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The science of alcohol
The science of alcohol

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1.4 Ethanol excretion

You have already learned that 90% of ethanol is removed from the bloodstream via the liver, while the remaining 10% is lost elsewhere. Around half of ethanol is removed from the body in breath and most of the rest is excreted in urine. In order for this to happen it must first pass through the kidneys.

  • Why is the loss by breath important?

  • Ethanol vaporises easily. Not only can we smell alcohol on someone’s breath when they have consumed an alcoholic drink, but this vapour can also be measured. Given we know roughly how much percentage-wise can be lost through the lungs, a breathalyser can not only tell that somebody has been drinking but also suggest an amount that they have had. You will learn more about how a breathalyser works in Week 8.

The kidneys are responsible for excretion of waste substances from the blood through the production of urine. ‘Excretion’ specifically means the separation of waste products of metabolism from the blood (as distinct from ‘egestion’ which refers to the ejection of undigested waste from the digestive system).

Drinking alcoholic beverages has a familiar and characteristic effect on this process – the kidneys excrete more urine than would be predicted from the volume of liquid consumed, a phenomenon familiar to all alcohol drinkers! The performance of the kidneys and controlling the amount of water and other molecules that are excreted is regulated by a substance called antidiuretic hormone (ADH) which is sometimes also referred to as vasopressin. Ethanol inhibits the release of antidiuretic hormone, thus causing an increase in the amount of water that is lost in the urine. This in turn causes a disproportionate loss of water from the body leading to dehydration, a well-known acute effect of ethanol consumption, which in turn creates a thirst for more drinks!