Air reaches the alveoli through a branching tree of airways. As the tubes get narrower, their walls lose cartilage, gain proportionally more smooth muscle and keep a ciliated, mucus-covered lining that cleans the air on the way in.
Air passes through the trachea, which divides into two bronchi, one to each lung; these branch into many bronchioles, which end in clusters of alveoli surrounded by a capillary network. Cartilage is found as C-shaped rings in the trachea and irregular plates in the bronchi, but not in bronchioles or alveoli. Ciliated epithelium with goblet cells lines the trachea, bronchi and larger bronchioles. Smooth muscle and elastic fibres are in the walls of all the airways, with relatively most smooth muscle in the bronchioles. Squamous epithelium forms the alveolar walls, which are wrapped in capillaries. Goblet cells and mucous glands secrete mucus that traps dust, pollen and bacteria, and the cilia beat to sweep the mucus up to the throat, where it is swallowed.
The lab opens on the whole system. Rotate it, then click each part, or pick a chip, and read the explanation for lungs, trachea, bronchi, bronchioles, alveoli and capillary network.
Pick trachea, bronchus, bronchiole and alveolus in turn and click each tissue in the wall. Compare what you see with the table in the panel: which tissues disappear as the airways get smaller?
| Tissue | Trachea | Bronchus | Bronchiole | Alveolus |
|---|---|---|---|---|
| cartilage | C-shaped rings | irregular plates | none | none |
| ciliated epithelium | yes | yes | larger ones only | no |
| goblet cells | yes | yes | few or none | no |
| smooth muscle | yes | yes | yes, relatively more | no |
| elastic fibres | yes | yes | yes | yes |
| squamous epithelium | no | no | no | yes |
| capillaries | in the wall | in the wall | in the wall | a dense network around each |
Mucous glands lie in the submucosa beneath the epithelium of the trachea and bronchi and open onto the surface through ducts.
Bronchioles have no cartilage; that is the quickest way to tell a bronchiole from a bronchus on a slide. Do not say the trachea has complete rings: the rings are C-shaped, and the gap at the back, next to the oesophagus, is bridged by smooth muscle.
Watch the particles fall onto the mucus and drift towards the throat. Read MUCUS MOVED and PARTICLES TRAPPED, then tick Cigarette smoke and watch what changes.
Cilia do not trap particles and they do not make mucus. Mucus (from goblet cells and mucous glands) traps; cilia move the mucus. Keep the two jobs separate in your answer.
Four quick checks. Each correct answer earns XP and records this skill on your profile.
In the gas exchange system tab, which parts have no cartilage according to the explanation panel?
In the airway walls tab, what does the table show for cartilage in the bronchus?
When you pick the alveolus chip in the airway walls tab, which tissues are present?
What happens to the MUCUS MOVED read-out when Cigarette smoke is ticked?
You must identify these tissues and airways in light micrographs and electron micrographs, and draw plan diagrams of the trachea and bronchus walls.
| Tissue | How to recognise it |
|---|---|
| cartilage | a large, smooth, glassy-looking region of matrix with scattered cells (chondrocytes) sitting in small spaces |
| ciliated epithelium | a layer of tall (columnar) cells lining the lumen, with a fringe of cilia on the free surface; nuclei at different heights |
| goblet cells | cup-shaped cells among the ciliated cells, with a pale, mucus-filled upper part and no cilia |
| smooth muscle | bundles of long spindle-shaped cells, each with one elongated central nucleus, around the airway |
| squamous epithelium | very thin, flattened cells with flattened nuclei forming the alveolar walls |
| capillaries | tiny vessels with a lumen about the width of one red blood cell, often with red blood cells inside |
Unlocks once the four checks above are done. Worth more XP, written to AS Paper 1 and Paper 2 standard.
Which tissue is present in the wall of a bronchus but absent from the wall of a bronchiole?
A micrograph shows a tube with a ciliated lining, no cartilage and a thick ring of smooth muscle. What is it?
What is the function of goblet cells in the trachea?
Which feature should a plan diagram of the trachea wall include?
This skill is now recorded on your profile. Keep the chain going.