In 1997, Americans drank an average of 2 gallons (7.57 liters) of alcohol per person. This translates roughly into one six-pack of beer, two glasses of wine and three or four mixed drinks per week (see this page for details). About 35 percent of adults don't consume alcohol, so the numbers are actually higher for those who do -- alcohol is an amazingly popular social phenomenon.
If you have ever seen a person who has had too much to drink, you know that alcohol is a drug that has widespread effects on the body, and the effects vary from person to person. People who drink might be the "life of the party" or they might become sad and droopy. Their speech may slur and they may have trouble walking. It all depends on the amount of alcohol consumed, a person's history with alcohol and a person's personality.
Even though you have seen the physical and behavioral changes, you might wonder exactly how alcohol works on the body to produce those effects. What is alcohol? How does the body process it? How does the chemistry of alcohol work on the chemistry of the brain? In this edition of HowStuffWorks, we will examine all of the ways in which alcohol affects the human body.
What is Alcohol?
In order to understand alcohol's effects on the body, it is helpful to understand the nature of alcohol as a chemical, so let's take a look...
Here are several facts:
Alcohol is a clear liquid at room temperature.
Alcohol is less dense and evaporates at a lower temperature than water (this property allows it to be distilled -- by heating a water and alcohol mixture, the alcohol evaporates first).
Alcohol dissolves easily in water.
Alcohol is flammable (so flammable that it can be used as a fuel).
Alcohol can be made by four different methods:
Fermentation of fruit or grain mixtures (See How Beer Works for details.)
Distillation of fermented fruit or grain mixtures (Spirits such as whiskey, rum, vodka and gin are distilled.)
Chemical modification of fossil fuels such as oil, natural gas or coal (industrial alcohol)
Chemical combination of hydrogen with carbon monoxide (methanol or wood alcohol)
The alcohol found in alcoholic beverages is ethyl alcohol (ethanol). The molecular structure of ethanol looks like this:
H3 C - C - O - H
In this structure, C is carbon, H is hydrogen, O is oxygen and the hyphens are the chemical bonds between the atoms. For purposes of clarity, the bonds between the three hydrogen atoms and the left carbon atom are not shown. The OH (O-H) group on the molecule is what gives it the specific chemical properties of an alcohol. For the remainder of this article, when we say "alcohol," we mean ethanol.
You will not find pure alcohol in most drinks; drinking pure alcohol can be deadly because it only takes a few ounces of pure alcohol to quickly raise the blood alcohol level into the danger zone. For various types of beverages, the ethanol concentration (by volume) is as follows:
Beer = 4 to 6 percent (average of about 4.5 percent)
Wine = 7 to 15 percent (average of about 11 percent)
Champagne = 8 to 14 percent (average of about 12 percent)
Most of the typical spirits purchased in liquor stores are 40 percent alcohol.
Some highly concentrated forms of rum and whisky (75 to 90 percent) can be purchased in liquor stores.
Some highly concentrated forms of whiskey (i.e. moonshine) can be made and/or purchased illegally.
In most U.S. states, you must be 21 years or older to buy alcoholic beverages, and there are penalties for serving or selling alcoholic beverages to minors.
How Alcohol Enters and Leaves the Body
When a person drinks an alcoholic beverage, about 20 percent of the alcohol is absorbed in the stomach and about 80 percent is absorbed in the small intestine. How fast the alcohol is absorbed depends upon several things:
The concentration of alcohol in the beverage - The greater the concentration, the faster the absorption.
The type of drink - Carbonated beverages tend to speed up the absorption of alcohol.
Whether the stomach is full or empty - Food slows down alcohol absorption.
Alcohol Effects: Men vs. Women
When you compare men and women of the same height, weight and build, men tend to have more muscle and less fat than women. Because muscle tissue has more water than fat tissue, a given dose or amount of alcohol will be diluted more in a man than in a woman. Therefore, the blood alcohol concentration resulting from that dose will be higher in a woman than in a man, and the woman will feel the effects of that dose of alcohol sooner than the man will.
After absorption, the alcohol enters the bloodstream and dissolves in the water of the blood. The blood carries the alcohol throughout the body. The alcohol from the blood then enters and dissolves in the water inside each tissue of the body (except fat tissue, as alcohol cannot dissolve in fat). Once inside the tissues, alcohol exerts its effects on the body. The observed effects depend directly on the blood alcohol concentration (BAC), which is related to the amount of alcohol consumed. The BAC can rise significantly within 20 minutes after having a drink.
Once absorbed by the bloodstream, the alcohol leaves the body in three ways:
The kidney eliminates 5 percent of alcohol in the urine.
The liver chemically breaks down the remaining alcohol into acetic acid.
As a rule of thumb, an average person can eliminate 0.5 oz (15 ml) of alcohol per hour. So, it would take approximately one hour to eliminate the alcohol from a 12 oz (355 ml) can of beer.
The Breakdown of Alcohol
The breakdown, or oxidation, of ethanol occurs in the liver. An enzyme in the liver called alcohol dehydrogenase strips electrons from ethanol to form acetaldehyde. Another enzyme, called aldehyde dehydrogenase, converts the acetaldehyde, in the presence of oxygen, to acetic acid, the main component in vinegar. The molecular structure of acetic acid looks like this:
H3 C - C - O - H
The || symbol is a double bond between the atoms. When ethanol is oxidized to acetic acid, two protons and two electrons are also produced. The acetic acid can be used to form fatty acids or can be further broken down into carbon dioxide and water.
The BAC increases when the body absorbs alcohol faster than it can eliminate it. So, because the body can only eliminate about one dose of alcohol per hour, drinking several drinks in an hour will increase your BAC much more than having one drink over a period of an hour or more.
The Effects of Alcohol
If you have seen someone who has had too much to drink, you've probably noticed definite changes in that person's performance and behavior. The body responds to alcohol in stages, which correspond to an increase in BAC:
Euphoria (BAC = 0.03 to 0.12 percent)
They become more self-confident or daring.
Their attention span shortens.
They may look flushed.
Their judgement is not as good -- they may say the first thought that comes to mind, rather than an appropriate comment for the given situation.
They have trouble with fine movements, such as writing or signing their name.
Excitement (BAC = 0.09 to 0.25 percent)
They become sleepy.
They have trouble understanding or remembering things (even recent events).
They do not react to situations as quickly (if they spill a drink they may just stare at it).
Their body movements are uncoordinated.
They begin to lose their balance easily.
Their vision becomes blurry.
They may have trouble sensing things (hearing, tasting, feeling, etc.).
Confusion (BAC = 0.18 to 0.30 percent)
They are confused -- might not know where they are or what they are doing.
They are dizzy and may stagger.
They may be highly emotional -- aggressive, withdrawn or overly affectionate.
They cannot see clearly.
They are sleepy.
They have slurred speech.
They have uncoordinated movements (trouble catching an object thrown to them).
They may not feel pain as readily as a sober person.
Stupor (BAC = 0.25 to 0.4 percent)
They can barely move at all.
They cannot respond to stimuli.
They cannot stand or walk.
They may vomit.
They may lapse in and out of consciousness.
Coma (BAC = 0.35 to 0.50 percent)
They are unconscious.
Their reflexes are depressed (i.e. their pupils do not respond appropriately to changes in light).
They feel cool (lower-than-normal body temperature).
Their breathing is slower and more shallow.
Their heart rate may slow.
They may die.
Death (BAC more than 0.50 percent) - The person usually stops breathing and dies.
How the Body Responds to Alcohol
Alcohol acts primarily on the nerve cells within the brain. Alcohol interferes with communication between nerve cells and all other cells, suppressing the activities of excitatory nerve pathways and increasing the activities of inhibitory nerve pathways.
How Nerve Cells Talk
Nerve cells talk to each other and to other cells (such as muscle or gland cells) by sending chemical messages. These messages are called neurotransmitters.
An electrical signal travels down one nerve cell, causing it to release the neurotransmitter into a small gap between cells called the synapse. The neurotransmitter travels across the gap, binds to a protein on the receiving cell membrane called a receptor, and causes a change (electrical, chemical or mechanical) in the receiving cell. The neurotransmitter and receptor are specific to each other, like a lock and key. Neurotransmitters can either excite the receiving cell to cause a response or inhibit the receiving cell from stimulation.
For example, this article talks about the ability of alcohol (and inhaled anesthetics) to enhance the effects of the neurotransmitter GABA, which is an inhibitory neurotransmitter. Enhancing an inhibitor would have the effect of making things sluggish, which matches the behavior you see in a drunk person. Glutamine is an excitatory neurotransmitter that alcohol weakens. By making this excitatory neurotransmitter less effective, you also get sluggishness. Alcohol does this by interacting with the receptors on the receiving cells in these pathways.
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.
The order in which alcohol affects the various brain centers is as follows:
Hypothalamus and pituitary gland
Medulla (brain stem)
Summary of alcohol's effects on the brain. Move your cursor over the colored bar in the lower left-hand corner to see which areas of the brain are affected by increasing BAC.
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.
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.
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 sexual behavior and urinary excretion.
Alcohol depresses the nerve centers in the hypothalamus that control sexual arousal and performance. As BAC increases, sexual behavior increases, but sexual performance declines. This observation has been known for a long time, and is referred to by William Shakespeare in "Macbeth" (Act 2 scene 3):
Macduff: What three things does drink especially promote? Porter: Marry sir, nose-painting, sleep, and urine. Lechery, sir it provokes, and unprovokes; it provokes the desire, but it takes away the performance...
The porter in the above excerpt also notes the effect of alcohol on 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.
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.
Alcohol's Effects on Other Body Systems
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").
All of alcohol's effects continue until the ingested alcohol is eliminated by the body.
In the United States, approximately 8 percent of people aged 18 and older suffer from alcohol abuse and/or dependence. This abuse or dependence costs upwards of $1.7 billion in medical treatment, lost earnings, casualty damages and criminal/legal costs.
Alcohol abuse has been a rising problem over the past three decades. With the continued exposure to alcohol, how does the human body respond or adapt? The body's increased tolerance to alcohol involves the following changes:
Increase in level of liver's enzymes that are used to break down alcohol
Increase in activity of brain and nervous-system neurons
These bodily adaptations change a person's behavior.
The levels of alcohol dehydrogenase and aldehyde dehydrogenase in the liver increase in response to long-term alcohol exposure. This means that the body becomes more efficient at eliminating the high levels of alcohol in the blood. However, it also means that the person must drink more alcohol to experience the same effects as before, which leads to more drinking and contributes to addiction.
The normal chemical and electrical functions of nerve cells increase to compensate for the inhibitory effects of alcohol exposure. This increased nerve activity helps people to function normally with higher BAC; however, it also makes them irritable when they are not drinking. Furthermore, the increased nerve activity may make them crave alcohol. Most certainly, the increased nerve activity contributes to hallucinations and convulsions (e.g. delirium tremens) when alcohol is withdrawn, and makes it difficult to overcome alcohol abuse and dependence.
In addition to these adaptations, there are many adverse physical effects that result from long-term exposure to alcohol:
The increased activity in the liver causes cell death and hardening of the tissue (cirrhosis of the liver).
The brain cells in various centers die, thereby reducing the total brain mass.
Stomach and intestinal ulcers can form because the constant alcohol use irritates and degrades the linings of these organs.
Blood pressure increases as the heart compensates for the initially reduced blood pressure caused by alcohol.
Male sex-cell (sperm) production decreases because of decreased sex-hormone secretion from the hypothalamus/pituitary and, possibly, direct effects of alcohol on the testes.
Because alcoholics lose balance and fall more often, they suffer more often from bruises and broken bones; this is especially true as they get older.
Finally, alcohol abuse and dependence cause emotional and social problems. Because alcohol affects emotional centers in the limbic system, alcoholics can become anxious, depressed and even suicidal. The emotional and physical effects of alcohol can contribute to marital and family problems, including domestic violence, as well as work-related problems, such as excessive absences and poor performance.
While alcoholism has devastating effects on a person's health and social environment, there are medical and psychological ways to treat the problem. See the links on the next page for more information.