When the concentration of oxygen is low myoglobin:oxygen dissociates to yield free oxygen (Rossi-Fanelli and Antonini 1958, Boulton and Huntsman 1972, Wang et al. 2004). The dissociation constant is about 1 micromole per liter. Myoglobin:oxygen may also transfer oxygen directly to cytochrome oxidase of mitochondria (inferred from rat homologs in Yamada et al. 2013).