ABSTRACT
Abnormalities of haem metabolism are important clinically. They include the porphyrias, which are rare, and disorders involving haem iron, such as iron deficiency, which are common in clinical practice. Most body tissues synthesize haem. In bone marrow it is incorporated into haemoglobin, an iron-containing pigment that carries oxygen from the lungs to tissues, and in muscle into myoglobin, which also binds oxygen. Carboxyhaemoglobin is cherry red in colour and is formed when carbon monoxide binds to haemoglobin or displaces oxygen from oxyhaemoglobin; haemoglobin has a greater affinity for carbon monoxide than for oxygen. Methaemoglobin is haemoglobin in which iron is in the ferric form. Sulphaemoglobin is similar to methaemoglobin but contains sulphur; unlike methaemoglobin, it cannot be reconverted to haemoglobin in vivo. The human haemochromatosis protein regulates the binding of transferrin to the transferrin receptor as well as regulating hepcidin.
