And so red blood cells produce many of these lactate molecules. So once again, lactate is the major precursor molecule that can be used to form glucose. In gluconeogenesis, skeleton muscle cells produce lactic acid which then dissociates into lactate and the H plus ions during exercise when the rate of glycolysis is greater than the rate of oxidative aspirulation that takes place in the mitochondria. Other types of cells, like red blood cells, only produce lactic acid because they don't have the machinery the mitochondria needed to actually regenerate those NAD plus coenzymes that we must have if we want to actually use glycolysis to form those ATP molecules. Now, notice that if the cells continually use lactic acid fermentation, there's going to be a build up in the H plus concentration in the cell and the surrounding tissue and that will decrease the PH and increase the acidity. And that can be very dangerous.