Answer:
Most heterotrophs have the ability to use an alternative metabolic pathway to continue producing small amounts of ATP even when oxygen is absent or in a limited supply. This route is called glycolysis and involves the oxidation of pyruvate to lactic acid.
Step-by-step explanation:
When animal tissues cannot be supplied of enough oxygen for aerobic oxidation of pyruvate (pyruvic acid) and NADH produced in glycolisis, lactic fermentation can occur. In lactic fermentation pyruvate is the terminal electron acceptor. So it picks up the electrons carried by the coenzyme NADH + H + (which in turn transforms into NAD +), from the oxidative reactions of the glycolysis, and is reduced to lactate (Acid. lactic).