Optimization of Oxygen use

While exercising, the human body tries to optimize the use of oxygen captured in the lungs.

During intense exercise, the oxygen consumption of the human body can increase up to 8 times, compared to the consumption at rest, before it plateaus. This higher demand for oxygen is usually covered through increased heart rate, which increases the cardiac output, and thus, the amount of blood (and oxygen) reaching the muscles.

However, cardiac output cannot increase as much as oxygen demand does. When this limit is surpassed, the human body has another mechanism to ensure oxygen supply to the muscles. Once cardiac output cannot increase more, the only solution is to remove more oxygen from hemoglobin, in this way, the same volume of blood will provide more oxygen. This happens because when muscles increase their metabolic rate, they produce high amounts of carbon dioxide and H⁺, which lower the pH. Lower pH reduces the affinity of hemoglobin for oxygen, making it easier for oxygen to be transferred from hemoglobin to the muscle tissue.