When a muscle exercises, it tends to build up lactic acid, because at some point, the muscle cannot get enough oxygen to activate other, more energy-efficient production systems. Athletes often blame lactic acid, also known as lactate, for their tired muscles, but the background to lactic acid production is more complex than that. This molecule has many different functions, some of which are actually beneficial to performance.
During exercise, as oxygen continues to be unavailable and lactic acid keeps building up, more and more lactic acid moves freely out of the cell that is accumulating it and into the blood. From the blood, the lactic acid travels into other cells, but primarily into liver cells, where it is converted into other molecules that are useful for metabolism.
Recycling for Energy
Lactic acid is produced from glucose metabolism when there is no oxygen available. When oxygen returns after an anaerobic period, the body converts most of the lactic acid back into more useful substances. In fact, according to the "Guyton and Hall Textbook of Medical Physiology," about 80 percent of lactic acid gets turned back into glucose with the help of oxygen and energy. The remainder is changed into pyruvate, which goes into the Krebs Cycle, which uses it to make more energy. Most of this occurs in the liver, with a minority happening in other cells.
According to Auburn University scientist L. B. Gladden in a 2004 article in the "Journal of Physiology," lactic acid was historically thought to be a cause of muscle fatigue, but many researchers think it is the H+ ion (a positively charged hydrogen atom) dissociating, or falling off, the lactic acid molecule that causes the fatigue. The definite cause of muscle fatigue is not yet known.
The heart is very good at using lactic acid to make pyruvate for energy, says the "Guyton and Hall Textbook of Medical Physiology." This is especially useful when a person exercises very hard because the blood is full of lactic acid and the heart can harness it to keep its own muscle cells going.
Lactic acid is, as it's name suggests, an acid. A buildup of acid can be dangerous for the body, which normally operates only within a safe limit of pH 7.35 to pH 7.45, as measured by an arterial blood sample. A pH lower than 7.35 in the blood is called "lactic acidemia," and this can occur when blood levels of lactic acid reach 5 millimoles per liter of blood. Lactic acidosis is associated with people who have certain serious medical conditions, though.
- BBC: Aerobic and Anaerobic Respiration
- Guyton and Hall Textbook of Medical Physiology; John E. Hall PhD.
- Medscape: Lactic Acidosis
- The Journal of Physiology: Lactate Metabolism: A New Paradigm for the Third Millennium
- Arterial Blood Gases Made Easy; Iain A.M. Hennessey and Alan G. Japp
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