Bile Explained

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The material that goes from the stomach to the small intestine is known as chyme. Chyme contains food that has been partly digested as well as stomach acid, and so it’s very acidic and must be neutralised.

Bile is produced in the liver and stored in the gall bladder before being released into the small intestine at a section called the duodenum, where it mixes with chyme. It has two main functions: neutralisation, and emulsification.

Bile is alkaline (meaning it has a high pH) and so it neutralises the excess stomach acid, raising its pH. This is important because the enzymes in the small intestine cannot function in acidic conditions.

It’s a good time here to recap enzymes - these are proteins that help speed up chemical reactions in living organisms. In the case of digestion, they speed up the break down of large food molecules into smaller ones. For example there are enzymes that speed up the breakdown of lipid molecules into fatty acids and glycerol.

The second job of bile is emulsification; bile emulsifies lipids. This means it takes large lipid globules and breaks them down into smaller droplets. This provides a larger surface area for enzymes to work.

What does that mean? Picture a single large globule of lipid with a volume of 1cm3. I’ll spare you the maths and tell you that the total area of its surface would be about 4.8 cm2. Now imagine we split that volume in half, so we have two spheres, each with a volume of 0.5cm3 (but still totalling 1cm3). If we calculate the surface area of each of these, we’d find its about 3cm2 each, giving a total surface area of around 6cm2 . So if we split large lipid globules into many smaller ones, we provide more area for enzymes to break down lipids, speeding up digestion.

Just to recap, bile neutralises excess stomach acid, and it emulsifies lipids to increase the surface area for enzymes to work.
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