When oxygen is not readily available in the blood, cells use fermentation as a means of anaerobic respiration. The process is used to generate NAD+, which can be used as a reactant in glycolysis to produce small amounts of ATP. Glycolysis still occurs in this environment, breaking glucose into pyruvate and producing two ATP per cycle.

Fermentation in humans converts pyruvate to lactate (lactic acid) and NADH to NAD+.

Fermentation is a form of anaerobic respiration where there is no oxygen available as the final electron acceptor to the electron transport chain. As such, pyruvate is reduced, yielding lactic acid and, in the presence of lactate decarboxylase, ethanol. However, the products of fermentation do not undergo the Krebs cycle nor electron transport chain. The purpose of fermentation is to regenerate NAD+ so that glycolysis can continue. The cell does this by transferring the electron from NADH to pyruvate. Fermentation is less efficient per glucose than is aerobic oxidation, generating a fraction of the ATP. Lactic acid and ethanol are actually quite toxic to the cell (in humans they are immediately sent to the liver to be detoxified).

Either ethanol or lactic acid can be produced through fermentation depending on the organism. Ethanol fermentation only occurs in bacteria and yeast. Lactic acid fermentation has been found to occur in multiple kingdoms.

Fermentation allows for the regeneration of which can allow for glycolysis to continue. Butanol is a four carbon alcohol and aspartic acid is an amino acid found in proteins.

Under anaerobic conditions, fermentation follows the process of glycolysis. While glycolysis is responsible for creating ATP, fermentation allows the body to regenerate the NAD+ that is reduced during glycolytic processes. This key step allows glycolysis to continue, and more ATP to be made.