The hemoglobin holding the oxygen will not release the oxygen it carries until it is in the presence of carbon dioxide. The incoming O2 and the outgoing CO2 molecules exchange places in the RBC. One gets off, and the other gets on. The O2 rides the hemoglobin through the bloodstream until it reaches the body part in need. That area of the body can only de-bond the hemoglobin to receive the oxygen it is carrying if there is a CO2 molecule to trade.
