Internal respiration is, in many ways, the reverse process of what occurs in external respiration. This process occurs between the blood and the bodies tissues and focuses once more on gas exchange. Here, oxygen gas, needed for cellular respiration, is delivered to cells, and CO2, a waste product from the same reaction, is removed. To accomplish this, oxy-haemoglobin surrenders its oxygen molecule adding to the concentration of O2 in the blood. This concentration gradient causes O2 to diffuse out of the capillaries and into the cells. The carbon dioxide released by the cells into the blood, where it has a lower concentration, is transported in three forms: free carbon dioxide in trace amounts, a predominance of bicarbonate ions whose generation is catalyzed by carbonic anhydrase, and in the form of carbamino-haemoglobin. When the bicarbonate ions are formed, the excess hydrogen ions (also called hydronium ions) are absorbed by haemoglobin which, in its transition to reduced haemoglobin, acts as a Bronsted-Lowry acid-base conjugate buffer system.