Lactic acid fermentation is the most common fermentation process carried out by animals. In this process, the organic molecule acting as the electron acceptor is pyruvate. Its reduction to lactate, or lactic acid, is accompanied by the oxidation of NADH to NAD+ (see image below). This serves the purpose of recycling the NAD+, making it available for the glycolytic pathway.
Pyruvic acid + NADH → lactic acid + NAD+
Lactic acid fermentation commonly occurs in muscle cells that have run out of oxygen.
If this metabolic pathway becomes the source of ATP for the cell because, for instance, oxygen levels remain low for a prolonged period, it can lead to a dangerous condition called lactic acidosis.