The Anatomy of the Bronchi

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The bronchi are the airways that lead from the trachea into the lungs and then branch off into progressively smaller structures until they reach the alveoli, the tiny sacs that allow for the exchange of oxygen and carbon dioxide in the lungs. While the bronchi function primarily as passageways for air, they also play a role in immune function. A number of different medical conditions can affect the bronchi, including bronchitis, asthma, chronic obstructive pulmonary disease (COPD), and lung cancer. Treatments are tailored to the specific ailment and range from medications to surgery.

Anatomy

The bronchi are made up of cartilage, smooth muscle, and mucous membranes. Together, the trachea and the structures of the bronchi are known as the tracheobronchial tree, or simply the bronchial tree.

Structure

The junction between the trachea and the bronchi begins at the level of the fifth thoracic verterbra. At the bottom of the trachea is a ridge of cartilage called the carina. The carina essentially divides into the two primary bronchi; the right bronchi travels into the right lung and the left one to the left lung.

Cartilage is what keeps the bronchi from collapsing during inhalation and exhalation. While the trachea and upper bronchi contain C-shaped cartilage, the smaller bronchi have "plates" of cartilage.

As the bronchi subdivide into smaller (subsegmental) bronchi, the amount of cartilage decreases, and the amount of smooth muscle increases. The vessels no longer contain cartilage as they divide into bronchioles, terminal bronchioles, respiratory bronchioles, alveolar sacs, and finally into the alveoli, where the exchange of oxygen and carbon dioxide takes place. 

The tracheabronchial tree is lined with mucosal membranes comprised of epithelial cells, mucus-secreting goblet cells, and hair-like projections called cilia that move foreign particles up and out of the airway.

Right main bronchus: The right main bronchus is shorter and more vertical than the left, approximately 1 inch (2.5 cm) in length. It divides into smaller bronchi to enter the three lobes of the right lung. Due to the angle at which the bronchi enter the lungs, fluids that are aspirated (breathed in) are more likely to enter the right lung—specifically, the smaller bronchus that enters the right middle lobe of the lung; aspiration pneumonia, for example, most commonly occurs in the right middle lobe.

Left main bronchus: The left bronchus is smaller and longer than the right main bronchus (approximately 5 cm or 1.5 inches.) It, in turn, divides into two secondary lobar bronchi which enter the two lobes of the left lung.

Function

The bronchi function primarily as a passageway for air to travel from the mouth and trachea, down to the alveoli, and back out of the body.

In this way, the body's tissues receive oxygen, and carbon dioxide is able to exit the body.

Because the bronchi bring in air from outside the body—potentially exposing the lungs to infectious agents—they are lined with mucous membranes. This mucus layer provides an important "barrier" to inhaled pathogens that can help prevent infections from taking hold.

Associated Conditions

Numerous medical conditions can involve the bronchi. Some of these involve other regions of the lungs, and others are restricted to the main bronchus and small bronchi.

Aspiration of foreign objects: If a foreign object is accidentally inhaled, it often becomes lodged in one of the bronchi. People who have trouble eating and swallowing—after a stroke, for example—are prone to aspirating food. In people who are unconscious, such as during surgery that requires general anesthesia, there is a risk the person could vomit and aspirate some of it. (This is why people are told to fast prior to surgery.) Inhaled substances can then attract bacteria, leading to aspiration pneumonia.
Acute bronchitis: In acute bronchitis, a viral infection usually starts in the nose or throat and then affects the cells of the bronchi, causing them to swell. Common symptoms of bronchitis include a cough that often involves coughing up phlegm, and wheezing.

Chronic bronchitis: Chronic bronchitis causes inflammation in the bronchioles, causing a heavy buildup of mucus in the lungs that is ongoing (versus acute). Symptoms include a chronic cough and a susceptibility to bacterial infections; eventually, breathing becomes increasingly difficult. Most cases of chronic bronchitis in the United States are causing by long-term cigarette smoking. Long-term exposure to secondhand smoke, air pollution, and chemical fumes can also play a role.

COPD: Chronic bronchitis is one manifestation of chronic obstructive pulmonary disease. Emphysema, which involves damage to the alveoli, often accompanies the bronchitis. COPD is a serious disease that can lead to lung cancer and is often fatal.

Asthma: Asthma is an illness characterized by constriction of the bronchi (bronchospasm), which in turn interferes with the passage of air from the environment to the alveoli of the lungs. Attacks of asthma are often triggered by allergies, exercise, or irritants.

Bronchiectasis: When the walls of the bronchi become irreversibly scarred, they can thicken, causing mucus to build up and become a breeding ground for bacteria. Over time, there is a decrease in lung function. Bronchiectasis is usually associated with other illnesses, such as COPD, cystic fibrosis, and recurrent cases of pneumonia.

Bronchiolitis: Bronchiolitis is caused by a viral infection, most commonly respiratory syncytial virus (RSV). The bronchioles swell and fill with mucus, making breathing difficult. Infants under three months old are the most likely to be affected by this disease. A rare and serious type of bronchiolitis, called bronchiolitis obliterans (also known as "popcorn lung"), is a chronic form of the illness that mainly affects adults.

Bronchopulmonary dysplasia: Bronchopulmonary dysplasia (BPD) can occur in newborns (usually those who are premature) who are being treated with oxygen or are on a ventilator for another breathing problem. The high amounts of oxygen and pressure used in these therapies can overstretch the alveoli, inflaming and damaging the inside lining of the airways. In some cases, BPD can have lifelong effects.

A bronchospasm occurs when your airways constrict, or narrow, causing wheezing and difficulty breathing. Bronchospasms are a symptom of many conditions that affect the airways, including asthma, seasonal allergies, and COPD.

Bronchogenic carcinoma: Bronchogenic carcinoma is an older term for cancers that arise in the bronchi and bronchioles. Now this term is used interchangeably with lung cancer of all types. Non-small cell lung cancer is most common, being responsible for 80% to 85% of cancers. This is the type of lung cancer more commonly found in non-smokers, women, and young adults. Small cell lung cancer is responsible for around 15% of lung cancers. These lung cancers tend to be aggressive and may not be found until they have already metastasized (spread to other parts of the body).

Bronchopleural fistula: A bronchopleural fistula is an abnormal passageway (a sinus tract) that develops between the bronchi and the space between the membranes that line the lungs (the pleural cavity). It is a serious complication often caused by lung cancer surgery, but may also develop after chemotherapy, radiation, or an infection. Bronchopleural fistula is a rare but dangerous condition that is fatal in 25% to 71% of cases. 

Diagnostic Procedures

Common procedures used to diagnose problems that involve the bronchi include:

X-rays: A chest X-ray is often the first procedure used to visualize the lungs. X-rays are useful for identifying causes of aspiration, pneumonia, and lung tumors.

Bronchoscopy: During a bronchoscopy, a tube called a bronchoscope is inserted through the mouth and into the bronchi. A bronchoscopy may be performed in order to evaluate symptoms such as a persistent cough or coughing up blood, but can also be used to treat some conditions, such as bleeding in the airways, or removal of a foreign body.

Endobronchial ultrasound: An endobronchial ultrasound can look at tissues that are deeper in the lungs past the bronchial walls. When a tumor is located, a needle biopsy may be performed under the guidance of the endobronchial ultrasound, making it possible to obtain tissue from a tumor without the need for an open lung biopsy.

Treatment and Rehabilitation

Because there are so many different types of conditions and diseases that can affect the bronchi, treatments vary widely, ranging from medications to surgery.

Bronchodilators: These medications help relax the muscles around the air passages, which makes breathing easier by widening the airway openings. They are a mainstay of asthma treatment, and usually administered via a nebulizer or an inhaler with a spacer.

Corticosteroids: These drugs reduce and/or prevent inflammation within the lungs. They help reduce swelling in the windpipe and decrease the amount of mucus that is produced. Like bronchodilators, they can be given via a nebulizer or an inhaler with a spacer.

Mucus thinning medications: When mucus builds up in the bronchi, mucus-thinning medications can help patients cough it up. These medications are administered via a nebulizer,

Bronchitis often goes away on its own or can be treated with over-the-counter medications, called expectorants, that loosen mucus. Antibiotics are usually prescribed when a bacterial infection affects the bronchial passages.

Oxygen therapy: When a bronchial condition leads to labored breathing, supplemental oxygen is often necessary, either on a temporary basis for an acute condition, or permanently, as with a chronic condition such as COPD. Oxygen therapy is provided in hospitals, but can also be used with home-based equipment. Oxygen is usually delivered through an oxygen cannula (nasal prongs) or a face mask.

Airway clearance devices: Hand-held devices, including Oscillating Positive Expiratory Pressure (PEP) and Intrapulmonary Percussive Ventilation (IPV) devices, can help break up mucus.

Chest physical therapy (CPT): CPT is a technique for loosening mucus that involves clapping on the chest in a certain way. Electronic chest clappers and vests are now available to help perform this technique.

Cancer treatments: People with lung cancer may receive radiation, chemotherapy, surgery, and/or immunotherapy medications, depending on the stage and extent of their cancer.

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