When oxygen runs out, cells switch to a backup pathway that yields 93% less ATP — yet it's the only reason you can sprint at all. Drag the slider to cut off oxygen and watch the energy output collapse.
When oxygen is absent, animal cells convert pyruvate into lactic acid through fermentation. This process doesn't generate any additional ATP beyond the 2 from glycolysis, but it serves a critical function: regenerating NAD⁺ so glycolysis can continue running. Without this regeneration step, glycolysis would stall completely, and even the meagre 2 ATP would be unavailable. Lactic acid buildup causes muscle fatigue and the burning sensation during intense exercise, but it's better than zero energy production.