Bronchioles

1. Overview

Bronchioles are small, thin-walled airways that branch from the bronchi and lead to the gas-exchanging regions of the lungs. Unlike the bronchi, bronchioles lack cartilage in their walls and are characterized by smooth muscle regulation of airway diameter. They play a key role in ventilation, air distribution, and resistance regulation within the lungs. Bronchioles mark the transition from the conducting zone to the respiratory zone of the respiratory tract.

2. Location

Bronchioles are located deep within the lung parenchyma, branching from the tertiary (segmental) bronchi. Their hierarchical order includes:

• Terminal bronchioles: Last part of the conducting zone, supplying air to lobules.

• Respiratory bronchioles: First structures to allow gas exchange via attached alveoli; part of the respiratory zone.

They progressively branch and decrease in size as they approach the alveolar ducts and alveoli.

3. Structure

Bronchioles differ structurally from bronchi and are classified based on size and function:

Terminal Bronchioles

• Epithelium: Simple ciliated columnar or cuboidal epithelium with Clara (club) cells replacing goblet cells distally.

• Wall composition: Lined by smooth muscle with no cartilage and no submucosal glands.

• Diameter: Typically <1 mm.

Respiratory Bronchioles

• Walls are interrupted by alveolar outpouchings allowing gas exchange.

• Epithelium becomes simple cuboidal, gradually replaced by squamous epithelium near alveoli.

Each terminal bronchiole divides into several respiratory bronchioles, which further lead to alveolar ducts and alveolar sacs.

4. Function

The bronchioles serve multiple vital functions:

• Conducting air: Deliver air from larger bronchi to the respiratory surfaces.

• Airflow regulation: Smooth muscle contraction or relaxation adjusts airway resistance and airflow distribution.

• Cleaning and humidifying: Ciliated cells and secretions trap and remove particles and pathogens.

• Transition to gas exchange: Respiratory bronchioles begin the process of alveolar gas exchange.

5. Physiological Role(s)

Bronchioles are integral to respiratory physiology in several ways:

• Airflow resistance: They are the primary site of variable resistance, influenced by autonomic control and chemical mediators (e.g., histamine, leukotrienes).

• Ventilation-perfusion matching: Bronchiolar tone adjusts to ensure proper distribution of air to perfused regions.

• Innate immunity: Club cells secrete protective proteins and surfactant-like substances that contribute to airway defense.

• Mucociliary clearance: Ciliated epithelium aids in transporting mucus and debris toward larger airways for expulsion.

6. Clinical Significance

Bronchiolitis

Inflammation of the bronchioles, common in infants and young children, usually caused by respiratory syncytial virus (RSV). Symptoms include:

• Wheezing and difficulty breathing

• Tachypnea and nasal flaring

• Crackles and reduced air entry on auscultation

Managed with supportive care, hydration, and oxygen therapy.

Asthma

Chronic inflammatory disease marked by bronchiolar hyperresponsiveness, leading to:

• Bronchospasm and narrowing of bronchioles

• Mucus overproduction

• Reversible airflow obstruction

Triggered by allergens, infections, or irritants. Treated with bronchodilators, corticosteroids, and leukotriene modifiers.

Chronic Bronchiolitis and Bronchiolitis Obliterans

Chronic inflammation and fibrosis of small airways, often post-infectious or due to inhalational injury. Leads to fixed obstruction, dyspnea, and hypoxia. In some cases, lung transplant is required.

Chronic Obstructive Pulmonary Disease (COPD)

Includes chronic bronchitis and emphysema; small airways (including bronchioles) are narrowed by inflammation, fibrosis, or mucus plugging. This contributes to:

• Airflow limitation

• Air trapping and hyperinflation

• Progressive dyspnea

Hypersensitivity Pneumonitis

Inflammation of the terminal bronchioles and surrounding alveoli due to repeated inhalation of organic antigens (e.g., mold, bird proteins). Manifests with cough, fever, and shortness of breath.

Airway Hyperresponsiveness Testing

Bronchoprovocation tests (e.g., methacholine challenge) assess bronchiolar responsiveness, helping diagnose asthma in unclear cases.