A very weak acid formed inside red blood cells when hydrogen ions combine with haemoglobin. The presence of the hydrogen ions, which are produced by the dissociation of carbonic acid (see carbonic anhydrase), encourages oxyhaemoglobin to dissociate into haemoglobin and oxygen (see bohr effect). The oxygen diffuses into the tissue cells and the haemoglobin acts as a buffer for the excess hydrogen ions, which it takes up to form haemoglobinic acid.