The following image from KhanAcademy.com shows the process of anaerobic respiration to make lactic acid in the muscles. When a person is exercising hard, the muscles do not have enough oxygen to continue the aerobic respiration pathway. This low oxygen causes the muscles to shift to an alternative pathway, anaerobic respiration that doesn’t require oxygen which is needed to accept electrons at the end of the electron transport chain.
The lactic acid produced in muscle cells is transported through the bloodstream to the liver, where it’s converted back to pyruvate and processed normally in the remaining reactions of cellular respiration. See the Cori cycle or the Lactic Acid Cycle.
