Lactic acid is a naturally occurring hydroxy-carboxylic acid. It occurs in many acidic fermentation products, e.g., silage, sour milk, sauerkraut, and pickles, and also has numerous commercial uses such as in drug preparation, acidification of wine for vinegar production, preparation of foods, plastics, and lacquers.
In physiology, lactic acid is produced as the end product of anaerobic respiration or glycolysis in muscle. Anaerobic respiration is generally defined as respiration in the complete absence of oxygen, but this is not the case for muscles. They are able to carry out anaerobic respiration even when oxygen is present. In skeletal muscle, respiration is normally aerobic and oxygen is used to metabolise glucose. As the skeletal muscle contracts during exercise, the rate of aerobic respiration increases so as to provide more energy for muscle contraction. This is facilitated by a concomitant increase in the supply of oxygen through the blood due to a greater ventilation and cardiac output. Glucose is taken up more rapidly and converted to pyruvate by enzymes of aerobic glycolysis and then to carbon dioxide and water by the enzymes of the Krebs cycle in mitochondria. The glycolytic breakdown of glucose to pyruvate yields a small amount of ATP while mitochondrial oxidation yields a great deal more.
As aerobic respiration reaches its maximum, as determined by the rate of oxygen supply to the skeletal muscle, anaerobic respiration is also mobilized simultaneously, to provide an additional energy source for maximum muscular exertion. Thus more ATP is provided for muscular contraction than is possible by aerobic respiration alone. The mobilization of anaerobic respiration in this way allows a rapid production of ATP within a short period for maximum muscular exertion. Under this situation, the muscular tissue is actually consuming a much greater amount of oxygen than at rest, and a large amount of glucose is oxidized aerobically. Thus anaerobic respiration takes place in the skeletal muscle even when there is an increased supply of oxygen. The products of anaerobic respiration in muscle are water and lactic acid. If allowed to accumulate in the body over long periods, lactic acid becomes toxic so anaerobic respiration can only be carried out for brief periods, after which the lactic acid has to be converted back to glucose or glycogen, or changed to carbon dioxide through aerobic respiration. Nevertheless, the maximum rate of muscular exertion made possible by the simultaneous occurrence of aerobic and anaerobic respiration is most essential for the survival of an animal at critical moments when an extra energy is needed for activity.
Lactic acid is utilized for the reformation of glucose in the process of gluconeogenesis and this occurs in the liver. There, lactic acid is converted first back to pyruvate and then through the reverse steps of glycolysis back to glucose. The events encompassing the production of lactic acid in the muscle, its conversion to glucose in the liver and the return of the glucose to the muscle with the eventual reformation of lactic acid constitutes the Cori cycle.
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