Most O2 is carried in the blood by erythrocytes (red blood cells) which contain haemoglobin (Hb). In adults, Hb is a protein formed of four polypeptide chains, called globins – there are two alpha and two beta chains (Figure 11). Attached to the interior of each globin chain is a small non-protein structure known as a haem group. The haem group has at its centre an iron ion (Fe2+) that binds to one O2 molecule. As there are four globin chains and four haem groups, each with one Fe2+, one Hb molecule can carry four O2 molecules.
When O2 is bound to Hb, the Hb is said to be oxygenated and the complex formed is called oxyhaemoglobin. Oxygenation occurs where there is a plentiful supply of O2; that is, in the capillaries surrounding the alveoli of the lungs.
O2 binding to Hb is governed by positive cooperativity, meaning that once one haem group binds O2, it becomes progressively easier for the other haem groups to also bind O2. This ensures that the Hb molecule can become quickly saturated (i.e. with four O2 molecules bound). Oxygen saturation levels (‘sats’) are used by doctors to detect respiratory distress or illness.
Binding of O2 to Hb is reversible, meaning that when oxyhaemoglobin reaches the capillaries within the tissues, where O2 is being consumed and the PO2 is low, the O2 is released and diffuses into the tissues. Hb that is not bound to O2 is termed deoxyhaemoglobin.
Question 8 Pulmonary arteries
Which form of Hb is predominant in the blood carried by the pulmonary arteries? (see Section 1.1.3)
The correct answer is b.
Pulmonary arteries carry blood coming from the peripheral organs into the lungs where CO2 will be exchanged for O2, so they carry CO2-rich blood. Therefore, the predominant form of Hb in the pulmonary arteries will be deoxyhaemoglobin.
The binding and dissociation of O2 to and from haemoglobin is dependent on the PO2. This is not surprising, because as you saw in Section 2.2, differences in partial pressures between tissue capillaries and pulmonary capillaries drive the exchange of O2 and CO2. However, if you look at the oxygen–haemoglobin dissociation curve in Activity 8, you will see that O2 binding to haemoglobin is not a linear relationship. Rather, the amount of Hb bound to O2 over a range of PO2 has a sigmoidal ‘S’-shaped curve.
Activity 8 Oxygen–haemoglobin dissociation curve
Take a look at this dissociation curve, then place the marker as directed and click ‘Enter answer’. If you place it correctly, one more question will then be posed to you.