So once we form our lactic acid, the lactic acid travels back into the liver. And inside the liver, the lactic acid undergoes a process known as the core cycle. In the core cycle, we basically use oxygen to transform our lactic acid back into Pyruvate. And the quantity of oxygen that we have to use to transform the lactic acid back into Pyruvate is known as the oxygen debt. So we see that the process of fermentation is the method by which we basically regenerate our NAD plus molecules from the NADH molecules, because we need our NAD plus molecules to basically undergo the process of glycolysis and produce our continual source of ATP. And our fermentation only takes place under anaerobic conditions.