[Haematology] Haemoglobin

Also known as hemoglobin.

Haemoglobin is the vital substance carried within erythrocytes (red blood cells). The red colour of these cells comes from the haemoglobin itself.

 

Structure

One haemoglobin molecule is formed from four subunits, two alpha and two beta.
Each subunit is a polypeptide with a prosthetic group, containing iron.
Each prosthetic group can carry one oxygen molecule (O₂).
This means that each molecule of haemoglobin can carry four oxygen molecules (4 x O₂).

 

Picking up oxygen

The alveoli of the lungs are surrounded by capillaries. Moving through these capillaries, red blood cells come as close as possible to the atmosphere without leaving the body.
At atmospheric pressure, the partial pressure of oxygen is higher than anywhere inside the body. In this relatively oxygen-rich environment, diatomic oxygen molecules bind to haem.
The haemoglobin-oxygen complexes are carried in the blood from the lungs to the tissues.

 

Dropping off oxygen

In peripheral tissues, cellular respiration results in high levels of CO₂ and low levels of O₂. An enzyme called carbonic anhydrase converts a certain proportion of CO₂ to carbonic acid (H₂CO₃), producing a relatively low pH (acidity).

In a low pH environment, the structure of haemoglobin changes, allowing oxygen to easily dissociate from the haem. This is called the Bohr effect, and it allows the blood to give oxygen optimally in the tissues which have used the most oxygen.

 

Biosynthesis

Haem is formed when ferrous iron (an Fe²⁺ ion) is chelated into protoporphyrin IX.

 

Pathophysiology 

• Anaemia   
• Sickle-cell anaemia
• Thalassemia  
• Porphyria