During muscular exercise, blood vessels in the muscles dilate and blood flow is increased in order to increase the available oxygen supply, to allow the muscles required to function properly. Up to a point, the available oxygen is sufficient to meet the energy needs of the body. However, when muscular exertion is very great, oxygen cannot be supplied to muscle fibres fast enough, and the aerobic breakdown of pyruvic acid cannot produce all the ATP required for further muscle contraction.
During such periods, additional ATP is generated by anaerobic respiration by glycolysis. In the process, most of the pyruvic acid produced is converted to lactic acid. This can only be done for a short time as the acid produced is toxic to the body, very quickly the muscles will begin to cramp and exercise cannot continue.
Ultimately, once adequate oxygen is available, lactic acid must be catabolised completely into carbon dioxide and water. After exercise has stopped, extra oxygen is......
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