As soon as the ethanol in an alcoholic beverage is absorbed into the blood, it finds its way to the brain and other organs, and begins its work as a depressant: compromising nerve cells' ability to transmit electrical pulses. There are nerve cells all throughout the body. Many are used to register senses -- nerve cells in the skin, on the tongue, in the eye, in the ear, and in the nose are the means by which we sense the world around us. Many more nerves are used to control muscles. The spinal cord is a center for nerves, the highway connecting the nerves for muscle control and senses, and the most important part of the human body, the brain. The brain is home to most of the nerves in your body. Here, through an as-of-yet undiscovered process, a mass of billions of simple cells is able to do cognitive thinking, processing of information, controlling of the body, and high-level decision making, just as millions of transistors thrown onto a chip can display to you the page you are reading now.
In this section we will discuss the general effects of alcohol on the body using the BAC convention discussed in the last section. In the next section we will examine the precise effects which make driving under alcohol's influence so dangerous. Though all nerves are affected by alcohol, the region hardest hit is the brain. Only at relatively high BAC's does the ethanol start to actually suppress the nerves used to sense pain -- at this point it can be used as an anaesthetic.
Beyond the effect on the brain, there are a few interesting effects that alcohol has on other parts of the body that are worth looking at.
Energy use. Alcoholic beverages tend to be high in calories but low in nutrients. These calories are readily available as energy for the body. Therefore, the body tends to take the nutrients and energy in other foods in the stomach during alcohol consumption and store them in body tissues for later use, while using alcohol's calories to power the body. Thus, the "beer belly" is typically a result of the foods eaten while drinking rather than the alcohol itself.
Circulatory system. The heart and circulation are also affected by alcohol. At lower doses, the heart rate and blood pressure may experience a slight increase. However, at higher levels of intoxication, a decrease of heart rate and blood pressure occurs, and irregularities in heart rhythym are common. In addition, alcohol tends to dilate blood vessels near the skin's surface, producing a reddening and a sense of warmth. Despite the sense of warmth, though, drinking actually decreases body temperature and increases heat loss. So drinking to warm up is not a good idea. You could freeze to death.
Urinary tract. Drinking causes increased urination, but not just because of the fluids being ingested. Alcohol stimulates the pituitary gland to release a hormone which reduces the kidneys' water retention. This effect is present only for rising BACs.
Blood sugar. In the presence of alcohol, the pancreas secretes an excess of insulin. As a result, sugars in the blood are broken down too quickly and hypoglycemia (low blood sugar) can result. Hypoglycemia is characterized by low energy, increased anxiety, and other psychological effects.