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Alcohol
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What is alcohol?
When we talk about "alcohol" we usually mean ethyl alcohol or ethanol (CH3CH2OH), which is a clear, colourless liquid with a characteristic spirituous odour and burning taste. Pure ethanol is lighter than water with a specific gravity of 0.7935. It is a small molecule (molecular weight 46.06) and mixes readily with water. These characteristics mean that alcohol passes readily into all parts of the body in direct proportion to the amount of water present.
Ethanol is produced in the brewing process by the fermentation of sugars. There are many other alcohols in brewed and distilled beverages of varying importance, but ethanol is the one in greatest concentration and is the prime intoxicant. Other alcohols and related compounds produced during fermentation and produce the unique flavours and aromas that characterise the beverage. These compounds are referred to as "congeners" and can be used to identify individual beverages.
The concentration of alcohol in beverages is usually expressed as a proportion of the volume (v/v). Commonly available drinks contain concentrations ranging less than one percent to more than fifty- percent (volume for volume).
The concentration of alcohol present in blood is expressed as grams of alcohol per 100 milliliters of blood. Concentrations measured indirectly (e.g. by breath or urine analysis) are frequently converted to the equivalent blood alcohol concentration (see breath analysis). For simplicity the phrase grams of alcohol per 100 milliliters of blood is often abbreviated to per cent (%) to improve readability.
Absorption and elimination
The level of alcohol in body fluids is dependent on three distinct processes - absorption, distribution, and elimination.
Absorption
Alcohol is absorbed into the blood stream from the gut. It travels to the liver, the right side of the heart, the lungs and then into the general circulation. It is rapidly distributed throughout the body water. The rate of absorption varies according to :
- The nature of the drink: carbonated drinks are absorbed faster, but very strong drinks sedate the stomach and are absorbed more slowly;
- The presence of food in the stomach which tends to delay absorption by delaying gastric emptying.
The peak level of alcohol in the blood stream is attained about forty-five minutes later. There is then an elimination phase during which the blood alcohol concentration is decreasing. In a person of average build (65-75 kilograms) a dose of 10 grams of alcohol will cause a rise in the blood alcohol concentration of 0.01% to 0.02%. This is the often called the "glass equivalent". Women will have a greater rise in blood alcohol concentration than a man of similar size, due to different body composition.
Distribution
Alcohol is mostly absorbed from the intestine. During passage throughout the circulation some alcohol will move in and out of the blood stream into the tissues until equilibrium is attained. Organs with a rich blood supply such as lungs, kidneys and brain will initially receive a proportionally higher amount of alcohol, and may have higher blood alcohol concentrations in the initial phase than tissues with a lower blood flow.
Metabolism and elimination
Alcohol is eliminated in two different ways, mostly by breakdown in the liver and other tissues but a small amount is excreted in breath, saliva, urine, faeces and sweat (~3%). The rate of elimination is not dependent on the concentration of alcohol present, but varies from 0.0103 to 0.026 gram per cent per hour in light to moderate drinkers and may be more than 0.27 grams % per hour in heavy drinkers.
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