When a runner experiences limited oxygen availability, such as during intense exercise, her body can still produce ATP (adenosine triphosphate) through anaerobic metabolism. Here’s how this process works:
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Anaerobic Glycolysis: In the absence of sufficient oxygen, the runner's muscles can rely on anaerobic glycolysis to generate ATP. This process breaks down glucose (derived from glycogen stored in muscles or obtained from blood) into pyruvate, producing a net gain of 2 ATP molecules per glucose molecule without using oxygen.
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Lactic Acid Production: Since aerobic respiration is not occurring due to the lack of oxygen, pyruvate is converted into lactate (or lactic acid). This process allows glycolysis to continue by regenerating NAD⁺, which is needed for the oxidation of glyceraldehyde-3-phosphate in the glycolytic pathway.
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Creatine Phosphate System: Additionally, ATP can be rapidly generated through the phosphocreatine system. Creatine phosphate (CP) stored in muscle cells can donate a phosphate group to ADP to form ATP. This process occurs quickly and provides a short burst of energy during high-intensity exercise.
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Short-Term Energy: It’s important to note that anaerobic metabolism is less efficient than aerobic metabolism in terms of ATP yield per glucose molecule. It can produce ATP quickly but is sustainable only for short periods (a couple of minutes) due to the accumulation of lactate and the depletion of glucose stores.
In summary, when oxygen is limited, the runner shifts to anaerobic pathways for ATP production, primarily through anaerobic glycolysis and the creatine phosphate system, allowing her to continue exercising even when oxygen supply is not sufficient for aerobic metabolism. However, prolonged reliance on these anaerobic processes can lead to fatigue as lactate accumulates and energy substrates become depleted.