Final answer:
When oxygen levels are insufficient during intense exercise, pyruvic acid is converted to lactic acid to continue ATP production through glycolysis, and this lactic acid can be processed back into glucose in the liver when oxygen is available.
Step-by-step explanation:
During vigorous exercise, when sufficient oxygen is not available to complete the breakdown of pyruvic acid through aerobic respiration, the body resorts to an alternative pathway. Here, pyruvic acid is converted into lactic acid. This conversion is crucial as it enables the recycling of NAD+ from NADH, which is necessary for glycolysis to continue producing ATP, thus supplying energy for muscle activity. The lactic acid produced can later be transported to the liver and converted back to glucose via the Cori cycle when oxygen becomes available again.
This is because oxygen is needed for aerobic respiration to occur, but in the absence of sufficient oxygen, anaerobic respiration takes place, resulting in the conversion of pyruvic acid to lactic acid. This process is important in supplying energy to the muscles when oxygen cannot be delivered fast enough, but it also contributes to muscle fatigue.