For years, during high intensity activity lactate or lactic acid in the blood is blamed for that burning sensation and not being able to maintain those intensities. While there's mild truth to it, lactate is not the enemy. It's a by-product of energy production during glycolysis under anaerobic conditions. Our bodies actually use this formed lactate as energy during this glycolysis cycle. Pyruvate is converted into lactate. This lactate is transported in the blood towards the liver where it is converted again to pyruvate for use in gluconeogenesis - the production of new glucose molecules from non-carbohydrate sources. At this point, glycolysis can start again.

The citric acid cycle, or the tricarboxylic acid cycle (TCA) or the Kreb's cycle as it's also known. The final resting place of the intial glucose molecule from glycolysis. At this point glycolysis has broken down glucose into pyruvate (see below). The citric acid cycle is another series of reactions with the help of oxygen that creates an abundance of products which are then used in the production of ATP molecules. We've seen via The Cori Cycle that in the absence of oxygen, pyruvate is converted to lactate and through gluconeogenisis new glucose molecules are produced for the continuation of glycolysis. When oxygen is present and in abundance, the citric acid cycle is used.