As the CO2 content of blood increases, the pH of the blood falls. This produces a decrease in oxygen affinity of haemoglobin. This is called the Bohr effect and is closely related to the fact that deoxygenated haemoglobin (deoxyhaemoglobin) binds hydrogen ion more actively than does the haemoglobin. This facilitates gaseous exchange because more oxygen is released in the tissues where the amount of carbon dioxide is rising due to metabolic activity. At the same time, more oxygen is taken up by the lungs or gills where the amount of carbon dioxide is low.