Haemoglobin and Oxygen Transport

Most oxygen is carried in the blood bound to haemoglobin (Hb) in erythrocytes. Only about 1.5% travels dissolved in plasma, because oxygen is not very soluble in water.Haemoglobin is a globular protein with a quaternary structure. Each molecule is made of four polypeptide chains: two α-globin and two β-globin chains. Each chain is attached to a haem group, a prosthetic group containing an iron ion (Fe²⁺). Each haem group can bind reversibly to one oxygen molecule, so one haemoglobin molecule can carry up to four oxygen molecules (eight oxygen atoms).Hb + 4O₂ ⇌ HbO₈Haemoglobin combined with oxygen is called oxyhaemoglobin. The reaction is reversible, which is essential:• in the lungs, the partial pressure of oxygen (pO₂) is high, so oxygen associates (binds) with haemoglobin. This is called loading.
• in respiring tissues, pO₂ is low because oxygen is being used, so oxyhaemoglobin dissociates and releases oxygen. This is called unloading.Partial pressure is a measure of the concentration of a gas in a mixture, measured in kilopascals (kPa). In the alveoli pO₂ is about 13 kPa, while in active muscle it may fall below 3 kPa.The affinity of haemoglobin for oxygen describes how readily it binds oxygen. A useful carrier must have a high affinity where oxygen is loaded and a lower affinity where it is unloaded. How saturation changes with pO₂ is shown by the oxygen dissociation curve, covered in the next lesson.