The science of alcohol
The science of alcohol

Start this free course now. Just create an account and sign in. Enrol and complete the course for a free statement of participation or digital badge if available.

1.3 Ethanol metabolism

The liver is the key organ responsible for removing ethanol from the bloodstream. In fact, around 90% of the ethanol we ingest is converted into other chemicals as part of the metabolic chemical reactions that go on in the liver. Only about 10% is excreted unchanged in body fluids (mainly in breath and urine).

The liver receives three-quarters of its blood supply from the portal vein, which carries blood directly from the gut, and only one quarter from the general circulation (the oxygen-rich blood pumped out from the heart via the hepatic artery). This means that all of the blood from the portal vein, complete with any absorbed nutrients or other substances (ethanol, toxins, drugs etc), passes through the liver before reaching the general circulation. This means the liver gets first pass at processing foreign substances but it also means the liver is highly exposed to high concentrations of ethanol.

If you cast your mind back to Week 1, you saw the experiment showing the surprising amount of energy within the ethanol molecule. A very small amount was able to fire a homemade ‘rocket’ a considerable distance.

The body needs to be able to safely break down ethanol without such a violent release of energy. The human body does this in the liver through enzymatic breakdown of ethanol rather than combustion, in three stages:

  1. Ethanol is converted to acetaldehyde. We can abbreviate this first conversion using the chemical formula:

    C2H5OH (ethanol) becomes CH3CHO (acetaldehyde)

  2. The acetaldehyde is then converted to acetic acid:

    CH3CHO (acetaldehyde) becomes CH3COOH (acetic acid)

  3. The third step breaks the acetic acid down into carbon dioxide and water:

    CH3COOH (acetic acid) becomes CO2 (carbon dioxide) and H2O (water).

If we look at this as one entire reaction, the overall breakdown of ethanol into carbon dioxide and water can be shown by the following unbalanced equation.

cap c sub two times cap h sub five times cap o times cap h plus cap o sub two right arrow cap c times cap o sub two plus cap h sub two times cap o
(Eqn 1)
  • Can you balance this equation?

  • A balanced version of this equation is:

    cap c sub two times cap h sub five times cap o times cap h plus three times cap o sub two right arrow two times cap c times cap o sub two plus three times cap h sub two times cap o
  • What is familiar about this equation?

  • It is exactly the same as the reaction that occurs when ethanol is burnt in air. Unlike the combustion of ethanol in air which is a single fast reaction, the metabolism in the liver goes through the two intermediate stages of forming acetaldehyde and acetic acid.

This ‘slow’ breakdown is controlled by a series of enzymes. At this point, it would be useful to explain what an enzyme is.

Take your learning further

Making the decision to study can be a big step, which is why you'll want a trusted University. The Open University has 50 years’ experience delivering flexible learning and 170,000 students are studying with us right now. Take a look at all Open University courses.

If you are new to University-level study, we offer two introductory routes to our qualifications. You could either choose to start with an Access module, or a module which allows you to count your previous learning towards an Open University qualification. Read our guide on Where to take your learning next for more information.

Not ready for formal University study? Then browse over 1000 free courses on OpenLearn and sign up to our newsletter to hear about new free courses as they are released.

Every year, thousands of students decide to study with The Open University. With over 120 qualifications, we’ve got the right course for you.

Request an Open University prospectus371