Final answer:
During intense exercise, lactic acid diffuses from muscle fibers into the blood and is taken up by the liver to be converted back into glucose through the Cori Cycle.
Step-by-step explanation:
During high-intensity exercise, when oxygen levels are insufficient for aerobic metabolism, muscle fibers produce lactic acid as a byproduct of anaerobic glycolysis. This lactic acid diffuses out of the muscle fibers and into the bloodstream. In the liver, the process known as the Cori Cycle occurs, wherein the liver absorbs this lactate and begins converting it back into glucose, which can be used again by the muscles for energy. This series of reactions helps maintain energy supplies during heavy exercise and manage blood lactate levels.