haemoglobin

Haemoglobin is an iron-containing protein within red blood cells that transports oxygen from the lungs to tissues and assists in carbon dioxide removal. It is a tetramer built from globin polypeptides with a heme prosthetic group on each subunit. In adults the most common form is haemoglobin A (HbA), composed of two α chains and two β chains (α2β2). Each heme contains a ferrous iron (Fe2+) that reversibly binds one molecule of O2. Fetal haemoglobin (HbF) has two γ chains and binds oxygen more tightly, facilitating transfer from mother to fetus.

Oxygen uptake occurs in the lungs where haemoglobin binds O2 to form oxyhaemoglobin; in tissues, O2 is released. Haemoglobin also participates in carbon dioxide transport: CO2 from tissues forms carbaminohaemoglobin and most is carried as bicarbonate in plasma after hydration. The affinity of haemoglobin for oxygen is modulated by factors such as pH (the Bohr effect), CO2, temperature, and 2,3-bisphosphoglycerate (2,3-BPG). These shifts optimize oxygen release where it is most needed.

Haemoglobin is synthesized during erythropoiesis in the bone marrow; mature red cells lack nuclei and mitochondria. The RBC lifespan is about 120 days. Senescent cells are removed by the spleen; the heme portion is degraded by heme oxygenase to biliverdin and bilirubin, which are excreted after conjugation in the liver, while iron is recycled.

Clinically, abnormal haemoglobins give rise to haemoglobinopathies such as sickle haemoglobin (HbS) and HbC, as well as conditions affecting chain production (thalassemias). Diagnosis involves measurements of haemoglobin concentration and electrophoretic or chromatographic analyses.