When oxygen is missing, or when pyruvate is briefly produced faster than it could be consumed via the citric acid cycle (as in extreme muscular exertion), pyruvate is transformed to lactate by the enzyme lactate dehydrogenase, a approach that also oxidizes NADH back to NAD+ for re-use in further glycolysis, https://skinrejuvenation77777.pages10.com/stress-reduction-for-dummies-64473283