During intense muscular activity, for instance, 100 meter sprint. The blood is unable to supply enough oxygen to contracting muscle cells. Under these conditions, glucose is broken down anaerobically to lactic acid. This helps the muscle cells to obtain some energy even in the absence of oxygen.
After the exercise, lactic acid is carried from the muscles to the liver where it is reconverted back to glucose. This requires energy in the form of adenosine triphosphate [ATP]-the energy carrying molecule. The panting observed after exercise is aimed at supplying the extra oxygen required for the formation of ATP necessary for the conversion of lactic acid to glucose. This extra oxygen is referred to as oxygen debt. The heart beat also increases under such conditions. This helps to remove lactic acid from the muscles and carry it to the liver and to supply extra oxygen to the tissues.