Lactic acid is a chemical produced naturally in the body. Production is increased with decreased oxygen availability to cells. The body responds to lactic acid in a number of ways, and the level of lactic acid can be useful for monitoring the metabolism and condition of a sick person.
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Glycolysis and Krebs Cycle
Energy metabolism is a complicated network of biochemical pathways in which molecules are built up and broken down. One of the main energy carriers in cells is adenosine triphosphate, or ATP. One of the primary fuels for making ATP is the blood sugar glucose. Glucose is metabolized into the the chemical pyruvate, which is further broken down into carbon dioxide in the Krebs cycle, and more ATP is made. Alternatively, pyruvate is converted into lactic acid.
Some cells such as red blood cells can break down glucose but cannot utilize the Krebs cycle, so they make lactic acid instead. Other times, cells make lactic acid when their capacity to use pyruvate in the Krebs cycle is overwhelmed, such as when cells break down glucose rapidly to provide fuel. The lactic acid from these sources is excreted into the blood, raising the blood level. Lactic acid can then migrate to tissues that can use it; the heart, liver and kidneys use the extra lactate for fuel, breaking it down into carbon dioxide. The liver and kidneys can also use it to produce glucose. This cycle of tissues producing lactic acid and other tissues using it is called the Cori cycle.
High lactic acid in the blood is called hyperlactatemia. It can occur, for example, during exercise when the muscles are working hard. Hard-working muscles require a lot of fuel, so they break down glucose to make ATP for muscle contractions. Lactic acid production overwhelms the ability of the muscles' cells to use it for the Krebs cycle, so it undergoes the Cori cycle to produce fuel for other organs.
Normally lactic acid in the blood does not make the blood acidic. However, a lot of lactic acid can do so, leading to what's called lactic acidosis. This condition is associated with poor outcomes in conditions such as shock because the tissues are not receiving enough oxygen, which in turn increases the body's production of lactic acid. However, lactic acid is not a cause of dysfunction; it is a symptom. Hyperlactatemia also does not cause disease within the body; it is a sign of dysfunction, not a disease itself.