In the absence of adequate oxygen the pyruvic acid is converted into lactic acid and alanine, which actually helps keep glycolysis going by removing excess hydrogen ions that would normally bring it to a halt. The lactic acid escapes into the bloodstream and away from the muscle. This escape mechanism is only temporary, however, because the level of lactic acid in the blood and muscle eventually increases. This increased acidity inactivates some of the enzymes used in glycolysis, which reduces the ability of the muscles to contract. Fatigue sets in and exercise must stop.