A level Biology exam revision resources written by A level Examiners


Some oxygen can be transported directly dissolved in plasma but this is minor and only accounts for about 3% of oxygen transport.

Most oxygen is transported bound to haemoglobin.

haemoglobin + oxygen = oxyhaemoglobin

Each haemoglobin molecule can transport four oxygen molecules. When oxygen molecules are bound to haemoglobin they are said to be associated. When they are released they are said to be disassociated.

The reaction where haemoglobin takes up oxygen occurs when the concentration of oxygen is high (e.g. in the lung capillaries) and the reverse reaction where the oxygen is released occurs when the concentration of oxygen is low (e.g. in the muscle capillaries).

Unfortunately the transport of oxygen is not simple because temperature and carbon dioxide concentration also effect the ability of haemoglobin to carry oxygen. Plus the fact that haemoglobin does not simply accept oxygen at high concentration and release it at low concentration it does so in a non linear fashion. The association and disassociation of oxygen from haemoglobin can be described with a sigmoidal graph. This graph is known as the oxyhaemoglobin dissociation curve.   Exam Advice

The following figures shows how oxygen is associated and dissasociated from haemoglobin in a variety of different conditions.

Figure 1
Oxyhaemoglobin dissociation curve

Explanation of figure 1

This graph shows when the partial pressure of oxygen is high (as it is in the lung capillaries) there is a very high saturation of haemoglobin with oxygen. When the partial pressure of oxygen is low (as it is in the capillaries of the tissues) there is a low saturation of haemoglobin with oxygen. This means the haemoglobin releases oxygen as it travels from the lungs to the tissues.

Figure 2
Oxyhaemoglobin dissociation curve
   Exam Advice   

Explanation of figure 2

The amount of oxygen carried and released by haemoglobin depends not only on the partial pressure of oxygen but also on pH. An increase in carbon dioxide causes a decrease in pH. An acidic environment causes oxygen to dissociate from haemoglobin. Just a small decrease in the pH results in a large decrease in the percentage saturation of the haemoglobin with oxygen. This shift of the curve is called the Bohr effect.

Figure 3
Oxyhaemoglobin dissociation curve

Explanation of figure 3

A fetus developing in the uterus must be able to extract oxygen from its mother's blood. Fetal haemoglobin therefore requires a higher affinity for oxygen than adult haemoglobin (it combines more readily with oxygen).

Myoglobin is a pigment found in muscles which acts as an oxygen store and it receives its oxygen from haemoglobin therefore it also has a high affinity for oxygen.

Figure 4
Oxyhaemoglobin dissociation curve

Explanation of figure 4

The higher the temperature, the less saturated the blood is with oxygen. This situation often arises during exercise (heat is produced by respiration in muscle cells)

The increased temperature causes haemoglobin to release oxygen more readily.

Carbon monoxide (CO) has a higher affinity for haemoglobin than oxygen and binds irreversibly to it. When air is breathed in containing CO haemoglobin molecules combine with it. If too much CO is inhaled the blood does not have a enough free haemoglobin molecule left to maintain the oxygen transport needed to maintain life and death from hypoxia occurs.

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