Your body has increased the flow of oxygen-rich blood to your muscles, but your muscles still need to get the oxygen out of the blood. An exchange of oxygen and carbon dioxide is the key to this. A protein called hemoglobin, which is found in red blood cells, carries most of the oxygen in the blood. Hemoglobin can bind oxygen and/or carbon dioxide; the amount of oxygen bound to hemoglobin is determined by the oxygen concentration, carbon dioxide concentration and pH. Normally, hemoglobin works like this:
- Hemoglobin in red blood cells entering the lungs has carbon dioxide bound to it.
- In the lungs, oxygen concentration is high and carbon dioxide concentration is low due to breathing.
- Hemoglobin binds oxygen and releases carbon dioxide.
- Hemoglobin gets transported through the heart and blood vessels to the muscle.
- In muscle, the carbon dioxide concentration is high and the oxygen concentration is low due to metabolism.
- Hemoglobin releases oxygen and binds carbon dioxide.
- Hemoglobin gets transported back to the lungs and the cycle repeats.
As you exercise, though, the metabolic activity is high, more acids (hydrogen ions, lactic acid) are produced and the local pH is lower than normal. The low pH reduces the attraction between oxygen and hemoglobin and causes the hemoglobin to release more oxygen than usual. This increases the oxygen delivered to the muscle.
Getting Rid of Waste
Your exercising body is using energy and producing waste, such as lactic acid, carbon dioxide, adenosine and hydrogen ions. Your muscles need to dump these metabolic wastes to continue exercise. All that extra blood that is flowing to the muscles and bringing more oxygen can also take the wastes away. The hemoglobin in the blood will carry away the carbon dioxide, for example.