How is exercise oxygen debt repaid?

During the period of resting, the breathing rate continues to be fast for a while. This provide sufficient oxygen to repay back the oxygen debt.

In the liver, some of the lactic acid is oxidized to produce energy. This energy is used up to convert the lactic acid back to glucose. Lactic acid is toxic and cannot be stored or kept in our body for too long. When lactic acid is used up, oxygen debt is being paid.