As an expert in the field of exercise physiology, I can explain the concepts of oxygen deficit and oxygen debt, which are both related to the body's use of oxygen during physical activity. Oxygen Deficit occurs at the onset of intense physical activity when the demand for oxygen by the muscles exceeds the supply that can be delivered by the circulatory system. This results in an immediate and temporary reliance on anaerobic metabolism, which does not require oxygen, to produce the necessary energy. During this phase, the body is said to have an oxygen deficit because it is not using oxygen at its full aerobic capacity. Oxygen Debt, on the other hand, refers to the additional amount of oxygen that is required after the exercise has ended to repay the oxygen that was used during the anaerobic phase and to restore the body to its resting state. This repayment period is characterized by an elevated oxygen consumption as the body continues to metabolize the lactic acid produced during anaerobic respiration and returns to its normal, pre-exercise oxygen consumption levels. To summarize, the oxygen deficit is the immediate shortfall of oxygen during the onset of exercise, while oxygen debt is the additional oxygen needed post-exercise to return the body to its pre-exercise state. read more >>

At the beginning the body works anaerobically leaving an oxygen deficit. Over time the oxygen consumption levels out to a steady state. After exercise the oxygen is pain back (oxygen debt). Notice the area of oxygen debt is greater than the area of oxygen deficit for the reasons stated above.read more >>