Myoglobin's high O2 affinity

Myoglobin is a non-circulating single subunit oxygen binding pigment found predominantly in cardiac and  slow twitch skeletal muscle. Due to its single subunit, it is incapable of allosteric modulation and demonstrates a hyperbolic dissociation curve rather than a sigmoidal one. It has exceptionally high affinity for oxygen compared to hemoglobin, but exists in a much lower PO2 environment, indicating that binding and releasing oxygen intracellularly in a "shuttle fashion" may be a primary role.