Michigan Drunk Driving Defense Book
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CHAPTER FOUR
UNDERSTANDING THE PHYSIOLOGY OF ALCOHOL
4.0 Physiology of Beverage Alcohol - With the advent of
chemical intoxication evidence in the 1960s, and its continued refinement in
the twentieth century, drunk driving prosecution has essentially become an
exercise of providing and proving the number. Most jurors give great weight
to scientific evidence, and certainly this is true as it relates to the way
most jurors view chemical intoxication evidence. Because of this the
unreliability of this evidence often becomes central to the defense of drunk
driving cases.
A successful attack on this evidence requires that the defense attorney have at least a general knowledge of how beverage alcohol (ethanol) is absorbed and used by the human body. This is particularly true when retrograde extrapolation evidence is provided by the prosecution to further demonstrate the drivers blood alcohol level at the time of the driving. This knowledge will also inform both the direct and cross examination of any intoxication evidence experts.
4.0.1 Alcohol’s properties - alcohol is a general term
denoting a family of organic chemicals with common properties. Members of
this family include ethanol, methanol, isopropanol, and others. Alcohol
(ethanol) is a clear, volatile liquid that burns (oxidizes) easily. It has a
slight, characteristic odor and is very soluble in water. Alcohol is an
organic compound composed of carbon, oxygen, and hydrogen; its chemical
formula is C2H5OH. Alcohol is a central nervous system depressant and it is
the central nervous system which is the bodily system that is most severely
affected by alcohol (see chart below). The degree to which the central
nervous system function is impaired is directly proportional to the
concentration of alcohol in the blood.
When ingested, alcohol passes from the stomach into the small intestine, where it is rapidly absorbed into the blood and distributed throughout the body. Because it is distributed so quickly and thoroughly the alcohol can affect the central nervous system even in small concentrations. In low concentrations, alcohol reduces inhibitions. As blood alcohol concentration increases, a person's response to stimuli decreases markedly, speech becomes slurred, and he or she becomes unsteady and has trouble walking. With very high concentrations - greater than 0.35 grams/100 milliliters of blood (equivalent to 0.35 grams/210 liters of breath ) - a person can become comatose and die.
4.0.2 Absorption - Alcohol is absorbed from all parts of
the gastrointestinal tract largely by simple diffusion into the blood.
However the small intestine is by far the most efficient region of the
gastrointestinal tract for alcohol absorption because of its very large
surface area. In a fasting individual, it is generally agreed that 20% to
25% of a dose of alcohol is absorbed from the stomach and 75% to 80% is
absorbed from the small intestine. Because of this peak blood alcohol
concentrations are achieved in fasting people within 0.5 to 2.0 hours,
(average 0.75 - 1.35 hours depending upon dose and time of last meal) while
non-fasting people exhibit peak alcohol concentrations within 1.0, and in
extreme cases up to as much as 6.0 hours (average 1.06 - 2.12 hours).
4.0.3 Distribution - Alcohol has a high affinity for water and is therefore
found in body tissues and fluids inasmuch as they contain water. Absorbed
alcohol is rapidly carried throughout the body in the blood and once
absorption of alcohol is complete an equilibrium occurs such that blood at
all points in the system contains approximately the same concentration of
alcohol.
4.0.4 Elimination - the liver is responsible for the elimination - through metabolism - of 95% of ingested alcohol from the body. The remainder of the alcohol is eliminated through excretion of alcohol in breath, urine, sweat, feces, milk and saliva. The body uses several different metabolic pathways in its oxidation of alcohol to acetaldehyde to acetic acid to carbon dioxide and water. Healthy people metabolize alcohol at a fairly consistent rate. As a rule of thumb, a person will eliminate one average drink or .5 oz (15 ml) of alcohol per hour. Several factors influence this rate. The rate of elimination tends to be higher when the blood alcohol concentration in the body is very high or very low. Also chronic alcoholics may (depending on liver health) metabolize alcohol at a significantly higher rate than average. Finally, the body's ability to metabolize alcohol quickly tend to diminish with age.
4.0.5 Body Weight and Body Type - In general, the less you weigh the more you will be affected by a given amount of alcohol. As detailed above, alcohol has a high affinity for water. Basically one's blood alcohol concentration is a function of the total amount of alcohol in one's system divided by total body water. So for two individuals with similar body compositions and different weights, the larger individual will achieve lower alcohol concentrations than the smaller one if ingesting the same amount of alcohol. However, 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.
4.0.6 Rate of Consumption - Blood alcohol concentration depends on the amount of alcohol consumed and the rate at which the user's body metabolizes alcohol. Because the body metabolizes alcohol at a fairly constant rate (somewhat more quickly at higher and lower alcohol concentrations), ingesting alcohol at a rate higher than the rate of elimination results in a cumulative effect and an increasing blood alcohol concentration.
4.0.7 Alcohol Content - The concentration of the drinks that one ingest can have a slight effect on the peak alcohol concentration due to the differences in absorption rate of different concentrations of alcohol. Alcohol is most rapidly absorbed when the concentration of the drink is between 10% and 30%. Below 10% the concentration gradient in the gastrointestinal tract is low and slows absorption and the added volumes of liquid involved slow gastric emptying. On the other hand concentrations higher than 30% tend to irritate the mucous membranes of the gastrointestinal tract and the pyloric sphincter, causing increased secretion of mucous and delayed gastric emptying.
4.0.8 Food - Food taken along with alcohol results in a lower, delayed blood alcohol concentration peak (the point of greatest intoxication). There are two major factors involved in this phenomenon. First, because alcohol is absorbed most efficiently in the small intestine, the ingestion of food can slow down the absorption of alcohol into one's system. The pyloric valve at the bottom of the stomach will close in order to hold food in the stomach for digestion and thus keep the alcohol from reaching the small intestine. While alcohol will be absorbed from the stomach it is a slower and less efficient transition. Second and equally important is the fact that alcohol elimination rates are inversely proportional to alcohol concentration in the blood. Therefore the suppressed levels of alcohol due to food ingestion cause the body to eliminate the alcohol that is absorbed at a faster rate. The type of food ingested (carbohydrate, fat, protein) has not been shown to have a measurable influence on this affect but the larger the meal and closer in time between eating and drinking, the greater the diminution of peak alcohol concentration. Studies have shown reductions in peak alcohol concentration (as opposed to those of a fasting individual under otherwise similar circumstances) of 9% to 23%.
4.0.9 Fatigue - Fatigue causes many of the same symptoms that are caused by alcohol intoxication. These and other symptoms will be amplified if alcohol intoxication is concurrent with fatigue.
4.0.11 Tolerance - Tolerance is the diminution of the
effectiveness of a drug after a period of prolonged or heavy use of that
drug or a related drug (cross-tolerance). There are two types of tolerance
at work with alcohol. The first is metabolic tolerance in which the alcohol
is metabolized at a higher rate (up to 72% more quickly) in chronic users.
Because of the higher metabolic rate for alcohol lower peak blood alcohol
concentrations are achieved by chronic alcohol users than the average
drinker when the same amount of alcohol is ingested. The second is
functional tolerance in which there is an actual change in the organ or
system's sensitivity to the drug. Studies have shown that chronic alcohol
users can have twice the tolerance for alcohol as an average person. It is
important to note however that even in light of these tolerance factors, it
has been shown conclusively that even in heavy alcohol users functional
impairment is clearly measurable at the blood alcohol concentration levels
that are currently used for traffic law enforcement and safety sensitive job
performance.
4.0.12 Gender Differences - As outlined above in the section on Body Weight and Body Type different body types coincide with different body water percentages. In general, but by no means in all cases, women tend to have a higher percentage of body fat and thus a lower percentage of body water. Therefore, in general, if a man and a woman of the same weight ingest the same amount of alcohol the woman will tend to achieve a higher alcohol concentration. This, of course, would not be true if the woman was very fit and the man was somewhat obese, but on average, this is the case. Furthermore, total body water tends to decrease with age, so an older person will also be more affected by the same amount of alcohol
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