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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