The Anatomy of the Bronchioles

Bronchioles are air passages inside the lungs that branch off like tree limbs from the bronchi—the two main air passages into which air flows from the trachea (windpipe) after being inhaled through the nose or mouth. The bronchioles deliver air to tiny sacs called alveoli where oxygen and carbon dioxide are exchanged. They are vulnerable to conditions like asthma, bronchiolitis, cystic fibrosis, and emphysema that can cause constriction and/or obstruction of the airways.

Anatomy

The bronchioles are part of the lower respiratory system. As they branch off from the bronchi, they become smaller and smaller, traversing the interior of each lung before ending at clusters of alveoli. There are three types, categorized by size:

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

The bronchioles themselves are tiny, ranging from 0.3 to 1 mm in diameter.

Structure

Bronchioles rely on bundles of protein fibers, called elastin, to hold their shape by anchoring themselves into lung tissue.

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

The walls of the bronchioles are also lined with tiny finger-like projections, called cilia, whose function is to move debris and microbes out of the airways. The density of cilia decreases as the bronchioles branch off and get smaller and smaller.

The bronchioles are lined with club cells that secrete surfactants—compounds that reduce surface tension within airways, allowing them to expand during inhalation and 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.

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 expelled from the lungs as you exhale.

Associated Conditions

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

Symptoms of bronchoconstriction include:

• Shortness of breath
• Wheezing
• Coughing
• Chest tightness
• Extreme tiredness with physical activity
• Cyanosis (bluish skin due to the lack of oxygen)

Bronchoconstriction may be accompanied by bronchiolar obstruction caused when the airways are blocked, as from the overproduction of mucus. Symptoms of bronchiolar obstruction can overlap with those of bronchoconstriction and include:

• A chronic productive cough
• Wheezing
• Chest tightness
• Cyanosis
• Repeated respiratory infections

Among conditions in which the bronchioles are involved are:

• Asthma: 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: 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: A rare inflammatory condition that causes scarring (fibrosis) of the bronchioles to the extent that the airways are blocked. Nicknamed "popcorn lung," bronchiolitis obliterans primarily affects adults and may be caused by excessive or prolonged exposure to toxins such as ammonia, chlorine, formaldehyde, sulfur dioxide, nicotine, diacetyl (used for butter flavoring), and acetaldehyde (found in e-cigarettes); prolonged inhalation of fiberglass particles or fly ash from coal; viral lung infections, including coronavirus (COVID-19); organ transplants; and rheumatoid arthritis and other autoimmune diseases.  Bronchiolitis obliterans is irreversible: In severe cases a lung transplant may be needed.
• Cystic fibrosis: 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 gastrointestinal 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 preventive vaccinations.
• Emphysema is one manifestation 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. The leading cause of emphysema is smoking. 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.

Treatment and Rehabilitation

The course and duration of bronchiolar rehabilitation can vary based on whether the condition is restrictive and/or obstructive. Some treatments are designed to alleviate acute episodic symptoms (called exacerbations), while others prevent the worsening or recurrence of symptoms.

Restrictive Disorders

With bronchoconstriction, rehabilitative and treatment options include:

• Bronchodilators, taken orally or inhaled to open airways, including beta-adrenergic agonists like albuterol, anticholinergics like Spiriva (tiotropium), and methylxanthines like aminophylline
• Inhaled corticosteroids, like Flovent HFA (fluticasone) or QVAR RediHaler (beclomethasone) to 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) that help control inflammation in the airway
• Biologic drugs, like Xolair (omalizumab) that block the release of histamine in people for whom antihistamines do not work
• Avoidance of triggers such as allergens or environmental pollutants. This is a key aspect of asthma treatment.

Obstructive Disorders

Acute conditions, like bronchiolitis, may only need short-term treatment to resolve the underlying infection. Chronic bronchiolar obstruction, such as occurs with COPD and cystic fibrosis, typically require medications, physical therapies, and supportive care to slow disease progression. Surgery sometimes is needed.

The rehabilitative and treatment options for bronchiolar obstruction include:

• Diaphragmatic breathing—inhaling and exhaling with the abdomen rather than the chest to increase lung capacity
• Mucolytics that thin mucus so it can be coughed up more easily
• Bronchodilators to open airways blocked by the accumulation of mucus
• Inhaled corticosteroids to reduce inflammation caused by obstruction
• Nasal suctioning to remove excess mucus from the nasal passages and upper respiratory tract
• Pulmonary rehabilitation, including techniques like postural drainage and percussion, to regularly clear mucus in people with chronic bronchiolar obstruction
• Airway clearance devices, including high-intensity oscillators or positive expiratory pressure (PEP) to aid in mucus clearance
• Oxygen therapy, to provide supplemental oxygen to those with chronic breathing difficulty, either on a full-time basis or as needed

In addition, many people with chronic obstructive disorders like COPD and cystic fibrosis may find 20 to 30 minutes of physical exercise five days per week is a useful part of treatment.