of 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 among the driver, the vehicle and the environment. It requires swift and accurate transfer of information from the environment to the driver, the processing of that information, decision-making on how to respond, and the translation of decisions into physical actions.

Impairment at low BAC levels

The scientific community is unable to replicate the real-world driving 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. It is clear, however, that most persons with a BAC of 100 mg% would suffer some impairment.

Experimental studies

There are two separate and distinct sources of data concerning the issue of alcohol impairment and driving ability. Experimental studies relate the effects of alcohol to some aspect of physiological function that may or may not relate to driving ability. This usually involves laboratory testing, driving simulators or actual in-vehicle, closed-course driving situations. These experiments usually attempt to replicate some aspect of the real driving exercise.

Epidemiological studies

Epidemiological studies attempt to relate BAC to the likelihood or risk of accident involvement. These studies attempt to observe as many factors as possible in order to develop a thesis about their relationship to an activity (such as driving) and about their interaction with each other. This kind of research attempts to define how alcohol and road accidents are associated.

The following features have been shown to be negatively influenced by alcohol.

  • Vision: (visual acuity, depth perception; peripheral vision; and glare recovery)
  • Reaction time: simple, choice and complex reaction times
  • Tracking tasks: compensatory and pursuit tracking
  • Cognitive functions: concentrated attention; divided attention; rates of information processing; judgement; and decision-making.
  • Psychomotor skills: coordination; body sway; manual dexterity; and    general walking
  • Driving simulators and closed course driving experiments: braking and stopping efficiency; steering; lane position; evasive manoeuvres; parking; and emergency response
  • Other aspects: memory; risk-taking; overcompensation
  • Epidemiological studies: increased risk of accident with increasing BACs
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