When oxygen is not available, muscle cells undergo metabolism, producing lactate as they break down glucose?

This type of metabolism is called anaerobic respiration. With the production of lactate, the process is called lactic acid fermentation.  In this process, NADH  transfers its electrons directly to pyruvate (produced in glycolysis), generating lactate. Muscle cells undergo lactic acid fermentation when they have too little O2 for aerobic respiration to continue, for example during the exercise. Lactic acid produced that way, is transported through the bloodstream to the liver, where it’s converted back to pyruvate and processed normally in the remaining reactions of aerobic cellular respiration.