2Learning outcomesDescribe the role of haemoglobin in the transport of oxygen and carbon dioxideDescribe and explain the oxygen dissociation curve for haemoglobin.
3Changes in the Partial Pressures of Oxygen and Carbon Dioxide
4HaemoglobinOxygen is transported around the body inside red blood cells in combination with the protein haemoglobinEach haemoglobin is made up of four polypeptides each containing one haem groupHaem group
5What is partial pressure? The pressure that one component of a mixture of gases would exert if it were alone in a container.Note: During this topic you will come across the term of partial pressure. Essentially it is a measure of the concentration of oxygen. It is written in shorthand as pO2 and is measured in kilopascals (kPa).
6How much oxygen can be carried? Overall each molecule can combine with four oxygen moleculesThis means that eight oxygen atoms can be carried by each haemoglobin moleculeHb O HbO8oxyhaemoglobinhaemoglobinThe binding of oxygen is a reversible reaction.
7The haemoglobin dissociation curve The balance can be shown by an oxygen dissociation curve for oxyhaemoglobin.The amount of oxygen held by the haemoglobin, i.e. its saturation level, is normally expressed as a percentage.
8The haemoglobin dissociation curve At high partial pressures of oxygen, the percentage saturation of haemoglobin is very high. It is combined with large amounts of oxygen.At low partial pressures of oxygen, the percentage saturation of haemoglobin is very low, that is the haemoglobin is combined with only a very little oxygen.
9Loading and unloading of oxygen Blood arriving at the lungs has a lower pO2 than that in the lungs. There is therefore a diffusion gradient and oxygen will move from the alveoli into the blood. The O2 is then loaded onto the Hb until the blood is about 96% saturated with oxygen. The Hb is now called oxyhaemoglobin (HbO2).The blood is then taken to tissues where the cells are respiring all the time, using oxygen. The pO2 will be low. As the red blood cell enters this region, the Hb will start to unload the O2, which will diffuse into the tissues and be used for further respiration. Since much of the Hb will have unloaded the O2, a much lower percentage of the blood will be saturated with O2.OxygenCarbon dioxide
10Why is the oxygen dissociation curve S-shape? It is S-shaped because of the behaviour of the Hb in different pO2: The first molecule of O2 combines with an Hb and slightly distorts it. The joining of the first is quite slow (the flatter part of the graph at the beginning) but after the Hb has changed shape a little, it becomes easier and easier for the second and third O2 to join. This is shown by the curve becoming steeper. It flattens off at the top because joining the fourth O2 is more difficult.Overall, it shows that at the higher and lower end of the partial pressures, there isn’t a great deal of change in the saturation of the Hb, but in the middle range, a small change in the pO2 can result in a large change in the percentage saturation of the blood.