9.19 · Gas Exchange
The Gas Exchange System — Cambridge AS & A Level Biology 9700 Revision Notes & Topical Questions
10 min read 16 parts Lesson 2 of 9 Unit quiz
Lesson summary
9.1 The Gas Exchange System — CAMBRIDGE AS & A LEVEL BIOLOGY 9700 — REVISION NOTES notes: Describe the structure of the gas exchange system, the functions of its tissues and how gas exchange occurs by diffusion.Airways — the trachea divides into two bronchi, which branch repeated…
- Pleural membranes
- Capillaries
- Path of air
- Ciliated epithelium and goblet cells
- Smooth muscle
- Elastic fibres
Section 1 of 16Structure of the system
Structure of the system
Airways
- Airways the trachea divides into two bronchi, which branch repeatedly into bronchioles ending in clusters of alveoli.
Pleural membranes
- Pleural membranes they enclose the lungs in the thorax.
Capillaries
- Capillaries each alveolus is surrounded by a dense capillary network.
Path of air
- Path of air air is warmed, moistened and filtered in the nose or mouth,
- then passes down the trachea, through the bronchi and bronchioles to the alveoli.
Section 2 of 16Tissues of the airways
1Cartilage
- Cartilage it keeps the trachea and bronchi open.
- In the trachea it is C-shaped; the gap at the back lets the oesophagus expand when food is swallowed.
- Bronchi have irregular cartilage plates.
- Bronchioles have none, so smooth muscle can change their diameter.
2Ciliated epithelium and goblet cells
- Ciliated epithelium and goblet cells they form a mucus escalator.
- Mucus traps dust and pathogens.
- Cilia beat to move it up to the throat, protecting the alveoli.
3Smooth muscle
- Smooth muscle contraction narrows airways; relaxation widens them during exercise, increasing airflow.
- (for example in asthma or response to irritants)
4Elastic fibres
- Elastic fibres they are stretched on breathing in and recoil on breathing out.
- This helps push air out of the alveoli.
- They also prevent alveoli bursting.
Section 3 of 16The alveolus and gas exchange
Wall
- Wall alveolar walls are squamous epithelium.
- With the capillary endothelium this gives a diffusion distance of less than 1 micrometre (two thin squamous cells).
Capillaries
- Capillaries they are narrow, so red cells are pressed against the wall, which slows blood flow and reduces the diffusion distance.
Diffusion
- O2 diffuses from alveolar air (high pO2) into blood (low pO2), and CO2 diffuses from blood (high pCO2) into alveolar air.
Gradients
- Gradients continuous blood flow removes oxygenated blood and brings deoxygenated blood, and ventilation brings fresh air.
- Both maintain steep concentration gradients.
Surfactant
- Surfactant it lowers surface tension so alveoli do not collapse at the end of expiration.
Macrophages
- Macrophages phagocytes in the alveoli engulf bacteria and particles that reach them.
Section 4 of 16Fick's law and surface area
Fick's law and surface area
Fick's law
- Fick's law rate of diffusion is proportional to ÷ diffusion distance.
- (surface area × concentration difference)
Surface area to volume ratio
Surface area to volume ratio surface area ÷ volume.
- Larger organisms have smaller ratios, so they need specialised gas exchange surfaces.
- For a cube of side L, surface area is 6L2, volume is L3, so the ratio is 6/L.
Size from images
- Size from images actual size = image size ÷ magnification, and 1 mm = 1000 micrometres.
Section 5 of 16Recognising cells and tissues
Goblet cells
- Goblet cells in micrographs they lack cilia, are cup-shaped with the nucleus near the base,
- and have a large Golgi apparatus and many mucus granules near the free surface.
- (with a few microvilli)
Cartilage
- Cartilage it looks pale and glassy and has no blood vessels or nerves.
Smooth muscle
- Smooth muscle closely packed spindle-shaped cells beneath the epithelium, with no stripes (unstriated).
Section 6 of 16Recognising airway sections
Cartilage test
- Cartilage test a C-shaped ring means trachea, irregular plates mean bronchus, and none means bronchiole or alveolus.
Lining
- Lining ciliated epithelium with goblet cells suggests trachea, bronchus or larger bronchiole.
Bronchioles
- Bronchioles the smallest are lined by simple cuboidal epithelium, and their wall is mainly smooth muscle and elastic fibres.
- (about 1 mm or less)
- Without cartilage they can narrow or close.
Alveoli
- Alveoli thin-walled air sacs with squamous cells and capillaries.
Section 7 of 16Damage by tobacco smoke
Tar
- Tar it paralyses cilia and increases mucus, leading to bronchitis.
Emphysema
- Emphysema loss of elastic fibres from alveoli reduces surface area and recoil.
Section 8 of 16More from the specification
More from the specification
Mucociliary clearance by mucous glands
- Mucociliary clearance by mucous glands secretions from submucosal mucous glands trap inhaled dust and microbes,
- which are swept upward by ciliated epithelial cells.
Human gas exchange system structure
- The human respiratory tract comprises the trachea, bronchi, bronchioles,
- and alveoli, specialised for efficient exchange of oxygen and carbon dioxide between air and blood.
Photomicrograph and electron micrograph analysis
- Recognising respiratory tissues in a light photomicrograph or transmission electron micrograph involves identifying ciliated epithelial cells, mucus-secreting goblet cells,
- and smooth muscle.
Protection against microorganisms
- Inhaled dust and microorganisms are trapped by sticky mucus from goblet cells and swept upwards away from the lungs by the synchronized beating of ciliated cells.
Carbon dioxide excretion across alveolar membranes
- Carbon dioxide diffuses down its concentration gradient from pulmonary capillary blood across the squamous alveolar epithelium into alveolar air to be exhaled.
Photomicrograph
- A photograph taken through a light microscope.
Carbon
- Element with four valence electrons that forms the backbone of organic molecules.
Microorganism
- A living organism so small that it can only be seen clearly with a microscope.
Electron micrograph
- A photograph taken through an electron microscope.
Mucous gland
- A gland that secretes mucus into the airway.
Section 9 of 16Airway structure revisited
Airway structure revisited
Bronchioles
- Bronchioles about 1 mm or less across, with smooth muscle and cuboidal epithelium in the walls.
Bronchi
- Bronchi lined with ciliated epithelium, with cartilage hoops for support and gland ducts that secrete mucus.
Trachea
- Trachea a lumen of about 15 to 20 mm, with a wall surrounded by strong flexible cartilage that flexes during breathing.
Alveoli
- Alveoli surrounded by an extensive capillary network and lined by squamous epithelium.
Section 10 of 16Structure and function of the tissues
Structure and function of the tissues
Protection
- Protection ciliated epithelial cells, goblet cells and mucous glands maintain the health of the system.
Structure
- Structure cartilage, smooth muscle, elastic fibres and squamous epithelium play structural roles.
Squamous epithelium
- Squamous epithelium flattened cells lining the alveoli;
- the layer is thin and permeable for easy gas diffusion.
Cilia versus microvilli
- Cilia versus microvilli cilia sweep mucus, while microvilli increase surface area for absorption in the small intestine.
Section 11 of 16Gas exchange by diffusion
1Process
- Process oxygen and carbon dioxide are exchanged between alveoli and capillaries by simple diffusion, a passive movement from high to low concentration.
2Oxygen
- Oxygen alveolar air has a high oxygen concentration and blood from respiring tissues a lower one,
- so oxygen diffuses into the blood and binds to haemoglobin.
3Carbon dioxide
- Carbon dioxide blood has a higher concentration, so it diffuses into the alveolus and is breathed out.
4Fick's law
- Fick's law rate of diffusion is proportional to ÷ diffusion distance.
- (surface area × concentration difference)
Section 12 of 16Alveolar features
Alveolar features
Large surface area
- Large surface area an adult has about 480 to 500 million alveoli with 40 to 75 m2 of surface, which greatly increases the space for diffusion.
Thin barrier
- Thin barrier the alveolar wall and capillary are each one cell thick, a combined thickness of about 0.5 micrometres, so diffusion is fast.
Gradient
- Gradient a rich blood supply and good ventilation keep it steep.
Section 13 of 16Partial pressure gradients
Partial pressure gradients
Oxygen
- Oxygen alveolar pO2 is 13.3 kPa and blood entering the capillary 5.3 kPa, a difference of 8.0 kPa that drives diffusion.
Maintaining the gradient
- Maintaining the gradient oxygen bound to haemoglobin is removed from solution along the capillary.
Carbon dioxide
- Carbon dioxide blood is about 6.1 kPa and the alveolus about 5.3 kPa, a smaller gradient.
- CO2 is much more soluble, so it still diffuses fast enough.
Exercise
- Exercise more ventilation brings alveolar pO2 closer to atmospheric levels and keeps the gradient steep.
Red cells
- Red cells they are squeezed against the capillary wall, reducing diffusion distance and increasing contact time.
Section 14 of 16More from the specification
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.
Bacteria
- Single-celled microorganisms, much smaller than plant or animal cells.
Electron micrograph
- A photograph taken through an electron microscope.
Section 15 of 16Key terms
| Term | Meaning |
|---|---|
| Surfactant | A substance that lowers surface tension in alveoli so they do not collapse. |
| Mucus escalator | Goblet-cell mucus swept upward by cilia to remove trapped particles. |
| Fick's law | Diffusion rate is proportional to surface area × concentration difference ÷ diffusion distance. |
| Surface area to volume ratio | Surface area divided by volume. |
| Macrophage | A phagocyte in the alveoli that engulfs bacteria and particles. |
| Emphysema | Loss of alveolar elastic fibres that reduces surface area and recoil. |
| Term | Meaning |
|---|---|
| Simple diffusion | Passive movement from high to low concentration. |
| Squamous epithelium | Flattened cells forming the thin alveolar lining. |
| Partial pressure gradient | Difference in pressure of a gas that drives its diffusion. |
| Fick's law | Diffusion rate is proportional to surface area × concentration difference ÷ distance. |
| Alveolar barrier | The alveolar wall plus capillary wall, about 0.5 micrometres thick. |
| Oxyhaemoglobin | Haemoglobin combined with oxygen. |
Section 16 of 16Exam technique
1
Saying bronchioles are held open by cartilage — they have none, so smooth muscle changes their diameter.
2
Saying a thick wall helps gas exchange — a very thin alveolar wall gives a short diffusion distance.
3
Saying oxygen diffuses into the alveoli — it diffuses from alveolar air into the blood.
4
Saying elastic fibres pump air out actively — they recoil passively after being stretched.
1
Saying gas exchange is active transport — it is passive diffusion.
2
Saying CO_{2} has a bigger gradient than O_{2} — its partial pressure gradient is smaller, but it is more soluble.
3
Saying the diffusion barrier is many cells thick — it is two thin cells, about 0.5 micrometres.
4
Confusing cilia and microvilli.
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