More energy can be obtained from pyruvate by degrading it all the way to C0₂ in the citric acid cycle:

The citric acid cycle, which consists of roughly as many successive reactions as glycolysis, is a more recent metabolic invention, found only in organisms that respire and oxidize their foods to completion. It also is physically segregated from the earlier steps: Glycolysis is carried out in the cell cytoplasm, but the steps of the citric acid cycle take place inside the mitochondria. Much more energy is saved in this process: 30 ATP are formed, storing 30 X 7.3 kcal = 219 kcal of free energy per mole of glucose.

If the oxygen supply is ample, then the process just outlined takes place. Glucose is degraded to pyruvate during glycolysis, and pyruvate is broken down to C0₂ in the citric acid cycle, with a yield of 38 molecules of ATP per molecule of glucose.

However, if oxygen is in short supply, then in human muscle the pyruvate is reduced to lactate, using up all of the NADH from glycolysis:

The overall reaction of converting one glucose molecule to two of lactate is not an oxidation at all, but only a rearrangement and cleavage: