AQA · A-Level · Biology
Course version: 7402 · Version from Sept 2016
Gas exchange: Ventilation and the exchange of gases in the lungs.
Ventilation is the movement of air into and out of the lungs; gas exchange is the diffusion that then takes place at the alveolus.
Checked against the AQA 7402 specification (opens in a new tab) on .
- Point 5 of 7
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Ventilation is the movement of air into and out of the lungs; gas exchange is the diffusion that then takes place at the alveolus. The two are linked by the concentration gradient. Breathing in brings air with a high concentration of oxygen and a very low concentration of carbon dioxide into the alveoli, and breathing out removes air that has become depleted of oxygen and enriched with carbon dioxide. This constant replacement keeps the oxygen concentration in the alveolus higher than that in the blood arriving in the pulmonary capillaries, and the carbon dioxide concentration lower, so oxygen diffuses from the alveolus into the blood and carbon dioxide diffuses the other way, each across the alveolar epithelium and the capillary endothelium.
What earns the marks
- Ventilation continually replacing the air in the alveoli.
- Maintaining a higher oxygen concentration in the alveolus than in the blood, and the converse for carbon dioxide.
- Oxygen diffusing across the alveolar epithelium and capillary endothelium into the blood, and carbon dioxide diffusing out.
- Blood flow removing oxygenated blood and bringing deoxygenated blood, so maintaining the gradient.
- The consequence if the gradient were not maintained, that equilibrium is reached and net diffusion stops.
Common mistakes and how to put them right
Using ventilation and gas exchange as if they were the same process.
Saying the alveoli take in oxygen rather than describing diffusion down a concentration gradient.
Claiming exhaled air contains no oxygen.
Explaining only the air side of the gradient and forgetting the blood side, which is half the answer.
Describing active transport of oxygen into the blood.
Exam tips
- Use the phrase 'maintains the concentration gradient' explicitly; it is the marking point that links ventilation to exchange.
- Deal with oxygen and carbon dioxide separately so both directions of diffusion are stated.
- When asked why breathing rate rises during exercise, link it to maintaining the gradient and to the increased demand of respiring muscle.
Try the lesson
The start of the Gas exchange: Ventilation and the exchange of gases in the lungs. lesson, made from checked teaching notes for this specification point.
Have a go first
Which process describes the movement of oxygen from the alveolar air into the blood?
Listen
Ventilation is the movement of air into and out of the lungs; gas exchange is the diffusion that then takes place at the alveolus. The two are linked by the concentration gradient. Breathing in brings air with a high concentration of oxygen and a very low concentration of carbon dioxide into the alveoli, and breathing out removes air that has become depleted of oxygen and enriched with carbon dioxide. This constant replacement keeps the oxygen concentration in the alveolus higher than that in the blood arriving in the pulmonary capillaries, and the carbon dioxide concentration lower, so oxygen diffuses from the alveolus into the blood and carbon dioxide diffuses the other way, each across the alveolar epithelium and the capillary endothelium. A strong answer earns marks for these points. ventilation continually replacing the air in the alveoli. maintaining a higher oxygen concentration in the alveolus than in the blood, and the converse for carbon dioxide. oxygen diffusing across the alveolar epithelium and capillary endothelium into the blood, and carbon dioxide diffusing out. blood flow removing oxygenated blood and bringing deoxygenated blood, so maintaining the gradient. Exam tip: Use the phrase 'maintains the concentration gradient' explicitly; it is the marking point that links ventilation to exchange.
Key words
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ventilation
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maintaining a concentration gradient
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diffusion at the alveolus
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role of blood flow
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See every definition
- ventilation
- The movement of air into and out of the lungs, continually replacing the air in the alveoli.
- maintaining a concentration gradient
- Keeping the oxygen concentration in the alveolus higher than in the blood arriving in the pulmonary capillaries, and the carbon dioxide concentration lower.
- diffusion at the alveolus
- Oxygen diffusing from the alveolus into the blood and carbon dioxide diffusing the other way, each across the alveolar epithelium and the capillary endothelium.
- role of blood flow
- Removing oxygenated blood and bringing deoxygenated blood, so maintaining the gradient.
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