One of a group of globular proteins occurring widely in animals as oxygen carriers in blood. Vertebrate haemoglobin is contained in the red blood cells (erythrocytes). It comprises two pairs of polypeptide chains, known as α‎-chains and β‎-chains (forming the globin protein), with each chain folded to provide a binding site for a haem group. Each of the four haem groups binds one oxygen molecule to form oxyhaemoglobin. Dissociation occurs in oxygen-depleted tissues: oxygen is released and haemoglobin is reformed (see bohr effect; haemoglobinic acid; oxygen dissociation curve). The haem groups also bind other inorganic molecules, including carbon monoxide (to form carboxyhaemoglobin). Carbon dioxide can bind to terminal –NH₂ groups of the α‎- and β‎-chains, forming carbamino groups (–NHCOO^-), in which form a significant fraction of carbon dioxide is transported in the red blood cells.