myoglobin (Mb)

myoglobin (Mb)

iron-containing haem protein present at high density in the cytoplasm of oxidative muscle fibres (skeletal and cardiac) and substantially contributing to their redness. Related to haemoglobin (Hb), with which it shares affinity for oxygen. In diving mammals (whose tissues it makes almost black) Mb serves as a significant oxygen store but in terrestrial ones, including humans, its predominant function is considered to be facilitation of diffusion of oxygen from the blood through the cytoplasm, by the transfer of oxygen from Hb which has a lower oxygen affinity than Mb at the oxygen tension ( P O2) encountered in the muscle vascular bed. See also muscle fibre types.