Oxygen Debt is the informal term for the condition of Excess post-exercise oxygen consumption, or the state of increased respiration after a period of strenuous labor. In simple terms, this is the period after exercise or other exertion when the body has used a significant portion of its blood-oxygen mix in order to create movement, and needs to increase its intake of oxygen to replenish that oxygen deficit. The body has a certain amount of oxygen dissolved in its blood, which is used to initialize respiration and create ATP, the basic energy unit that muscles use. When the load on the muscles is increased, for example while running, the oxygen in the blood is used up faster and so the muscles start to create ATP through anaerobic glycolysis; the stored glucose is converted to ATP without oxygen with the byproduct of lactic acid, which is a factor in muscular soreness. After the period of exertion is over, the body seeks to replenish its base supply of dissolved oxygen, and so oxygen intake increases to "refill" the "debt" of blood-oxygen created by the exercise. This is marked by heavy, deep breathing, gasping, and an increased heart-rate that slows down as the blood-oxygen returns to equilibrium.