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 to do so oxygen consumption (VO) may be measured using a spirometer to detect gaseous concentrations in exhaled air compared to inhaled air, while cardiac output can be determined using a Doppler ultrasound. Arterial blood will generally contain an oxygen concentration of around 20 mL/100 mL. Venous blood with an oxygen concentration of 15 mL/100 mL would therefore lead to typical values of the a-vO diff at rest of around 5 mL/100 mL. During intense exercise, however, the a-vO diff can increase to as much as 16 mL/100 mL due to the working muscles extracting far more oxygen from the