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.

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