Cori cycle:

Under the limiting oxygen conditions experienced in during vigorous exercise, the configuration of NADH through glycolysis exceeds the ability of the respiratory chain to oxidize it back to NAD⁺. The pyruvate produced through glycolysis in muscle is then converted to lactate through lactate dehydrogenase, a reaction which regenerates NAD⁺ and so permits glycolysis to continue to produce ATP. Moreover, lactate is a metabolic dead-end in which it cannot be metabolized further until it is converted back to pyruvate. The Lactate diffuses out of the muscle and is carried in the bloodstream to the liver. Here it spread into liver cells and is converted back to pyruvate through lactate dehydrogenase.  The pyruvate is then transformed to glucose through gluconeogenesis and the glucose is released back into the bloodstream ready to be taken up through skeletal muscle and brain. This cycle of reactions shown in the figure is known as the Cori cycle.

Figure:   The Cori cycle.