Lactic acid

• Lactic acid is the waste product produced during anaerobic respiration.
• Running fast can lead to a build-up of lactic acid in your muscles, causing cramp.
• Lactic acid also causes tooth decay.

Cells get the energy they need from the chemical reactions of glucose. When you run fast there is a chemical reaction called anaerobic respiration. This reaction transfers energy from glucose to your cells without oxygen. There is just one waste product called lactic acid. Lactic acid is a molecule with the formula C₃H₆O₃. A molecule of lactic acid contains atoms of carbon, hydrogen and oxygen.

When you run fast lactic acid builds up in the muscles, causing painful cramps. After running you continue to breathe quickly. The extra oxygen you breathe in reacts with the lactic acid in your muscles, breaking it down to make carbon dioxide and water. As the lactic acid breaks down the cramps will begin to disappear.

Lactic acid is also made in the mouth, where specialised bacteria convert glucose and other sugars to lactic acid. Lactic acid in your mouth can cause tooth decay.