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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 1 quiz

📚 Cambridge AS & A Level Biology 9700 Hub / Topical Revision
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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

Part 1 of 8Structure 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.

Part 2 of 8Tissues 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.

Part 3 of 8The 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.

Part 4 of 8Fick'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.

Part 5 of 8Recognising 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).

Part 6 of 8Recognising 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.

Part 7 of 8Damage 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.

Part 8 of 8More 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.

Key terms

TermMeaning
SurfactantA substance that lowers surface tension in alveoli so they do not collapse.
Mucus escalatorGoblet-cell mucus swept upward by cilia to remove trapped particles.
Fick's lawDiffusion rate is proportional to surface area × concentration difference ÷ diffusion distance.
Surface area to volume ratioSurface area divided by volume.
MacrophageA phagocyte in the alveoli that engulfs bacteria and particles.
EmphysemaLoss of alveolar elastic fibres that reduces surface area and recoil.

Avoid these mistakes

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.

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