Haemoglobin is an iron-containing protein in red blood cells that carries oxygen from the lungs to tissues. It also helps carry carbon dioxide and contributes to blood buffering. Haemoglobin gives red blood cells much of their colour and is central to the diagnosis of anaemia and many inherited blood disorders.
The abbreviation Hb is commonly used in medicine and laboratory reports.
Structure
Adult haemoglobin is mainly haemoglobin A. It is made from four protein subunits: two alpha-globin chains and two beta-globin chains. Each chain is attached to a haem group containing iron. Each haem group can bind one oxygen molecule, so one haemoglobin molecule can carry up to four oxygen molecules.
MedlinePlus Genetics describes beta-globin as a subunit of haemoglobin and notes that adult haemoglobin typically contains two beta-globin subunits and two alpha-globin subunits.
Function
Haemoglobin binds oxygen in the lungs, where oxygen concentration is high, and releases oxygen in tissues, where oxygen concentration is lower. This allows red blood cells to deliver oxygen throughout the body.
Haemoglobin also helps carry carbon dioxide from tissues back to the lungs and helps buffer blood pH. Its oxygen-binding behaviour changes with acidity, carbon dioxide, temperature, and 2,3-bisphosphoglycerate, allowing oxygen delivery to increase in active tissues.
Main Forms
Important forms include:
- Haemoglobin A: the main adult form, made from two alpha and two beta chains.
- Haemoglobin A2: a smaller adult fraction, made from alpha and delta chains.
- Fetal haemoglobin: the main fetal form, made from alpha and gamma chains. It binds oxygen more strongly than adult haemoglobin, helping oxygen transfer from mother to fetus.
- Variant haemoglobins: inherited forms such as haemoglobin S, C, and E.
Different forms can be measured by haemoglobinopathy screening, electrophoresis, high-performance liquid chromatography, or related laboratory methods.
Measurement
Haemoglobin concentration is usually measured as part of a full blood count. The result helps assess anaemia, polycythaemia, bleeding, nutritional deficiency, chronic disease, and response to treatment.
Interpretation depends on age, sex, pregnancy, altitude, hydration, smoking, medical history, and laboratory reference ranges. A low haemoglobin result is not a diagnosis by itself. The cause must be investigated.
Disorders
Haemoglobin disorders include:
- Sickle cell disease, usually caused by haemoglobin S.
- Thalassaemia, caused by reduced production of alpha or beta globin chains.
- Haemoglobin C disease.
- Haemoglobin E disease.
- Methaemoglobinaemia, where haem iron is in a form that cannot carry oxygen effectively.
These disorders can cause anaemia, pain crises, jaundice, enlarged spleen, pregnancy risk, stroke risk, infection risk, iron overload, or other complications, depending on the condition.
Iron and Haem
Iron is essential for haemoglobin function because oxygen binds to iron in the haem group. Iron deficiency can reduce haemoglobin production and cause iron deficiency anaemia.
Too much iron can also be harmful. People receiving repeated transfusions, such as some patients with thalassaemia major, may develop iron overload and need chelation treatment.
See Also
References
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