Final answer:
During intense exercise, the absence of oxygen leads to anaerobic glycolysis in muscle cells, resulting in the production and accumulation of lactate. This allows glycolysis to continue and provides ATP to the muscle cells.
Step-by-step explanation:
During intense exercise when oxygen is not readily available, muscle cells undergo anaerobic glycolysis, a process that converts glucose into pyruvate. In the absence of oxygen, pyruvate is converted into lactate, resulting in an increase in lactate levels in the blood. This increase in lactate levels occurs because the rate of glycolysis exceeds the rate at which pyruvate can be metabolized by the Krebs cycle. Therefore, the production of lactate allows glycolysis to continue, supplying ATP to the muscle cells even without oxygen.