Normal response to exercise

Changes in oxygen uptake (V_O2), carbon dioxide production (V_CO2), and respiratory gas exchange ratio (R) after incremental treadmill exercise in a normal subject. Maximal oxygen uptake (V_O2 max) and onset of the anaerobic threshold are identified; the latter occurs as oxygen uptake reaches a plateau. Carbon dioxide is derived from both oxidative metabolism (metabolic CO₂) and, once the anaerobic threshold is reached, from the buffering of lactate by bicarbonate (nonmetabolic CO₂).