A transformation product of oxyhemoglobin resulting from the oxidation of the normal Fe2+ to Fe3+, a process that converts heme to hematin; because methemoglobin contains water in firm union with ferric iron, it is chemically different from oxyhemoglobin; found in sanguineous effusions and in the circulating blood after poisoning with acetanilid or potassium chlorate, among other substances. SYN: ferrihemoglobin.
