Maximal oxygen uptake is the gross criterion of aerobic endurance. It is understood as the measure of oxygen supply (respiration), oxygen transport (cardiovascular) and oxygen utilization (muscle cell) in the state of exertion.
Maximal oxygen uptake captures both the respiratory system and the cardiovascular system, which is why both systems can be addressed equally during endurance exercise. The respiratory and cardiovascular systems together form the cardiopulmonary system.
When the body requires more oxygen during exercise than the maximum oxygen uptake allows, oxygen debt occurs.