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Lactic acid is an organic acid produced as a normal part of many metabolic processes in the body. It is an important component of milk. It is continuously produced when the body metabolizes glucose in the absence of oxygen. It is then either recombined with oxygen to form pyruvates to produce energy within a muscle cell, or is released into the bloodstream where it is processed by the liver to produce more glucose. In solution, it an lose a proton and in such form is referred to as lactate.

Lactic acid is also the acid that is directly responsible for tooth decay.

It is also an important component of Ringer's solution, a fluid given to trauma and burn victims that is superior in these situations to saline solution.

Lactic acid at Wikipedia