Alcohol in the body

Alcohol is classed as a ‘sedative hypnotic’ drug,[2] which means it acts to depress the central nervous system at high doses. At lower doses, alcohol can act as a stimulant,[3] inducing feelings of euphoria and talkativeness, but drinking too much alcohol at one session can lead to drowsiness, respiratory depression (where breathing becomes slow, shallow or stops entirely), coma or even death.[4-6]

As well as its acute and potentially lethal sedative effect at high doses, alcohol has effects on every organ in the body, and these effects depend on the blood alcohol concentration (BAC) over time.[7]

After a drink is swallowed, the alcohol is rapidly absorbed into the blood (20 percent through the stomach and 80 percent through the small intestine), with effects felt within 5-10 minutes after drinking.[6] It usually peaks in the blood after 30-90 minutes,[6] and thus is carried through all the organs of the body.

Most (90%) of the metabolism (or breaking down) of alcohol from a toxic substance to water and carbon dioxide is performed by the liver,[6] with the rest excreted through the lungs (allowing alcohol breath tests), the kidneys and in sweat.[8]

The liver can only break down a certain amount of alcohol per hour, which for an average person is around one standard drinkA drink containing 10 grams of alcohol. (which raises the BAC about 15-20 mg/dLmilligrams of alcohol per 100 millilitres of blood [8] – the current limit for driving in New Zealand is 80 mg/dL).

The BAC rises, and the feeling of drunkenness occurs, when alcohol is drunk faster than the liver can break it down. The table below shows the relationship between BAC and symptoms of drunkenness – the higher the BAC, the greater the effects on the body. However, BAC does not correlate exactly with symptoms of drunkenness and different people have different symptoms even after drinking the same amount of alcohol. The BAC level, and every individual’s reaction to alcohol, is influenced by: [1, 2, 7]

• the ability of the liver to metabolise alcohol (which varies due to genetic differences in the liver enzymes that break down alcohol)[7]
• the presence or absence of food in the stomach (food dilutes the alcohol and dramatically slows its absorption into the bloodstream by preventing it from passing quickly into the small intestine)
• the concentration of alcohol in the beverage (highly concentrated beverages such as spirits are more quickly absorbed)
• how quickly alcohol is drunk
• body type (heavier and more muscular people have more fat and muscle to absorb the alcohol)[2]
• age, sex, ethnicity (e.g. women have a higher BAC after drinking the same amount of alcohol than men due to differences in metabolism and absorption – since men have on average more fluid in their body to distribute alcohol around than women do; some ethnic groups have different levels of a liver enzyme responsible for the break down of alcohol)
• how frequently a person drinks alcohol (someone who drinks often can tolerate the sedating effects of alcohol more than someone who does not regularly drink)[6]

Symptoms of drunkenness at different levels of blood alcohol concentration (BAC)

Adapted from: Table 163.1 in Brust, J. C. M. (2005). Alcoholism. In L. P. Rowland (Ed.), Merritt’s neurology (11th ed.). Philadelphia: Lippincott Williams and Wilkins.and Table 1 in Vonghia, L., Leggio, L., Ferrulli, A., Bertini, M., Gasbarrini, G., Addolorato, G., et al. (2008). Acute alcohol intoxication. European Journal of Internal Medicine, 19(8), 561-567.