The Purpose and Common Disorders of the Bronchioles

The bronchioles are an important part of the respiratory system. They are the passages through which air is directed from the nose and mouth to the alveoli (air sacs) at the end of the respiratory tree, literally like the larger branches in the respiratory system. Bronchioles contain no cartilage and are instead surrounded by smooth muscle that contracts and expands to control air flow. In addition to this function, special cells in the bronchioles produce surfactants (that hold the airways open) and proteins that break down toxins that enter the lungs. Disorders such as bronchiolitis, asthma, and bronchiolitis obliterans involve these passageways.

As air is inhaled, it is drawn into the trachea and enters the main bronchus. The main bronchus then divides into two bronchi (one for each lung) which continue to subdivide before becoming bronchioles. As the bronchioles progressively fan out, literally like branches of a tree, they are each capped with an alveolus. This is where the transfer of oxygen and carbon dioxide to the blood take place.

The bronchioles are divided into three types, each becoming progressively smaller:

• Lobular bronchioles (larger lobes)
• Terminal bronchioles (think terminals for transfer)
• Respiratory bronchioles (responsible for directing air to the alveoli)

The lobular and terminal bronchioles are known as "dead space" simply because no air exchange occurs in these passes. The bronchioles themselves are tiny, ranging from 0.5 to 1 mm in diameter.

Whereas the bronchi have rings of cartilage that serve to keep them open, the bronchioles are lined with smooth muscle tissue. This allows them to contract and dilate, effectively controlling the flow of air as it makes its way to the alveoli.

On the downside, it also leaves them vulnerable to environmental elements that can cause them to contract and, in some cases, permanently narrow. Called bronchoconstriction, this response can be caused by cigarette smoke, toxic fumes, cold air, and allergens, among other things.

While the narrowing of bronchiole is meant to keep irritants out of the airways, it can cause respiratory distress, sometimes serious. When this happens, adrenaline (epinephrine) and other hormones are released by the body to alleviate the stress and allow air to flow back into the passages.

One type of cell found in the bronchioles (called Type 2 alveolar cells) is responsible for the secretion of substances (called surfactants) that ensure the bronchioles so they don’t collapse during exhalation and effectively increases the surface area of the lungs. It is the lack of these surfactants in premature infants that contributes to respiratory distress. Another type of cell (called club cells) secrete proteins that break down any toxins that may have found their way down the respiratory tree.

As the bronchioles do not have cartilage to support them, they are more likely to be affected by conditions that cause constriction and/or obstruction of the airways. When this happens, symptoms can including wheezing, shortness of breath, difficulty breathing (retractions), and cyanosis (a bluish skin color caused by reduced oxygen intake) occur.

There are several conditions that can directly impair the function of the bronchioles. Among them:

Bronchiolitis is the inflammation of the bronchioles usually seen in children between the ages of three and six months. It is often caused by viruses such as respiratory syncytial virus (RSV) and influenza. Treatment is primarily focused on managing symptoms until the infection has run its course. In severe cases, a ventilator may be needed.

Asthma is caused primarily by allergens or toxins in the air. Respiratory infections can also cause asthma in children. When allergens enter the bronchioles, a type of immune cell (mast cells) releases substances that cause contraction of the smooth muscle surrounding the bronchioles. Treatment consists of medications to dilate the airways (bronchodilators) as well as the elimination or avoidance of any known allergen.

Bronchiolitis obliterans is a rare and serious condition primarily affecting adults in which the bronchioles become scarred and fibrous, impairing a person’s ability to breathe. The causes can include exposure to toxic fumes, viral infections, organ transplants, and rheumatoid arthritis. Bronchiolitis obliterans is irreversible and often requires oxygen therapy and the use of steroids. In severe cases, a lung transplant may be needed.

We often think of the bronchioles as serving only as a conduit for air in the lungs, but as noted, they have many important functions and are involved in several serious conditions affecting the airways. Protecting the bronchioles involves the same measures used to prevent lung diseases affecting other structures in the lungs, such as avoiding tobacco smoke, toxic fumes, and practicing measures such as hand washing to reduce the risk of respiratory infections.