Bronchioles and terminal bronchiole

Bronchioles · Under 1 mm No cartilage No glands Club cells Gen 4-16

Beyond the subsegmental bronchi the airways become bronchioles: under 1 mm in diameter, with no cartilage, no submucosal glands and a relatively thick ring of smooth muscle, held open only by the elastic recoil of the surrounding lung. About 16 generations of branching end in the terminal bronchiole, the last purely conducting airway, where goblet cells have disappeared and club cells take over.

Traced from the start

Detail

Where cartilage stops: The last cartilage plates are in the smallest bronchi; bronchioles (under 1 mm) have none, so patency depends on radial traction from the alveolar septa
Where glands stop: Submucosal glands end with the cartilage; mucus now comes from goblet cells only, and then only from club cells
Where goblet cells stop: Decline through the bronchioles and are absent by the terminal bronchiole, which has simple ciliated cuboidal epithelium with non-ciliated club (Clara) cells that secrete a surfactant-like film, detoxify inhaled toxins and act as progenitor cells
Smooth muscle: Thickest relative to the wall of any airway; vagal M3 stimulation constricts, beta-2 stimulation (circulating adrenaline, salbutamol) dilates; sympathetic nerves reach the muscle only sparsely
Generations and dead space: Generations about 4 to 16 (Weibel); terminal bronchiole about 0.5 mm; the conducting zone ends here, so the anatomical dead space is about 150 mL (2 mL/kg)
Resistance: Airways under 2 mm contribute less than 20 per cent of total airway resistance (the silent zone); most resistance sits in the medium-sized bronchi

When it goes wrong

Asthma

Smooth-muscle spasm, mucosal oedema and mucus plugging of bronchi and bronchioles: wheeze, prolonged expiration and a reduced FEV1/FVC ratio that reverses with a beta-2 agonist; goblet-cell metaplasia and muscle hypertrophy with chronic disease

Emphysema (loss of elastic recoil)

Bronchioles lose their radial traction and collapse during expiration: air trapping, hyperinflation, pursed-lip breathing and a fixed reduction in FEV1/FVC

Bronchiolitis (RSV, infant under 2 years)

Oedema and necrotic debris plug bronchioles of tiny absolute calibre: wheeze, widespread crackles, hyperinflation and apnoea in the very young

Obliterative bronchiolitis (chronic lung transplant rejection, post-viral, inhaled toxins, rheumatoid disease)

Fibrous obliteration of terminal bronchioles: progressive fixed airflow obstruction unresponsive to bronchodilators

Chronic bronchitis

Goblet-cell metaplasia extends into bronchioles where goblet cells are normally absent; mucus hypersecretion and small-airway obstruction

Practise this structure

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