Lactic acid is a by-product, produced by the body's tissues, primarily muscle, in order to obtain energy by metabolising glucose in the absence of oxygen. The glucose is either supplied from the blood stream or from it's stored form, glycogen, in the muscles. This is called anaerobic respiration (without oxygen).
*Lactic acid (in large amounts) is usually formed during strenuous exercise, and a large amount of lactic acid in muscle leads to fatigue and can cause muscle cramps.
*Lactic acid also causes the souring of milk, by fermentation of lactose. This makes Lactic acid useful commercially, as it is used in preparing cheese, sauerkraut, soft drinks, and various other food products.
Oxygen System
A.T.P-CP System
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