Animals respire anaerobically as a means of overcoming a temporary shortage of oxygen. This is useful when the amount of oxygen is very low, for an animal living in water. However, anaerobic respiration most often occurs in muscles during strenuous exercise when oxygen is used up more rapidly than it is supplied. Pyruvate is reduced to lactate using the hydrogen from reduced NAD and NAD is regenerated so that glycolysis can continue.Lactate lowers the pH of cells and may stop enzymes from working properly so it is taken away in the blood to the liver. In the liver lactate is either converted to glycogen for storage or oxidised to release energy once oxygen is again available.