The bronchioles are a part of the respiratory system that are not discussed as often as, say, the bronchi, or alveoli, yet they play an important role in respiration. There are also several medical conditions that occur primarily in the bronchioles. Where are they, what is their function, and what are the symptoms of conditions that affect them?
AnatomyThe bronchioles are passages in the respiratory system that begin at the end of the bronchi. After taking a breath, air passes through the nose or mouth and into the trachea. It then enters the main bronchus. The main bronchus divides into 2 bronchi (one enters each lung) which in turn go through several divisions before becoming bronchioles. The bronchioles continue the process of fanning out (going through divisions noted below) like the branches of a tree until the final place in the respiratory system is reached – the alveoli. The alveoli are the air sacs at which the exchange of oxygen and carbon dioxide takes place.
The bronchioles are divided into 3 types, each becoming progressively smaller. These are:
- Lobular bronchioles
- Terminal bronchioles, and
- Respiratory bronchioles
The terminal bronchioles are the final airways in the conducting zone, with the respiratory bronchioles beginning the respiratory zone.
Sequence of Respiration
It may help to quickly review the respiratory system in order during inspiration:
When you take a breath, the air first enters through your mouth or your nose. It then passes into your trachea followed by the main bronchus. The main bronchus divides into 2 bronchi – one of which enters each lung. The bronchi then become successively smaller, branching out like tree limbs. This begins with primary bronchi, then secondary bronchi, and then the tertiary bronchi. At this point the air enters your bronchioles – going from the lobular, to terminal, and then to respiratory bronchioles. Each of these smallest of bronchioles brings air to a pulmonary nodule which is a cluster of alveoli. Here the air travels from the smallest bronchiole to the alveolar ducts to alveolar sacs and finally to the alveoli.
Structure of the BronchiolesStructurally, the bronchioles are different from the bronchi. Whereas the bronchi have rings of cartilage that serve to keep them open, the bronchioles are instead lined with muscular walls. The bronchioles are tiny, with sizes ranging from approximately 0.5 to 1 mm in diameter. Both the muscular walls and diameter of the bronchioles play an important role in diseases affecting these structures.
Since the bronchioles are lined by a greater amount of smooth muscle than other bronchial tubes, they are more subject to processes that affect smooth muscle. In a healthy person this muscular lining serves to dilate and constrict the airways, thus controlling the delivery of air to the alveoli. Yet this also leaves the bronchioles vulnerable to environmental irritants that can affect the diameter.
Narrowing of the bronchioles - something called bronchoconstriction - may occur in response to toxic fumes, cold air, agents that cause allergic reactions and other processes. This narrowing is the body’s way of keeping irritants out of the airways, but may cause significant symptoms (more about this below.) In contrast, an increase in diameter of the bronchioles is called bronchodilation. Hormones such as adrenaline (epinephrine) that are released in response to stress (or given to someone suffering from respiratory distress) serve to dilate these airways.
One type of cell found in the bronchioles, Type 2 alveolar cells, are responsible for secreting surfactant. Surfactant works to maintain the surface tension of the bronchioles so they don’t collapse during expiration. Another type of cell are club cells, which secrete proteins that break down toxins that have made it this far in the bronchial tree.
FunctionThe primary function of the bronchioles is to conduct air from the bronchi to the alveoli, and to control the amount of air distributed through the lung by constricting and dilating. The lobular and terminal bronchioles are known as “dead space” – simply meaning that there is no air exchange that occurs in these vessels. The respiratory bronchioles in turn begin the respiratory zone in which air exchange takes place in the alveoli.
Diseases That Affect the Bronchioles and Possible SymptomsMany conditions can affect these tiny airways. Since 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 the bronchioles are inflamed or infected, symptoms may include:
- A rapid respiratory rate
- Retractions (pulling in of the chest muscles with a deep breath),
- Nasal flaring (wide open nostrils,)
- Cyanosis (a bluish tinge to the skin if the oxygen level in the blood is low)
Medical conditions affecting the bronchioles include:
Bronchiolitis - Bronchiolitis in children is often caused by viruses such as RSV and influenza. The goal of treatment is primarily to control the symptoms until the viral infection runs its course. Cool mist humidifiers, hydration, and careful monitoring for worsening of symptoms is usually recommended. In severe cases support with a ventilator may be required until the condition has resolved.
Asthma – Asthma results from a decrease in diameter of the airways (bronchoconsriction.) Causes may include allergens in the air as well as respiratory infections common in children. Treatment consists of medications to dilate the airways (bronchodilators,) as well as avoiding known allergens.
- Bronchiolitis obliterans - This is a rare and serious condition most often affecting adults, in which the bronchioles become scarred and fibrous. This scarring limits exhalation and results in air trapping. There are many causes including exposure to toxic fumes, viral infections, organ transplants, and conditions such as rheumatoid arthritis. Bronchiolitis obliterans is irreversible and treatment is directed at symptoms and limiting the damage, often using oxygen and steroids. In severe cases a lung transplant may be needed.
American Thoracic Society. Anatomy and Function of the Normal Lung. Accessed 09/15/13. http://www.thoracic.org/clinical/copd-guidelines/for-patients/anatomy-and-function-of-the-normal-lung.php
Ohio State University, Wexner Medical Center. Anatomy of the Respiratory System. Accessed 09/15/13. http://medicalcenter.osu.edu/patientcare/healthcare_services/lung_diseases/about/anatomy/Pages/index.aspx
National Cancer Institute. SEER Training Modules. Introduction to the Respiratory System. Accessed 09/15/13. http://training.seer.cancer.gov/anatomy/respiratory/
National Heart, Lung, and Blood Institute. The Respiratory System. Updated 07/17/12. http://www.nhlbi.nih.gov/health/health-topics/topics/hlw/system.html