Physiology 101The expression "lactic acid" is used most commonly by athletes to describe the intense pain felt during exhaustive exercise, especially in events like the 400 metres and 800 metres. When energy is required to perform exercise it is supplied from the breakdown of Adenosine Triphosphate (ATP). The body has a limited store of about 85 grms of ATP and would use it up very quickly if we did not have ways of resynthesising it. There are three systems that produce energy to resynthesise ATP: ATP-PC, lactic acid and aerobic.
The lactic acid system is capable of releasing energy to resynthesise ATP without the involvement of oxygen and is called anaerobic glycolysis. Glycolysis (breakdown of carbohydrates) results in the formation of pyruvic acid and hydrogen ions (H+). A build up of H+ will make the muscle cells acidic and interfere with their operation so carrier molecules, called nicotinamide adenine dinucleotide (NAD+), remove the H+. The NAD+ is reduced to NADH which deposit the H+ at the electron transport gate (ETC) in the mitrochondria to be combined with oxygen to form water (H2O).
If there is insufficient oxygen then NADH cannot release the H+ and they build up in the cell. To prevent the rise in acidity pyruvic acid accepts H+ forming lactic acid which then dissociates into lactate and H+. Some of the lactate diffuses into the blood stream and takes some H+ with it as a way of reducing the H+ concentration in the muscle cell. The normal pH of the muscle cell is 7.1 but if the build up of H+ continues and pH is reduced to around 6.5 then muscle contraction may be impaired and the low pH will stimulate the free nerve endings in the muscle resulting in the perception of pain (the burn). This point is often measured as the lactic threshold or anaerobic threshold or onset of blood lactate accumulation (OBLA).
The process of lactic acid removal takes approx. one hour, but this can be accelerated by undertaking an appropriate warm down which ensures a rapid and continuous supply of oxygen to the muscles.
The normal amount of lactic acid circulating in the blood is about 1 to 2 millimoles/litre of blood. The onset of blood lactate accumulation (OBLA) occurs between 2 and 4 millimoles/litre of blood. In non athletes this point is about 50% to 60% VO2 max and in trained athletes around 70% to 80% VO2 max.
Lactic acid (lactate) is not:
• responsible for the burn in the leg muscles when exercising very fast
• responsible for the soreness you experience in the 48 hours following a hard session
• a waste productLactate, which is produced by the body all day long, is resynthesized by the liver (Cori Cycle) to form glucose which provides you with more energy. Sounds like a friend to me.
Lactate Shuttle
The lactate shuttle involves the following series of events:
• As we exercise pyruvate is formed
• When insufficient oxygen is available to breakdown the pyruvate then lactate is produced
• Lactate enters the surrounding muscle cells, tissue and blood
• The muscle cells and tissues receiving the lactate either breakdown the lactate to fuel (ATP) for immediate use or use it in the creation of glycogen
• The glycogen then remains in the cells until energy is required
65% of lactic acid is converted to carbon dioxide and water, 20% into glycogen, 10% into protein and 5% into glucose
Read more here:
Essentials of Exercise Physiology (2nd Edition) - W.D. McArdle et al - page 108
Principals of Anatomy and Physiology (6th Edition) - G. J. Tortora & N. P. Anagnostakos - page 241
Disposal of Lactate during and after Strenuous Exercise in Humans, Journal of Applied Physiology, Vol 61(1), pp338-343, 1986
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