Respiratory Bronchioles

1. Overview

Respiratory bronchioles are small airway passages in the lungs that represent the transition zone between the conducting and respiratory portions of the respiratory tract. Unlike terminal bronchioles, respiratory bronchioles have alveoli budding directly off their walls, making them the first airway segments involved in gas exchange. They play a critical role in both air conduction and oxygen-carbon dioxide exchange, marking the beginning of the respiratory zone of the lungs.

2. Location

Respiratory bronchioles are located:

• Distal to the terminal bronchioles

• Proximal to the alveolar ducts and alveolar sacs

• Within the lung parenchyma, embedded in pulmonary lobules

Each terminal bronchiole gives rise to several respiratory bronchioles, which continue branching into alveolar ducts and sacs within the lung’s acinar units.

3. Structure

Respiratory bronchioles have a hybrid structure that reflects their dual function:

• Wall composition: Composed of simple cuboidal epithelium, transitioning to squamous epithelium near the alveolar outpouchings

• Epithelium: Contains club (Clara) cells, ciliated cells, and few goblet cells in proximal regions

• Alveolar outpouchings: The walls are interrupted by scattered alveoli, enabling gas exchange

• Minimal cartilage and smooth muscle: Unlike larger bronchi, respiratory bronchioles lack supportive cartilage; smooth muscle is sparse and discontinuous

4. Function

Respiratory bronchioles serve two primary functions:

• Air conduction: Continue delivering air from the upper airway to alveolar ducts and sacs

• Gas exchange: Through alveolar openings in their walls, they participate in the exchange of oxygen and carbon dioxide with adjacent pulmonary capillaries

5. Physiological Role(s)

Within pulmonary physiology, respiratory bronchioles contribute to:

• Initiation of gas exchange: Being the first airway segment where alveoli appear, they enable oxygen diffusion into and carbon dioxide out of the bloodstream

• Mucociliary clearance: Ciliated cells help clear inhaled debris and pathogens, although this function decreases with proximity to alveoli

• Ventilation-perfusion matching: Their position in the acinar structure allows close matching of air delivery and blood flow

• Host defense: Club cells secrete surfactant components and detoxify harmful substances

6. Clinical Significance

Respiratory Bronchiolitis

A condition often seen in smokers, characterized by inflammation of the respiratory bronchioles. It may be asymptomatic or present with:

• Chronic cough

• Dyspnea

Can progress to respiratory bronchiolitis–interstitial lung disease (RB-ILD), requiring smoking cessation and corticosteroids.

Bronchiolitis Obliterans

Also known as constrictive bronchiolitis, this is a fibrotic narrowing of bronchioles, often involving respiratory bronchioles. Causes include:

• Post-infectious inflammation (e.g., adenovirus)

• Toxic inhalation (e.g., diacetyl, nitrogen dioxide)

• Chronic graft-versus-host disease (after lung or stem cell transplant)

Leads to airflow obstruction, wheezing, and irreversible pulmonary damage.

Emphysema (Centrilobular Type)

Smoking-related emphysema often begins at the level of respiratory bronchioles, causing:

• Destruction of alveolar walls

• Enlargement of airspaces

• Impaired gas exchange and air trapping

Characterized by decreased diffusion capacity and hypoxemia in advanced stages.

Viral Bronchiolitis (Infants)

Though primarily involving smaller bronchioles, severe viral bronchiolitis (e.g., RSV) may extend to respiratory bronchioles. This can cause:

• Airway obstruction

• Wheezing

• Respiratory distress in infants

Managed supportively with oxygen and hydration.

Environmental Exposure Damage

Inhalation of toxins, dust, or fumes can cause direct injury to respiratory bronchioles. Long-term exposure (e.g., in coal workers or farmers) may lead to:

• Bronchiolitis

• Fibrosis

• Reduced lung compliance