The Cori Cycle

During brief, intense muscular activity, the demand for ATP can outstrip the rate at which oxygen can be delivered to the muscle. Under these low-oxygen conditions, muscle fibres rely on anaerobic glycolysis: stored glycogen or glucose is broken down to pyruvate, and because there is not enough oxygen to feed pyruvate into aerobic respiration, the pyruvate is instead reduced to lactic acid (lactate). This anaerobic route yields ATP quickly, but the build-up of lactic acid in the muscle is associated with the fatigue and soreness felt after strenuous exercise.

This lactic acid does not stay trapped in the muscle. It diffuses out into the bloodstream and is carried to the liver. There, the liver re-oxidises lactate back to pyruvate and uses gluconeogenesis to resynthesise it into glucose, which can either be released back into the blood for the muscle to use again or stored as liver glycogen. Glucose returning from the liver can be taken up once more by the muscle to replenish its own glycogen reserves. …