(Brent S. Rushall)

Acidosis: a condition in which blood hydrogen ion concentration is higher than normal. During exhaustive exercise, large amounts of lactate accumulate in both muscle and blood and the corresponding accumulation of H+ ions produces metabolic acidosis. Acidosis in muscle at exhaustion has been thought to be an important factor in the development of fatigue. Acidosis, marked by a decrease in pH, has an adverse effect on respiration, metabolism, and other functions.

Enzyme: any of numerous complex proteins that are produced by living cells and catalyze specific biochemical reactions at body temperature. Specific proteins, the enzymes, control all biochemical reactions. They are responsible for accelerating chemical reactions in cells. Enzymes are generally named by adding the suffix "-ase" to the name of the substrate or to the type of reaction catalyzed by the enzyme. For example, the enzyme lactic dehydrogenase catalyzes the removal of hydrogen atoms from lactic acid.

Fatigue: one way of defining fatigue is to say that fatigue is the failure to generate required or anticipated force during muscular exercise. When skeletal muscle is contracting and ATP production increases, the breakdown of glucose occurs too rapidly for the blood to supply oxygen for pyruvic acid to be completely catabolized into carbon dioxide and water. The pyruvic acid is then transformed into lactic acid that accumulates in muscle tissue and body fluids eventually lowering the pH to a level incompatible with cellular function. Work becomes subjectively more strenuous as the accumulation of lactic acid in muscle and blood is associated with the feeling of muscle fatigue. Usually, exercise requiring the steady expenditure of 20 of more kilocalories (Kcal) per minute for periods greater than 90 minutes will deplete the normal supply of stored glycogen and give rise to a pronounced sense of fatigue, known to endurance athletes as "the wall".

Glumatic oxaloacetic transminase (GOT): an enzyme normally present in serum and various tissues, especially the heart and liver. It is released into the serum because of tissue injury and is present in increased concentration in myocardial infarction or acute damage to liver cells.

Glumatic pyruvic transminase (GPT): an enzyme present in the body, especially the liver, observed in higher concentration in the serum of patients with acute damage to liver cells.

Glycolysis: glycolysis is a sequence of reactions resulting from the action of enzymes that leads from a molecule of glucose (sugar) to two molecules of either pyruvic acid or lactic acid. If glycolysis occurs in the absence of oxygen, it is called anaerobic. If it occurs in the presence of oxygen, it is called aerobic. Glycolysis is a form of catabolic metabolism.

Homeostasis: An organism is said to be in homeostasis when its internal environment: (1) contains the appropriate physiological ranges of gases, nutrients, and ions; (2) has an optimal temperature; and (3) has an optimal pressure for the health of all its cells. Innumerable control mechanisms are at work in cells and tissues to maintain static or constant conditions in the internal environment. Through these a pathway can be activated as needed or can be inhibited as products accumulate.

Internal environment: The body fluid that bathes the body cells is termed the extra-cellular fluid. Essentially, the same fluid environment surrounds all body cells; and for this reason, extra-cellular fluid is often called the internal environment of the body. Cells are automatons that are capable of living, growing, and performing their functions so long as the proper concentrations of oxygen, glucose, the different electrolytes, amino acids, and fatty substances are available.

Lactate: (a salt of lactic acid) is the negatively charged ion formed when lactic acid is in solution. Carbohydrate lactate is a metabolite (product of metabolism) which can be produced in large amounts by exercising skeletal muscle.

Lactic acid: A compound formed in the body in anaerobic metabolism of carbohydrate. If there is a shortage of oxygen, the pyruvic acid resulting from glycolysis is transformed into lactic acid instead of providing energy to the cells. When an adequate supply of oxygen becomes available, the chemical reaction for formulation of lactic acid reverses itself, the lactic acid again becoming pyruvic acid. Lactic acid that forms during anaerobic glycolysis does not become lost to the body for when sufficient oxygen is again available it is recycled into the aerobic metabolic pathway or is used directly for energy. If the body did not have a homeostatic mechanism for reducing the amount of lactic acid, the extra-cellular fluid would become so acidic that it would destroy cells and eventually the entire body.

Metabolism: the chemical changes in living cells by which energy is provided for vital processes and activities and new material is assimilated to repair the waste. If the metabolic process is breaking down compounds, it is called catabolic metabolism or catabolism. If it is building up compounds, it is called anabolic metabolism, or anabolism.

Pyruvic acid: a compound formed in the body in aerobic metabolism of a carbohydrate. When oxygen is absent or in short supply, pyruvic acid can be converted into lactic acid. Otherwise, pyruvate is directed into the mitochondria of cells for oxidative metabolism.

RER: respiratory exchange ratio (RER).

Serum: The clear, straw-colored, liquid portion of plasma that does not contain fibrinogen. It remains fluid after blood clots.

Stress: stress is any stimulus that creates an imbalance in the internal environment. It may come from the external environment in many forms such as heat, cold, loud noises, or lack of oxygen. It may originate within the body as for example, high blood pressure, pain, tumors, or even unpleasant thoughts. Most muscular exercise creates temporary chemical and temperature imbalances in the internal environment. Homeostasis in all organisms is disturbed by stress.

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