Learner Resource 3 -Oxygen Transport
Haemoglobin Structure
•
Haemoglobin is a globular protein, with a quaternary structure. Each molecule is made of 4
subunits, 2α and 2β polypeptide chains.
•
Each of these chains is attached to a haem group which contains iron
•
There are four haem groups in each haemoglobin molecule each of which can bind to an
oxygen molecule..
Write out the chemical equation for the formation of oxyhaemoglobin
•
When oxygen combines with haemoglobin it is said to associate. This occurs in the lungs.
•
When haemoglobin gives up its oxygen, it is said to dissociate. This occurs in tissues
where the oxygen is needed for respiration.
•
The partial pressure of oxygen (pO2) is a measure of the oxygen concentration. The
greater the concentration of dissolved or gaseous oxygen, the higher its partial pressure.
Where do red blood cells pick up oxygen?
Why does the haemoglobin associate with oxygen here?
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Where does oxyhaemoglobin give up its oxygen?
Why?
•
To summarise, in high concentrations of oxygen, haemoglobin will combine with large amounts
of the gas. In low concentrations haemoglobin dissociates from the gas, giving it up.
Samples of haemoglobin can be exposed to different partial pressures of oxygen. The amount
of oxygen combining at the different partial pressures can then be estimated. The percentage
saturation of each sample can then be plotted against the partial pressure.
Use the following data to draw an oxygen dissociation graph:
Partial pressure of oxygen in kpa(pO2 )
% saturation of haemoglobin
0
0
1.3
10
2.7
22
4.0
42
5.3
67
6.7
81
8.0
88
9.3
93
10.7
94
12.0
95
13.3
96
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1. In the lungs, will the pO2 be high or low?
2. In the muscles/tissues, will the pO2 be high or low?
3. Mark on your graph, the point at which the Hb is 50% saturated. Label this ‘unloading tension’.
4. Where in the body would this situation be found?
5. Mark on your graph, the point at which the Hb is 95% saturated. Label this ‘loading tension’.
6. Where in the body would this situation be found?
7. What would happen as blood moves from the lungs into an active muscle?
To summarise:
•
•
Hb becomes almost fully saturated with oxygen at high partial pressures such as those
found in the lungs – around 13 kPa.
Oxyhaemoglobin dissociates under low partial pressures such as those found in respiring
tissue – between 2 – 6 kPa.
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Why is the curve S shaped?
•
Each molecule of Hb has four haem groups.
•
When the first oxygen combines with the first haem group the shape of the Hb
molecule becomes distorted.
This makes it easier for the other three oxygen molecules to bind to the other haem
groups.
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Carbon Dioxide Transport
During respiration, CO2 is produced. This diffuses into the blood plasma and into the red blood
cells. Inside the red blood cells are many molecules of an enzyme called carbonic anhydrase. It
catalyses the reaction between CO2 and H2O. The resulting carbonic acid then dissociates into
HCO3− + H+. (Both reactions are reversible.)
CO2
carbon dioxide
+
H2O
water
→
H2CO3
carbonic acid
H2CO3
Carbonic acid
→
HCO3−
hydrogencarbonate ion
+
H+
hydrogen ion
85% of the carbon dioxide in the blood is carried in this way. The other 15% is either dissolved
in plasma or attaches directly to the haemoglobin molecules to form carbamino compounds.
If the hydrogen ions were allowed to accumulate, a decrease in pH would occur.
Why does the pH inside cells need to be kept constant?
The pH does not change because the hydrogen ions are combined with haemoglobin form
haemoglobinic acid. The haemoglobin acts as a buffer preventing a drop in pH. It also causes
haemoglobin to give up its oxygen more easily. This is known as the Bohr effect.
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Now draw a graph of the following data:
% saturation of blood with oxygen
Partial pressure of O2 (kPa)
3kPa CO2
11kPa CO2
0.0
0
0
1.3
10
5
2.7
22
12
4.0
60
24
5.3
84
42
8.0
94
78
10.7
97
90
13.3
98
95
What effect does carbon dioxide have on the oxygen haemoglobin dissociation curve?
Use data from the table to calculate the percentage decrease in saturation of oxygen at a partial
pressure of oxygen of 4.0kPa, as the partial pressure of CO2 rises from 3 kPa to 11 kPa.
What tissues would you expect to have higher carbon dioxide partial pressures than the blood?
The effect of carbon dioxide on the oxygen saturation of haemoglobin is known as the Bohr effect.
Its effect on the oxygen dissociation curve is known as the Bohr shift.
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To summarise:

Hb becomes saturated easily in the lungs. But as the amount of O2 drops, oxygen is rapidly
released – as the blood enters the tissues.

Actively respiring tissues will have a lower amount of oxygen which will lead to more
oxygen being released.

Higher levels of CO2 will mean more O2 is released, eg in working muscles where more
oxygen is needed
Write out a summary of the changes that occur leading to the Bohr effect:
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Fetal Haemoglobin
On the first graph you drew, add the oxygen dissociation curve for fetal haemoglobin.
Why is it necessary, in order to keep the fetus supplied with oxygen, that the oxygen dissociation
curve for fetal haemoglobin lies to the left of that for adult haemoglobin?
Myoglobin
1. Describe the structure of myoglobin.
2. Where is myoglobin found?
3. Draw the oxygen dissociation curve for myoglobin on your graph.
4. Can you suggest what use myoglobin may have if the oxygen content of exercising muscle is
near zero?
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Now let’s link what we have learnt to exercise:
When exercise is taken, muscles need more ATP which can be supplied as follows:



Oxygen is supplied by oxyhaemoglobin in the blood and this allows efficient production of
ATP by aerobic respiration.
The Bohr effect ensures oxygen is delivered at a faster rate than normal in tissues with a
high rate of respiration.
When oxygen levels in the muscles become very low, oxymyoglobin dissociates supplying
additional oxygen for aerobic respiration.
ATP can be produced anaerobically producing lactic acid.
After strenuous exercise, additional oxygen must be taken into the body to restore all systems to
their normal states. This is called oxygen debt.
The additional oxygen is needed to
1.
2.
3.
4.
Replenish ATP.
Remove lactic acid.
Replenish haemoglobin and myoglobin with oxygen.
Provide the extra oxygen needed to meet the extra demands of body changes that occur
during exercise such as increase in metabolic rate and body temperature.
The need for oxygen to replenish ATP and remove lactic acid is referred to as the "Oxygen Debt" or
"Excess Post-exercise Oxygen Consumption" (EPOC) - the total oxygen consumed after exercise in
excess of a pre-exercise baseline level.
In low intensity, primarily aerobic exercise, complete recovery can be achieved within several
minutes. However, recovery from more strenuous exercise, which is often accompanied by increase
in blood lactate and body temperature, may require 24 hours or more before re-establishing the preexercise oxygen uptake. The amount of time will depend on the exercise intensity and duration and
fitness of the individual.
The oxygen deficit is the difference between the oxygen needed during exercise and the volume of
oxygen that is actually obtained.
Oxygen debt is the additional oxygen that must be taken into the body after vigorous exercise to
restore all systems to their normal states.
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Glossary
Haemoglobin
Myoglobin
Myoglobin
Partial pressure
Oxygen Dissociation
Sigmoid curve
Carbonic anhydrase
Hydrogen carbonate ions
Haemoglobinic acid
Buffer
Bohr Shift
Oxygen deficit
Oxygen debt
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