So in certain cells, including human cells, bacterial cells as well as fungi cells when oxygen supply runs low, for example during some type of vigorous exercise too many Pyruvate molecules and too many NADH molecules are produced. And because not all pyruvates and NADH molecules can be placed through the citric acid cycle and the electron transport chain at the same exact time we have a build up of NADH molecules that takes place. And so we have a very little amount of NAD plus present in the cell and we need NAD plus for glycolysis to take place. So what the cell actually does is it undergoes this lactic acid fermentation process in which a special enzyme known as lactate dehydrogenase transforms our Pyrulbate. It reduces the Pyrulbate into lactic acid as well as forming our oxidized version of NADH, our NAD plus. And this NAD plus can then be placed into the glycolysis cycle to produce our continual source of ATP.