What Happens to Lactic Acid After Exercise?
当乳酸在你的肌肉细胞内聚集时，它进入你的血液。你的肝脏吸收了血循环的乳酸。当你休息的时候，你的肝脏正忙着将乳酸氧化（需要氧气）成丙酮酸（pyruvic acid），通过一种叫做乳酸脱氢酶(lactic dehydrogenase)的酶催化反应。该酶利用从乳酸中去除的电子，将NAD的分子还原为NADH。丙酮酸通过转运体进入被称为线粒体的小的胶囊状结构，在那里它可能会遇到一对不同的命运。
三羧酸循环 (Citric Acid Cycle)
葡萄糖（Glucose) 转化为乳酸(Lactate)，乳酸转化为葡萄糖的一系列事件被称为Cori循环（Cori Cycle）。你的肌肉最终从葡萄糖分解和乳酸发酵获得的能量要比你的肝脏消耗乳酸，使乳酸重新转化为葡萄糖。因此，Cori循环需要净能量消耗。
What Happens to Lactic Acid After Exercise?
BY JOHN BRENNAN SEPT. 11, 2017
When your muscles can't get enough oxygen during a short burst of exercise, they start to make use of a pathway called lactic acid fermentation, which generates a small three-carbon compound called lactic acid or lactate as a byproduct of glucose breakdown. Lactic acid is not useful to your muscle cells, but your liver turns it back into glucose later on after exercise.
As lactic acid accumulates inside your muscle cells, it enters your bloodstream. Your liver soaks up the circulating lactate. Later on while you are resting, your liver is busy oxidizing the lactic acid to pyruvate through a reaction catalyzed by an enzyme called lactate dehydrogenase. The enzyme uses the electrons removed from lactate to reduce a molecule of NAD to NADH. Pyruvate enters small capsule-shaped structures called mitochondria via a transporter, where it may meet with one of a couple different fates.
Citric Acid Cycle
Inside the mitochondria, pyruvate may be converted to acetyl-CoA and CO2 by an enzyme called the pyruvate dehydrogenase complex. In this case, the acetyl-CoA will feed into a biochemical pathway called the citric acid cycle, and your liver cell will use the energy it extracts by oxidizing these carbons to store energy in the form of adenosine triphosphate or ATP. By so doing, however, the liver merely satisfies its own requirements and not those of other cells. The liver also needs to turn the lactic acid into glucose. It does so through a process called gluconeogenesis.
When lactic acid is abundant in your liver cells after exercise, the gluconeogenesis pathway is a little bit different from the one your liver employs at other times. It begins in the mitochondria, where an enzyme called pyruvate carboxylase adds a molecule of bicarbonate to pyruvate and converts it to oxaloacetate. This reaction requires energy expenditure in the form of a molecule of ATP. Next, another enzyme called mitochondrial PEP carboxykinase converts oxaloacetate into phosphoenolpyruvate or PEP and free carbon dioxide. This step also requires energy investment in the form of a molecule of GTP. The PEP produced by PEP carboxykinase is exported from the mitochondria and converted back to glucose through a series of nine enzyme-catalyzed reactions inside the cell.
The series of events by which glucose is converted into lactate and back again is called the Cori cycle. Your muscles ultimately gain less energy from glucose breakdown and lactic acid fermentation than your liver must expend to make the lactate back into glucose. Consequently, the Cori cycle entails a net energy loss. Your body makes use of it during intense workouts, when your bloodstream can't furnish your muscles with all the oxygen they need. At times like these, lactic acid fermentation becomes the only way your muscles can keep metabolizing glucose for fuel.
Limitations of the Cori Cycle
Using the Cori cycle, the human body is able to convert metabolic by products into a source of energy for the muscles. However, it cannot continue to do so infinitely.
Similar to many other natural cycles, the Cori cycle isn't a completely closed loop. In the muscles, glycolysis results in the production of two units of ATP. However, the liver uses up six units of ATP to carry out the process of gluconeogenesis. The Cori cycle also requires the initial introduction of oxygen, without which it cannot begin. As such, eventually, the muscles are bound to require a new supply of glucose as well as oxygen.
If a physical activity is too strenuous, the energy requirements of the muscles will exceed the capacity of the Cori cycle to regenerate glucose from lactate. This will result in a condition known as lactic acidosis, which is an accumulation of excess lactic acid in the system. Lactic acidosis brings down the pH level of the blood, which can lead to tissue damage. It also induces symptoms associated with panic, such as hyperventilation, abdominal cramps, vomiting, etc., all of which are the body's natural defense mechanisms designed to slow down the rigorous activity, and prevent permanent damage from occurring. A Brief Explanation of the Importance of Cori Cycle in Metabolism https://biologywise.com/brief-explanation-of-cori-cycle
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