Final answer:
When exercise outpaces oxygen supply, the body switches to anaerobic respiration, specifically lactic acid fermentation, which produces ATP less efficiently but allows continued muscle function.
Step-by-step explanation:
When there is not enough oxygen to support exercise, the body switches from aerobic respiration to anaerobic respiration, specifically a process called anaerobic glycolysis or lactic acid fermentation. During this anaerobic process, glucose can still be used to produce ATP but in a much less efficient manner. Unlike aerobic respiration, which generates approximately 36 ATP per molecule of glucose, anaerobic respiration produces only a small amount (2 ATP per molecule of glucose). Despite being less efficient, anaerobic glycolysis is faster, supplying ATP when aerobic respiration cannot keep up with the energy demand due to a lack of oxygen.
During high-intensity exercise, muscles consume ATP more quickly than oxygen can be supplied, forcing them to rely on anaerobic respiration. Here, pyruvate is converted into lactic acid, which not only helps in generating an additional ATP but also allows the glycolysis cycle to continue by oxidizing NADH to NAD+. This is critical because it provides the NAD+ necessary for the continuation of glycolysis, albeit at a lower energy yield. Lactic acid then diffuses into the bloodstream, transported to the liver, and eventually reconverted into pyruvate or glucose through the Cori cycle.
In summary, in the absence of sufficient oxygen, aerobic respiration is bypassed for anaerobic glycolysis, allowing prolonged energy generation, though less efficiently, via lactic acid fermentation, which sustains muscle function during intense physical activities.