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Alcohol And The Human Body

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).

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.

Once absorbed, 96% of the alcohol is then metabolised by the liver through the oxidation of alcohol to acetaldehyde then to acetic acid and finally to carbon dioxide and water The remaining 4% is eliminated through the excretion of alcohol in breath, urine, sweat, feces, breast milk and saliva.

A healthy person will metabolise alcohol at a fairly consistent rate of 7ml to 12ml of alcohol per hour, with an average of about 10ml/hr (the figure we use for our BAC calculator).

How Alcohol Affects The Brain

Alcohol affects various centers in the brain, both higher and lower order. The centers are not equally affected by the same BAC -- the higher-order centers are more sensitive than the lower-order centers. As the BAC increases, more and more centers of the brain are affected.

  • Cerebral Cortex: The cerebral cortex is the highest portion of the brain. The cortex processes information from your senses, does your "thought" processing and consciousness (in combination with a structure called the basal ganglia), initiates most voluntary muscle movements and influences lower-order brain centers. In the cortex, alcohol does the following:

    • Depresses the behavioral inhibitory centers - The person becomes more talkative, more self-confident and less socially inhibited.
    • Slows down the processing of information from the senses - The person has trouble seeing, hearing, smelling, touching and tasting; also, the threshold for pain is raised.
    • Inhibits thought processes - The person does not use good judgement or think clearly.
    • These effects get more pronounced as the BAC increases.

  • Limbic System: The limbic system consists of areas of the brain called the hippocampus and septal area. The limbic system controls emotions and memory. As alcohol affects this system, the person is subject to exaggerated states of emotion (anger, aggressiveness, withdrawal) and memory loss.

  • Cerebellum: The cerebellum coordinates the movement of muscles. The brain impulses that begin muscle movement originate in the motor centers of the cerebral cortex and travel through the medulla and spinal cord to the muscles. As the nerve signals pass through the medulla, they are influenced by nerve impulses from the cerebellum. The cerebellum controls fine movements. For example, you can normally touch your finger to your nose in one smooth motion with your eyes closed; if your cerebellum were not functioning, the motion would be extremely shaky or jerky. As alcohol affects the cerebellum, muscle movements become uncoordinated. In addition to coordinating voluntary muscle movements, the cerebellum also coordinates the fine muscle movements involved in maintaining your balance. So, as alcohol affects the cerebellum, a person loses his or her balance frequently. At this stage, this person might be described as "falling down drunk."

  • Hypothalamus and Pituitary Gland: The hypothalamus is an area of the brain that controls and influences many automatic functions of the brain through actions on the medulla, and coordinates many chemical or endocrine functions (secretions of sex, thyroid and growth hormones) through chemical and nerve impulse actions on the pituitary gland. Alcohol has two noticeable effects on the hypothalamus and pituitary gland, which influence: and

    • Sexual Behavior: Alcohol depresses the nerve centers in the hypothalamus that control sexual arousal and performance. As BAC increases, sexual behavior increases, but sexual performance declines.
    • Urine Excretion: Alcohol inhibits the pituitary secretion of anti-diuretic hormone (ADH), which acts on the kidney to reabsorb water. Alcohol acts on the hypothalamus/pituitary to reduce the circulating levels of ADH. When ADH levels drop, the kidneys do not reabsorb as much water; consequently, the kidneys produce more urine.

  • Medulla: The medulla, or brain stem, controls or influences all of the bodily functions that you do not have to think about, like breathing, heart rate, temperature and consciousness. As alcohol starts to influence upper centers in the medulla, such as the reticular formation, a person will start to feel sleepy and may eventually become unconscious as BAC increases. If the BAC gets high enough to influence the breathing, heart rate and temperature centers, a person will breathe slowly or stop breathing altogether, and both blood pressure and body temperature will fall. These conditions can be fatal.

How Alcohol Affects Other Parts Of The Body

In addition to the brain, alcohol can affect other body tissues. It has the following effects on other systems in the body:

  • Irritates the linings of the stomach and intestine - This can lead to vomiting.
  • Increases blood flow to the stomach and intestines - This increases secretions by these organs, most notably stomach acid secretion.
  • Increases blood flow to the skin - This causes a person to sweat and look flushed. The sweating causes body heat to be lost, and the person's body temperature may actually fall below normal.
  • Reduces blood flow to muscles - This can lead to muscle aches, most notably when a person recovers from the alcohol (the "hangover").

Eliminating Alcohol From The Body

All of alcohol's effects continue until the ingested alcohol is eliminated by the body.

95% of ingested alcohol is metabolised by the liver through the oxidation of alcohol to acetaldehyde then to acetic acid and finally to carbon dioxide and water The remainder is eliminated through the excretion of alcohol in breath, urine, sweat, feces, breast milk and saliva.

Healthy people eliminate alcohol at a fairly consistent rate. As a rule of thumb, a person will eliminate between 7ml and 12ml of alcohol from their body per hour, with an average of about 10ml/hr (the figure we use for our BAC calculator).

The ability of the body to eliminate alcohol is affected by several factors, with most relating to the water content and fat content of the individual's body:

  • Weight: In general, the less you weigh the more you will be affected by a given amount of alcohol. Alcohol has a high affinity for water and one's blood alcohol concentration is a function of the total amount of alcohol in one's system divided by total body water. In 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.

  • Level Of Fitness: For people of the same weight, a well muscled individual will be less affected by the same volume of alcohol than someone with a higher percentage of fat. Fatty tissue does not contain very much water and will not absorb very much alcohol, hence more alcohol makes its way into the bloodstream.

  • Gender: Generally women tend to have a higher percentage of body fat, and thus a lower percentage of body water, and 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. The differences in alcohol concentration due to average body composition differences based on gender would be between 16% and 10% depending on age.

    Although not completely supported by clinical studies, another gender based difference is in the elimination of alcohol. Studies appear to show that women eliminate alcohol from their bodies at a rate 10% greater than that of men which may counteract gender differences in body composition.

  • Age: Total body water also tends to decrease with age, so an older person will be more affected by the same amount of alcohol. Blood alcohol concentrations may be up to 10% higher in a 60 year old individual compared to a 30 year old individual where their body compositions are similar.

  • Exercise: Strenuous exercise increases the body's metabolic rate on the one hand, while associated increases in breathing rates and air volumes increase the flow of alcohol across the lung membranes, caused the alcohol to be expelled through breathing at a greater rate. In one study the blood alcohol content of subjects before and after running up a flight of stairs decreased 11-14% after one trip, and 22-25% after two such trips. In another study, a 15% decrease in blood alcohol was reported in subjects following vigorous exercise or hyperventilation.

  • 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.

Other factors that effect elimination and metabolism rates

  • Alcohol Concentration: It's not how many drinks that you have, but how much alcohol that consume. 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.

  • 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%.

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