Lactic acid fermentation occurs as an alternative to that of aerobic respiration, this occurs in cases where oxygen levels or concentrations within the cells are very low and as a result, is not available for the acceptance of electrons as the final electron acceptor. The cells undergo this process, where essentially, rather mr than using oxygen as the final electron acceptor after producing ATP. NAD+ I believe is able to accept electrons and thus we can produce ATP, again. Glycolysis occurs but rather than moving on to the next stage, Pyruvate molecules are ultimately converted to lactic acid which releases ATP, the extra electrons would go to NAD+ which is needed to keep the reactions going.
