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3.4.3Exchange and Transport

The Role of Haemoglobin — AQA A Level Biology 7402 Revision Notes & Topical Questions

5 min read 9 parts Lesson 29 of 48 1 quiz

📚 AQA A Level Biology 7402 Hub / Topical Revision
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3.4.3 The Role of Haemoglobin — AQA A Level Biology 7402 notes: Explain how haemoglobin loads, transports and unloads oxygen, using ideas of partial pressure and affinity.The partial pressure of oxygen (pO2) is a measure of the concentration of oxygen in a mixture of gases or dis…

  • Percentage saturation = (oxygen carried ÷ maximum oxygen capacity) × 100
  • Ventilation
  • Haemoglobin concentration
  • Cardiac output
  • Capillary density
  • Exam-style question 1

Section 1 of 9Partial pressure and loading

The partial pressure of oxygen (pO2) is a measure of the concentration of oxygen in a mixture of gases or dissolved in a liquid; it is measured in kilopascals (kPa).

Partial pressure and loading
In the alveoli, pO2 is about 13 kPa.
In resting tissues it is lower, about 5 kPa, and in actively respiring tissues, such as exercising muscle, it can be lower than 2 kPa.

Haemoglobin has to do two opposing jobs: load oxygen in the lungs and unload it in the tissues. Affinity is the tendency of haemoglobin to combine with oxygen. High affinity means it takes up oxygen readily but releases it less easily; low affinity means it releases oxygen easily.

Section 2 of 9Loading, transport and unloading

Loading
(association): in the lungs the high pO2 leads to oxygen diffusing into red blood cells and binding to haemoglobin. Haemoglobin has a high affinity under these conditions, and becomes about 95–98% saturated.
Transport
Transport blood carries oxyhaemoglobin in the arteries to the capillary networks of the tissues.
Unloading
(dissociation): in respiring tissues the pO2 is low and the pCO2 is high. The haemoglobin has a lower affinity under these conditions, releases oxygen, and oxygen diffuses into the cells for aerobic respiration.

The change in affinity occurs because of both the lower partial pressure of oxygen and the effect of carbon dioxide, which is explained in 3.4.5.

Section 3 of 9Percentage saturation

Percentage saturation = (oxygen carried ÷ maximum oxygen capacity) × 100. Blood leaving the lungs is nearly 100% saturated, and in a resting person returning venous blood is about 75% saturated. This means that at rest only about a quarter of the available oxygen is released to the tissues, leaving a large reserve for exercise.

RegionpO2 (kPa)Saturation
Alveolusabout 13about 97%
Resting muscleabout 5about 75%
Active muscle2 or lessabout 20–30%

Section 4 of 9Haemoglobin and oxygen supply to tissues

The amount of oxygen released depends on the difference between saturation at the lungs and saturation at the tissue.

The difference increases during exercise because more oxygen is released at lower pO2.
Myoglobin
Muscle also contains myoglobin, a single-chain oxygen-binding protein with a very high affinity, which acts as an oxygen store and releases oxygen only when pO2 is very low, during heavy exercise.

Oxygen delivery therefore depends on several things: the amount of haemoglobin, the affinity of the haemoglobin, the cardiac output and the ventilation rate, as well as the diffusion gradient in the tissues.

Section 5 of 9Link to other topics

The process relies on the diffusion of oxygen from alveolar air to the blood (3.2) and on the flow of blood through capillaries to tissues (3.5).

Link to other topics
It leads directly to the dissociation curve (3.4.4) and its shifts (3.4.5 and 3.4.6).
In terms of respiration, the oxygen released is the final electron acceptor in the electron transport chain and is combined with hydrogen to form water.

Section 6 of 9Factors that change oxygen delivery

The amount of oxygen delivered to the tissues in a given time depends on several things, and a good answer links them.

Factors that change oxygen delivery
Ventilation
Ventilation keeps alveolar pO2 high, so the diffusion gradient into the blood is steep.
Haemoglobin concentration
Haemoglobin concentration sets the capacity of the blood; anaemia, from a lack of iron or of red cells, reduces it.
Affinity
Affinity determines how much is loaded and how much is released.
Cardiac output
Cardiac output is the rate at which blood is pumped; at a higher output, more oxygen reaches the tissues each minute.
Capillary density
Capillary density in the tissue determines the diffusion distance and surface area.

Carbon monoxide lowers delivery in two ways: it occupies haem sites, and it makes the remaining haem groups hold oxygen more tightly, which shifts the curve left.

Factors that change oxygen delivery
For this reason, even a small proportion of carboxyhaemoglobin has a large effect on oxygen supply.
The treatment is to remove the person from the source of CO and give pure oxygen, which displaces CO more rapidly.

Remember that oxygen released in a capillary diffuses out through the capillary wall and the tissue fluid before it enters the cell. A steep gradient is maintained because cells are using oxygen continuously in the mitochondria.

Section 7 of 9Exam-style questions

  1. Explain how haemoglobin is able to both pick up oxygen in the lungs and release it in the tissues. [4]4 marks

  2. Blood leaving the lungs is 98% saturated and the blood returning from the muscles is 40% saturated. The blood carries 1.34 cm3 O2 per gram of haemoglobin, and contains 15 g per 100 cm3. Calculate the volume of oxygen released by 100 cm3 of blood. [3]3 marks

  3. Suggest the role of myoglobin in muscle. [2]2 marks

Section 8 of 9Key terms

TermMeaning
Partial pressureThe pressure contributed by one gas in a mixture, a measure of its concentration.
AffinityThe tendency of a molecule to bind to another.
LoadingAssociation of oxygen with haemoglobin.
UnloadingDissociation of oxygen from haemoglobin.
Percentage saturationProportion of haemoglobin binding sites occupied by oxygen.
MyoglobinSingle-chain oxygen-store protein in muscle.

Section 9 of 9Exam technique

1
Saying that haemoglobin always has a high affinity.
2
Mixing up loading with unloading.
3
Using concentration of oxygen instead of partial pressure.
4
Writing that carbon dioxide is the cause of oxygen loading in the lungs.
5
Forgetting that returning blood is still about 75% saturated at rest.

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