What Are the Bronchioles?

Bronchioles are air passages inside the lungs. They branch off like tree limbs from the bronchi—the two main air passages through which air flows from the trachea (windpipe) after being inhaled through the nose or mouth.

As they do, the bronchioles become smaller and smaller, passing across the interior of each lung. They end at clusters of tiny air sacs called alveoli, where oxygen and carbon dioxide are exchanged.

Bronchioles are vulnerable to conditions like asthma, bronchiolitis, cystic fibrosis, and emphysema that can cause constriction and/or obstruction of the airways.

This article discusses the anatomy and function of the bronchioles, as well as conditions that can affect them.

Bronchiole Anatomy and Types

The bronchioles themselves are tiny, ranging from 0.3 to 1 mm in diameter. There are three types, categorized by size:

• Lobular bronchioles: Larger passages that first enter the lobes of the lungs
• Respiratory bronchioles: Two or more branches from each terminal bronchiole that, in turn, lead to two to 10 alveolar ducts
• Terminal bronchioles: 50 to 80 smaller passages in each lung

Bronchiole Structure

Bronchioles rely on bundles of stretchy protein fibers called elastin to anchor themselves into lung tissue and hold their shape.

The lining of the bronchioles, called lamina propria, is thin and surrounded by a layer of smooth muscle. It contracts when the flow of blood is decreased and dilates when the flow of blood is increased.

The walls of the bronchioles have two notable components:

• Cilia: These tiny finger-like projections line the bronchioles and work to move debris and germs out of the airways. The number of cilia in the airway decreases as the bronchioles branch off and get smaller and smaller.
• Club cells: These cells in the lining of the bronchioles secrete surfactants, substances that reduce surface tension within airways. This allows the airways to expand during inhalation, preventing them from collapsing during exhalation. Club cells also secrete enzymes that break down debris and irritants so that they can be readily excreted from the lung.

Bronchiole Function

The function of the bronchioles is to deliver air to a diffuse network of around 300 million alveoli in the lungs. As you inhale, oxygenated air is pulled into the bronchioles. Carbon dioxide collected by the alveoli is then released from the lungs as you exhale.

The smooth muscles that surround the airways will automatically constrict (close) and dilate (open) to control the flow of air in and out of the lungs.

Dilation of the airways (bronchodilation) occurs when the lungs need more oxygen, such as during exercise or at higher altitudes.

Narrowing of the airways (bronchoconstriction) can occur when irritants or allergens are inhaled. While this is meant to keep foreign substances from entering the lungs, it can restrict breathing, sometimes severely. Certain medications, inflammation, and disease can do the same.

Conditions That Affect Bronchioles

Lung disease can be classified as either obstructive or restrictive. Either type can result in breathing difficulty and shortness of breath.

Obstructive lung diseases cause difficulty with exhaling air due to some form of blockage. Conditions that are considered obstructive include:

• Asthma
• Bronchiolitis
• Emphysema
• Bronchiolitis obliterans
• Cystic fibrosis

Restrictive lung disease can restrict one's ability to fill the lungs with air during inhalation. Conditions that are considered restrictive include:

• Bronchopneumonia
• Interstitial lung disease (ILD)

Asthma

Asthma is an allergic condition triggered primarily by allergens or irritants in the air and, in children, respiratory infections.

When allergens enter the bronchioles, a type of immune cell called a mast cell releases a substance called histamine that causes smooth muscles of the bronchioles to contract.

Bronchiolitis

Bronchiolitis is inflammation of the bronchioles. It's most common in babies between 3 months and 6 months of age who become infected with a virus such as respiratory syncytial virus (RSV) or influenza.

Bronchiolitis Obliterans

Bronchiolitis obliterans is a rare inflammatory condition that causes scarring (fibrosis) of the bronchioles to the extent that the airways are blocked.

Nicknamed "popcorn lung," bronchiolitis obliterans mainly affects adults and may be caused by:

• Excessive or prolonged exposure to toxins such as ammonia, chlorine, formaldehyde, sulfur dioxide, nicotine, diacetyl (used in butter flavoring), and acetaldehyde (found in e-cigarettes)
• Organ transplants
• Prolonged inhalation of fiberglass particles or fly ash from coal
• Rheumatoid arthritis and other autoimmune diseases
• Viral lung infections, including COVID-19

Bronchiolitis obliterans is not reversible. A lung transplant may be needed in severe cases.

Cystic Fibrosis

Cystic fibrosis is an inherited, life-threatening disorder that damages the lungs and digestive system.

Cystic fibrosis causes goblet cells to produce excessive amounts of mucus that clogs tubes, ducts, and passages in the respiratory and intestinal tracts. As the disease advances, it can cause scarring of the bronchioles as well as the collapse of the lung (atelectasis).

There is no cure for cystic fibrosis, but it can be slowed with medications, antibiotics, physical therapies, and vaccinations.

Emphysema

Emphysema is one form of congestive obstructive pulmonary disease (COPD), a group of lung diseases that block airflow and make it difficult to breathe. In emphysema, damage to the alveoli and lung tissue causes the bronchioles to collapse.

Smoking is the leading cause of emphysema. Others include air pollution, secondhand smoke, chronic respiratory infections, and occupational exposure to dust and chemicals.

Emphysema can't be reversed, but it can be treated with bronchodilators, anti-inflammatory drugs, supplemental oxygen, and surgery to remove damaged tissue.

Bronchopneumonia

Bronchopneumonia is a widespread lower-respiratory lung infection that affects the bronchi and bronchioles.

It's commonly caused by bacteria such as Streptococcus pneumoniae and Haemophilus influenzae type b (Hib), and can be treated effectively with antibiotics.

Interstitial Lung Disease

Interstitial lung disease refers to a large group of conditions that cause inflammation and scarring of lung tissue.

There are a number of interstitial lung diseases that can affect the bronchioles, including hypersensitivity pneumonitis and sarcoidosis.

Lung Function Testing

Lung function tests, also known as pulmonary function tests (PFTs), are frequently used to check how well your lungs are working. They can be used to diagnose certain conditions that affect the bronchioles, as well as monitor chronic lung diseases.

PFTs include:

• Spirometry: This test is used to determine how much and how quickly you can move air in and out of your lungs. It can be used to diagnose conditions such as asthma and COPD.
• Lung volume test: Also known as lung plethysmography, this test measures how much air you can hold in your lungs and the amount of air that remains after you breathe out as much as you can. It can help tell the difference between obstructive and restrictive lung diseases.
• Gas diffusion test: This type of testing measures how well you are able to pass oxygen from the air sacs in the lungs to the bloodstream. Conditions that can lead to abnormal results include asthma, emphysema, sarcoidosis, and inflammation of the lungs.

Depending on PFT results and the suspected diagnosis, other types of testing and imaging may be done, including:

• Chest X-ray
• Computed tomography (CT) scan 
• Bronchoscopy
• Lung biopsy
• Bronchoalveolar lavage
  

Treatment

The course and duration of treatment can vary based on whether the condition is restrictive and/or obstructive, as well as the underlying cause. Some treatments are designed to lessen acute episodic symptoms called exacerbations, while others prevent the worsening or recurrence of symptoms.

Acute conditions, like bronchiolitis, may only need short-term treatment to resolve the underlying infection.

Chronic bronchiolar obstruction, which occurs with COPD and cystic fibrosis, typically require medications, physical therapies, and supportive care to slow disease progression. Surgery sometimes is needed.

Some common treatments for conditions that affect the bronchioles are described below.

Trigger Avoidance

Avoidance of triggers is essential to keep reducing symptoms and is a key aspect of asthma treatment in particular. Some common triggers include:

• Smoking and secondhand smoke
• Air pollution
• Cleaning solutions
• Dust
• Pollen
• Pet dander
• Dust mites

Since infections can also trigger respiratory conditions, do your best to avoid those who are sick, use good hand hygiene, and stay up-to-date on your vaccinations.

Medications

Several medications may be used to treat conditions that affect the bronchioles:

• Bronchodilators, taken orally or inhaled to open airways, including beta-adrenergic agonists like albuterol, anticholinergics like Spiriva (tiotropium), and methylxanthines like aminophylline
• Biologic drugs like Xolair (omalizumab), which block the release of histamine in people for whom antihistamines do not work
• Inhaled corticosteroids, like Flovent HFA (fluticasone) or QVAR RediHaler (beclomethasone) to decrease inflammation and reduce the frequency and severity of exacerbations
• Oral corticosteroids such as prednisone to treat acute exacerbations
• Oral leukotriene modifiers like Singulair (montelukast) and Zyflo (zileuton), which help control inflammation in the airway
• Mucolytics that thin mucus so it can be coughed up more easily

Procedures and Therapies

These may be used in tandem with other options and at various points in the course of treatment:

• Nasal suctioning to remove excess mucus from the nasal passages and upper respiratory tract
• Oxygen therapy to provide supplemental oxygen to those with chronic breathing difficulty, either on a full-time basis or as needed
• Airway clearance devices, including high-intensity oscillators or positive expiratory pressure (PEP), to aid in mucus clearance
  
• Diaphragmatic breathing—inhaling and exhaling with the abdomen rather than the chest to increase lung capacity, the amount of air your lungs can hold
  
• Pulmonary rehabilitation, including techniques like postural drainage and percussion, to regularly clear mucus in people with chronic bronchiolar obstruction