The haemoglobin dissociation curve is further to the right in the presence of carbon dioxide. The greater the concentration of carbon dioxide, the lower the affinity for oxygen. This is known as the Bohr effect.At the surface of the lungs there is a high partial pressure of oxygen and the concentration of carbon dioxide is low. The affinity of haemoglobin for oxygen is higher and oxygen is loaded onto the haemoglobin.At respiring tissue there is a low partial pressure of oxygen and the concentration of carbon dioxide is high. The affinity of haemoglobin for oxygen is lower and oxygen is unloaded from the haemoglobin.An enzyme in red blood cells called carbonic anhydrase catalyses the reaction of carbon dioxide with water to form carbonic acid. This lowers the pH, causing the haemoglobin to change shape and have less affinity for oxygen.