This phenomenon is called the Bohr effect , in which Hb gives up oxygen more easily at low pH, areas of high cell metabolism. The reason for a lower pH is active tissues is the production of carbon dioxide as a bi product of the Krebs cycle. The above chemical reaction explains the situation. Tissues are producing carbon dioxide. There are three methods by which carbon dioxide is transported in the blood from the tissues to the lungs.
Carbon dioxide leaves the cell, diffuses through the interstitial fluid and enters a capillary.
Most of it enters an erythrocyte. The bicarbonate ion leaves the red blood cell and travels to the lungs in the plasma of the blood. The bicarbonate ion reenters the cell, it combines with the hydrogen ion, forming carbonic acid, which dissociates to form water and carbon dioxide. The carbon dioxide diffuses across the capillary wall, enters the alveolus, and is exhaled. There are control centers for respiration in both the medulla and the pons of the hindbrain.