Oxygen–Haemoglobin Dissociation Curve
The oxygen–haemoglobin dissociation curve (OHDC) is a graph relating the percentage of haemoglobin saturated with oxygen to the partial pressure of oxygen. When oxygen binds to haemoglobin, the two ß chains move closer together and change the position of the haem moieties that assume a ‘relaxed’ or R state. When oxygen dissociates from haemoglobin, the reverse happens and the haem moieties take up a ‘tense’ or T state. Myoglobin also has a higher affinity for oxygen than haemoglobin, and so its dissociation curve lies to the left of the OHDC. Myoglobin takes up oxygen from the circulating haemoglobin and releases it into exercising muscle tissues at very low partial pressure of oxygen, thus providing a source of oxygen during periods of sustained muscle contractions when blood flow to these muscles may be constricted due to blood vessel compression.