Glycolysis involved in glycolysis. The burning sensation in muscles during hard exercise can be attributed to the release of hydrogen ions during the shift to glucose fermentation from glucose oxidation to carbon dioxide and water, when aerobic metabolism can no longer keep pace with the energy demands of the muscles. These hydrogen ions form a part of lactic acid. The body falls back on this less efficient but faster method of producing ATP under low oxygen conditions. This is thought to have been the primary means of energy production in earlier organisms before oxygen reached high concentrations in the atmosphere between
Arteriovenous oxygen difference blood than they do at rest. Alternatively, in order to find the efficiency of the lungs in replenishing blood oxygen levels, the a-vO diff may instead be taken by comparing blood from the pulmonary artery and the pulmonary vein; in this case a negative value for a-vO diff would be obtained as the oxygen content of the blood would have increased. Physical exercise leads to an increase in the arteriovenous oxygen difference in all individuals. As exercise intensities increase, the muscles increase the amount of oxygen they extract from the blood, and this therefore results in further increases in a-vO