Bronchioles

Bronchioli

Also known as: bronchiole, club cells, Clara cells

Small airways (< ≈1 mm) without cartilage or glands, held open by the elastic pull of surrounding lung and controlled by a relatively thick layer of smooth muscle.

Show on body map Respiratory Tract

Overview

Diameter< ≈1 mm

Cartilage / glandsAbsent

When bronchi narrow to about 1 mm they lose their cartilage plates and submucosal glands and become bronchioles. The epithelium changes from ciliated pseudostratified to simple ciliated columnar and then simple cuboidal; goblet cells disappear and club (Clara) cells become prominent. Club cells are dome-shaped, non-ciliated secretory cells that produce club-cell secretory protein (CC16), components of the bronchiolar lining fluid, detoxify inhaled xenobiotics with cytochrome P450 enzymes and act as progenitors for the bronchiolar epithelium.

Without cartilage, bronchiolar patency depends on radial traction by the elastic fibres of attached alveolar walls. Relative to their size, bronchioles have the thickest smooth muscle in the airway, so they are the main site of bronchoconstriction in asthma. A lobular bronchiole enters each secondary pulmonary lobule and divides into several terminal bronchioles.

Location: Throughout the lung parenchyma, between the smallest bronchi and the terminal bronchioles (generations ≈12–16).

Parts & subdivisions

Lobular bronchioles
Supply secondary pulmonary lobules (1–2.5 cm polyhedral units bounded by interlobular septa).
Terminal bronchioles
The last purely conducting airways.
Club (Clara) cells
Secretory, detoxifying and progenitor cells of the bronchiolar epithelium.

Anatomy

Histology: Simple ciliated columnar → cuboidal epithelium with club cells; no goblet cells (distally), no glands, no cartilage; prominent spiral smooth muscle; elastic fibres; surrounded by alveolar attachments.

Function

Conduct air to the respiratory zone.
Regulate distribution of airflow via smooth muscle tone.
Secrete protective lining fluid and detoxify inhaled substances (club cells).

Clinical significance

Bronchiolitis

Viral infection of the small airways in infants (most often RSV) causes wheeze, crackles, feeding difficulty and hypoxaemia; management is supportive. Bronchiolitis obliterans (constrictive bronchiolitis) causes fixed obstruction after lung transplantation, toxic inhalation or connective tissue disease.

Small airway collapse in emphysema

Destruction of alveolar attachments removes radial traction, so bronchioles collapse during expiration, causing air trapping and the need for pursed-lip breathing.

Test yourself

Which airway segment is the first to contain alveoli opening from its wall?

Terminal bronchiole
Segmental bronchus
Respiratory bronchiole
Lobar bronchus

Answer: Respiratory bronchiole

Respiratory bronchioles (generations ≈17–19) have scattered alveoli budding from their walls and mark the start of the respiratory zone. Terminal bronchioles (≈generation 16) are the last purely conducting airways.