9.1.69 · Gas Exchange
Gas Exchange Processes — Cambridge AS & A Level Biology 9700 Revision Notes & Topical Questions
5 min read 9 parts Lesson 9 of 9 Unit quiz
Lesson summary
9.1.6 Gas Exchange Processes — CAMBRIDGE AS & A LEVEL BIOLOGY 9700 — REVISION NOTES notes: Describe how oxygen and carbon dioxide are exchanged in the alveoli, how gradients are kept steep and how ventilation is controlled.Process — gas exchange is simple diffusion, a passive pro…
- Blood arriving
- Alveolar air
- Circulation
- Haemoglobin
- In the lungs
- Expired air
Part 1 of 7Diffusion across the alveolar barrier
1Process
- Process gas exchange is simple diffusion, a passive process needing no ATP, from higher to lower partial pressure.
2Blood arriving
- Blood arriving deoxygenated blood from respiring tissues has low pO2 and high pCO2.
3Alveolar air
- Alveolar air it has higher pO2 and lower pCO2.
4Oxygen
- Oxygen it dissolves in the moisture lining the alveolus.
- It diffuses across the squamous epithelium and basement membrane, then the capillary endothelium.
- It enters plasma and then red cells, where it binds to haemoglobin.
5Carbon dioxide
- Carbon dioxide it moves the opposite way, from blood into alveolar air, to be breathed out.
Part 2 of 7Keeping the gradient steep
Ventilation
- Ventilation it replaces alveolar air with fresh air.
Circulation
- Circulation it constantly replaces the blood.
Haemoglobin
- Haemoglobin it removes oxygen from plasma as oxyhaemoglobin, so plasma oxygen stays low and more diffuses in.
Carbon dioxide
- Carbon dioxide about 85 per cent travels as hydrogencarbonate ions formed in red cells by carbonic anhydrase.
- CO2 + H2O ⇌ H2CO3 ⇌ H+ + HCO3-.
In the lungs
- In the lungs the reaction reverses and CO2 diffuses into the alveoli.
- H+ is taken up by haemoglobin, forming haemoglobinic acid.
- Loss of CO2 raises pH, so haemoglobin loads oxygen more readily (a Bohr effect feature).
Part 3 of 7Composition of air
| Gas | Inspired air (%) | Alveolar air (%) | Expired air (%) |
|---|---|---|---|
| Oxygen | 21 | about 14 | about 16 |
| Carbon dioxide | 0.04 | about 5.5 | about 4 |
| Nitrogen | 78 | about 80 | about 79 |
| Water vapour | variable | saturated | saturated |
Expired air
- Expired air it has less oxygen and more CO2 than inspired air, and is saturated with water vapour.
Dead space
- Dead space expired air mixes alveolar air with air from the airways that took no part in gas exchange,
- so it lies between inspired and alveolar composition.
Part 4 of 7Investigating breathing
Spirometer trace
- Spirometer trace the number of peaks per minute gives breathing rate and the height of each peak gives tidal volume.
Exercise
- Exercise both rise because muscles make more CO2, which lowers blood pH.
- Chemoreceptors detect this and the medulla increases ventilation.
Oxygen consumption
- Oxygen consumption it is found from the falling trace as oxygen is used and CO2 is absorbed by soda lime.
Safety
- Safety avoid overbreathing and fit a fresh, dry mouthpiece for each user.
Part 5 of 7Control of ventilation
Ventilation centre
- Ventilation centre it is in the medulla oblongata.
Chemoreceptors
- Chemoreceptors they are in the medulla and in the carotid and aortic bodies, and detect a rise in CO2 and the associated fall in pH.
Response
- Response impulses go to the medulla, then along motor neurones to the diaphragm and intercostal muscles, increasing rate and depth of breathing.
Feedback
- Feedback faster, deeper breathing removes CO2 and returns pH to normal, which is negative feedback.
Oxygen
- Oxygen it has a much smaller effect on normal control; CO2 is the main stimulus.
Stretch receptors
- Stretch receptors they prevent over-inflation by sending inhibitory signals to the medulla.
Breath holding
- CO2 builds up until the urge to breathe cannot be resisted.
Part 6 of 7Gas exchange in exercise
Gradients
- Gradients muscles use more oxygen and make more CO2, so gradients between alveolar air and blood become steeper.
Deeper breaths
- Deeper breaths they recruit more alveoli.
Cardiac output
- Cardiac output it rises, carrying blood faster through capillaries and keeping gradients steep.
Overall
- Overall together these can raise oxygen uptake several-fold.
Part 7 of 7More from the specification
More from the specification
Bronchial secretion by mucous glands
- Bronchial secretion by mucous glands goblet cells and submucosal mucous glands produce protective mucus, trapping inhaled pathogens away from alveoli.
Cartilage in the Respiratory Tract
- Incomplete C-shaped rings of cartilage support the trachea and irregular cartilage blocks reinforce the bronchi, keeping airways open and preventing collapse under negative pressure during ventilation.
Mucociliary Escalator
- Goblet cells secrete sticky mucus to trap inhaled pathogens and particulate matter,
- while the rhythmic beating of cilia on ciliated epithelial cells sweeps the mucus up toward the pharynx to be swallowed.
Bronchiole Structure and Airway Regulation
- Narrower bronchioles lack cartilage and are surrounded by smooth muscle and elastic fibres,
- allowing constriction and dilation to regulate airflow into the gas exchange alveoli.
Cartilage in the Respiratory Tract
- Incomplete C-shaped rings of cartilage support the trachea and irregular cartilage blocks reinforce the bronchi, keeping airways open and preventing collapse under negative pressure during ventilation.
Mucociliary Escalator
- Goblet cells secrete sticky mucus to trap inhaled pathogens and particulate matter,
- while the rhythmic beating of cilia on ciliated epithelial cells sweeps the mucus up toward the pharynx to be swallowed.
Bronchiole Structure and Airway Regulation
- Narrower bronchioles lack cartilage and are surrounded by smooth muscle and elastic fibres,
- allowing constriction and dilation to regulate airflow into the gas exchange alveoli.
Ciliated epithelium
- The tissue with cilia that lines the tubes leading to the lungs.
Goblet cells
- Cells in the airway lining that secrete mucus.
Smooth muscle
- Involuntary muscle that narrows or widens airways.
Electron micrograph
- A photograph taken through an electron microscope.
Elastic fibres
- Fibres allowing the lungs to stretch and recoil.
Mucous gland
- A gland that secretes mucus into the airway.
Trachea
- The windpipe, held open by C-shaped cartilage rings.
Cartilage
- The strong flexible material in rings that keeps the trachea open.
Bronchiole
- A small tube carrying air from a bronchus to the alveoli.
Photomicrograph
- A photograph taken through a light microscope.
Micrograph
- A photograph taken through a microscope.
Microorganism
- A living organism so small that it can only be seen clearly with a microscope.
Bronchus
- One of two branches of the trachea leading to each lung.
Bacteria
- Single-celled microorganisms, much smaller than plant or animal cells.
Key terms
| Term | Meaning |
|---|---|
| Simple diffusion | Passive movement of molecules down a concentration gradient. |
| Partial pressure | The pressure exerted by one gas in a mixture. |
| Chemoreceptor | A receptor detecting a chemical change such as a fall in pH. |
| Negative feedback | A change triggers a response that reverses it. |
| Soda lime | A chemical that absorbs CO_{2} in a spirometer. |
| Dead space | Airway volume where no gas exchange occurs. |
Avoid these mistakes
1
Saying gas exchange needs ATP — diffusion is passive.
2
Saying expired air has no oxygen — it still has about 16 per cent.
3
Saying low oxygen is the main stimulus for breathing — rising CO_{2} (falling pH) is.
4
Saying gradients stay steep without blood flow and ventilation — both are needed.
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