9.1.49 · Gas Exchange

Recognising Structures — Cambridge AS & A Level Biology 9700 Revision Notes & Topical Questions

5 min read 9 parts Lesson 6 of 9 Unit quiz

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

0%read

Lesson summary

9.1.4 Recognising Structures — CAMBRIDGE AS & A LEVEL BIOLOGY 9700 — REVISION NOTES notes: Identify the trachea, bronchus, bronchiole, alveoli and associated vessels in micrographs and relate their ultrastructure to function.Trachea (transverse section) — a thick-walled ring with…

Alveolar duct and alveoli
Alveolus under the microscope
Smooth muscle
Appearance
Type I pneumocytes
Type II pneumocytes

Part 1 of 7Airways in low-power views

Airways in low-power views

Trachea (transverse section)

Trachea (transverse section) a thick-walled ring with a large lumen lined by tall ciliated epithelium.
- (about 15 to 20 mm)
A C-shaped band of cartilage is embedded in the wall, with muscle across the gap.
The cartilage flexes during breathing.

Bronchus

Bronchus smaller than the trachea but similar, with several irregular cartilage plates and ciliated epithelium.
- (lumen about 8 to 9 mm)
Gland ducts secrete mucus into the tube.

Bronchiole

Bronchiole about 1 mm or less across, with a small round or folded lumen, no cartilage, a notable ring of smooth muscle and cuboidal or low columnar epithelium.

Alveolar duct and alveoli

Alveolar duct and alveoli clusters of thin-walled sacs resembling a bunch of grapes at the end of a bronchiole.

Alveolus under the microscope

Alveolus under the microscope a sponge-like appearance because of the air spaces.

Part 2 of 7Wall of the trachea

Lining

Lining ciliated epithelium with goblet cells, with cells tightly packed.

Support

Support strong flexible cartilage surrounds the wall.

Smooth muscle

Smooth muscle it constricts and lets air be expelled with more force.

Cilia

Cilia their density is clear in an electron micrograph, and they sweep mucus carrying bacteria and dust up the trachea.
The mucus is swallowed and pathogens are hopefully destroyed by stomach acid.

Part 3 of 7Alveoli and blood vessels

Appearance

Appearance a mesh of tiny, thin-walled, air-filled spaces.

Septum

Septum the thin partition between alveoli contains capillaries with red cells and a few elastic fibres.

Barrier

Barrier each capillary lies against the alveolar epithelium.
Two layers of flat cells and their fused basement membranes make a barrier only about 0.5 micrometres thick.

Vessels

Vessels a branch of the pulmonary artery runs beside each bronchiole carrying deoxygenated blood.
Arteries have a thick wall of smooth muscle and elastic fibres and a small round lumen.
Veins have a thinner wall and a larger, often irregular lumen.

Part 4 of 7Alveolar cells in electron micrographs

Alveolar cells in electron micrographs

Type I pneumocytes

Type I pneumocytes extremely flat cells that form most of the alveolar surface and are the site of gas exchange.

Type II pneumocytes

Type II pneumocytes small cuboidal cells that secrete surfactant, a mixture of phospholipid and protein.
They contain lamellar bodies, which look like dense whorls of membrane.

Surfactant

Surfactant it lowers the surface tension of the water film lining the alveolus.
Without it, water molecules would pull the walls together and the alveoli would collapse.
Premature babies may lack surfactant and develop respiratory distress.

Part 5 of 7Ultrastructure of airway cells

Ultrastructure of airway cells

Cilia

Cilia each has a 9+2 arrangement of microtubules.

Mitochondria

Mitochondria many lie near the cell surface and supply ATP for ciliary beating.

Goblet cells

Goblet cells they show many mucus vesicles, extensive Golgi apparatus and rough endoplasmic reticulum for making and packaging the glycoprotein mucin.

Part 6 of 7Diseased tissue

Emphysema

Emphysema alveoli look larger and fewer, with walls broken down.

Fibrosis

Fibrosis walls are thickened with scar tissue, so the diffusion distance is greater.

Tuberculosis

Tuberculosis nodules (tubercles) of dead tissue and immune cells form and reduce the gas exchange area.

Lung cancer

Lung cancer uncontrolled cell growth makes a tumour that disrupts tissue structure and blocks airways, and it may spread to other organs.

Comparing

Comparing look at the size and number of air spaces, wall thickness, mucus or fluid in the lumen, lining cell types and amount of connective tissue.

Part 7 of 7More from the specification

1Histological identification of mucous glands

Histological identification of mucous glands light micrographs of tracheal cross-sections reveal distinct submucosal mucous glands beneath pseudostratified ciliated columnar epithelium.

2Mucous gland

A gland that secretes mucus into the airway.

3Photomicrograph

A photograph taken through a light microscope.

4Carbon

Element with four valence electrons that forms the backbone of organic molecules.

5Microorganism

A living organism so small that it can only be seen clearly with a microscope.

6Squamous epithelium

A single layer of very flat cells forming the alveolar wall.

Key terms

Term	Meaning
Type I pneumocyte	Extremely flat alveolar cell that is the site of gas exchange.
Type II pneumocyte	Cuboidal alveolar cell that secretes surfactant.
Surfactant	Phospholipid and protein mixture that lowers surface tension in alveoli.
Septum	The thin partition between neighbouring alveoli.
Lamellar body	A whorl of membrane in type II cells that stores surfactant.
Tubercle	A nodule of dead tissue and immune cells in tuberculosis.

Avoid these mistakes

Saying bronchioles have cartilage — they have none.

Saying the trachea and bronchi differ in lining — both are lined by ciliated epithelium; the bronchus lumen is narrower.

Saying type I pneumocytes make surfactant — type II cells do.

Confusing cilia of the airway with microvilli of the small intestine.

Last updated: 15 October 2026