Bronchus

The bronchus is a major airway in the respiratory system that carries air from the trachea into the lungs. It is a part of the bronchial tree, which further branches into smaller airways called bronchioles. The bronchi are tubular structures reinforced with cartilage rings, ensuring they remain open for air to pass through. There are two main bronchi: the left and right bronchus, each serving one lung.

Location

The bronchi are located in the chest cavity. They begin at the lower end of the trachea (windpipe) and branch into the left and right lungs. The right bronchus is shorter, wider, and more vertical, while the left bronchus is longer and more horizontal, accommodating the heart’s position.

Structure and Anatomy

The bronchus is a crucial part of the respiratory system, providing the pathway for air to move from the trachea into the lungs. Its anatomy is designed to ensure the efficient transport of air and to maintain its structural integrity throughout the respiratory process. Below is a detailed description of the anatomy of the bronchus.

Main Bronchi (Primary Bronchi)

• Left Main Bronchus: The left main bronchus is longer and narrower than the right bronchus. It runs from the trachea and travels more horizontally into the left lung due to the presence of the heart on the left side. The left main bronchus measures about 4 to 5 centimeters in length before it divides into the lobar bronchi.
• Right Main Bronchus: The right main bronchus is shorter, wider, and more vertical compared to the left bronchus. It is approximately 2.5 centimeters in length and provides a more direct path for air into the right lung. The vertical orientation of the right bronchus makes it more prone to foreign body aspiration.

Both the left and right bronchi are supported by C-shaped cartilaginous rings, which keep the airways open and maintain their structural integrity.

Lobar Bronchi (Secondary Bronchi)

After entering the lungs, each main bronchus divides into lobar bronchi (also called secondary bronchi), which correspond to the lobes of the lungs.

• Right Lung: The right main bronchus divides into three secondary bronchi, one for each lobe of the right lung: superior, middle, and inferior lobar bronchi.
• Left Lung: The left main bronchus divides into two secondary bronchi, one for each lobe of the left lung: superior and inferior lobar bronchi.

The lobar bronchi are also supported by irregularly shaped cartilage plates, allowing them to remain open while being flexible enough to move with the lungs during breathing.

Segmental Bronchi (Tertiary Bronchi)

The lobar bronchi further divide into smaller airways known as segmental bronchi (or tertiary bronchi), which supply air to specific segments of each lung, called bronchopulmonary segments. These bronchopulmonary segments are functionally independent units of the lung, each with its own air supply and blood vessels.

• Right Lung: The right lung has ten bronchopulmonary segments, each supplied by its own tertiary bronchus.
• Left Lung: The left lung has eight to ten bronchopulmonary segments, depending on anatomical variations, and each segment is served by a tertiary bronchus.

Segmental bronchi are smaller in diameter than the lobar bronchi but continue to be reinforced with cartilage plates.

Bronchial Wall Layers

The walls of the bronchi consist of several layers that provide structural support and aid in the passage of air.

• Mucosa: The inner lining of the bronchi is composed of pseudostratified ciliated columnar epithelium. The cilia in this layer help to move mucus and trapped particles out of the airways, keeping the respiratory passages clean. The mucosa also contains goblet cells, which secrete mucus to trap dust, pathogens, and other foreign particles.
• Submucosa: Beneath the mucosa is the submucosa, which contains seromucous glands that contribute to mucus production. This layer also houses blood vessels, nerves, and smooth muscle fibers.
• Smooth Muscle Layer: Surrounding the submucosa is a layer of smooth muscle. This smooth muscle can contract or relax to adjust the diameter of the bronchial passageways, regulating airflow into the lungs. The smooth muscle becomes more prominent as the bronchi branch into smaller airways.
• Cartilage Plates: The bronchi are supported by cartilage plates rather than the full C-shaped rings seen in the trachea. These plates provide rigidity and help maintain an open airway while allowing flexibility for movement during breathing.
• Adventitia: The outermost layer of the bronchi is called the adventitia, which is composed of connective tissue. This layer connects the bronchi to surrounding structures in the lungs and offers additional support.

Vascular Supply

The bronchi are supplied with blood from two different circulatory systems:

• Bronchial Arteries: These arteries branch from the aorta and provide oxygenated blood to the bronchial walls and lung tissues. They supply the bronchi and surrounding lung tissues with the oxygen and nutrients they need to function.
• Pulmonary Circulation: The pulmonary arteries supply deoxygenated blood to the lungs for oxygenation. While the pulmonary circulation primarily serves the alveoli for gas exchange, small branches from the pulmonary arteries also contribute to the blood supply of the bronchi.

The venous blood from the bronchi drains into the bronchial veins, which empty into the azygos venous system, and into the pulmonary veins, which carry oxygenated blood back to the heart.

Nervous Innervation

The bronchi are innervated by both the autonomic nervous system, which includes the sympathetic and parasympathetic branches.

• Sympathetic Innervation: Sympathetic nerve fibers cause bronchodilation by relaxing the smooth muscles in the bronchial walls, which increases the diameter of the bronchi and enhances airflow into the lungs.
• Parasympathetic Innervation: Parasympathetic fibers from the vagus nerve cause bronchoconstriction, which reduces the diameter of the bronchi and limits airflow, typically as part of the body’s response to irritants or allergens.

Lymphatic System

The bronchi are closely associated with an extensive lymphatic network that drains excess fluid and helps protect against infections.

• Bronchopulmonary Lymph Nodes: Located at the hilum of each lung, these lymph nodes filter lymph fluid from the bronchi and surrounding tissues. They play a crucial role in immune defense by capturing foreign particles and pathogens before they enter the bloodstream.
• Lymphatic Drainage: Lymph from the bronchi and lungs drains into the hilar lymph nodes, and from there, it flows into larger lymphatic vessels that eventually join the thoracic duct or right lymphatic duct.

Function

The bronchus plays a critical role in the respiratory system, facilitating the movement of air from the trachea into the lungs and ensuring that gas exchange can occur efficiently in the alveoli. Below is a detailed explanation of the functions of the bronchus.

Air Conduction

The primary function of the bronchi is to conduct air between the trachea and the lungs.

• Pathway for Airflow: The bronchi serve as the main passageways through which inhaled air moves from the trachea into the left and right lungs. Once inside the lungs, the bronchi continue to branch into smaller airways (lobar and segmental bronchi), distributing air to each lobe and bronchopulmonary segment of the lungs.
• Branching of the Bronchial Tree: The bronchi branch into lobar bronchi (secondary bronchi) and then further into segmental bronchi (tertiary bronchi), ensuring that air reaches all parts of the lungs. This division increases the surface area for gas exchange and ensures even distribution of air across the lung tissue.

Control of Airflow Resistance

The bronchi play a vital role in controlling the flow of air into the lungs by adjusting the diameter of the bronchial passages.

• Bronchoconstriction: In response to irritants, allergens, or certain nervous system signals, the smooth muscle in the walls of the bronchi can contract, causing bronchoconstriction. This narrowing of the airway reduces airflow, limiting the amount of air entering the lungs. Bronchoconstriction can occur during allergic reactions or respiratory conditions such as asthma.
• Bronchodilation: Conversely, during times of increased oxygen demand, such as during physical exercise, the bronchi can undergo bronchodilation, where the smooth muscle relaxes and the bronchi widen. This increases airflow into the lungs and enhances oxygen delivery to meet the body’s needs. Bronchodilation is controlled by signals from the sympathetic nervous system.

This dynamic control of airway diameter is essential for maintaining proper respiratory function in varying conditions and environments.

Protection of the Lower Respiratory Tract

The bronchi serve as a key defense mechanism, protecting the lungs from harmful particles, pathogens, and irritants that are inhaled from the external environment.

• Mucociliary Clearance: The bronchi are lined with pseudostratified ciliated columnar epithelium and goblet cells that produce mucus. This mucus traps dust, bacteria, viruses, and other foreign particles. The cilia move the mucus and trapped particles upward toward the throat, where they can be swallowed or expelled by coughing. This process, known as mucociliary clearance, prevents harmful substances from reaching the alveoli.
• Cough Reflex: The bronchi are equipped with sensory receptors that can trigger the cough reflex in response to the presence of irritants, such as smoke, dust, or excess mucus. Coughing expels these irritants from the airways, protecting the delicate lung tissues from damage and infection.

Humidification and Warming of Air

The bronchi help condition the air that enters the respiratory system by warming, humidifying, and filtering it before it reaches the delicate alveoli.

• Humidification: As air passes through the bronchi, it is moistened by the mucus secreted by goblet cells and seromucous glands in the bronchial walls. This ensures that the air reaching the alveoli is properly humidified, preventing the drying out of lung tissues.
• Warming of Air: Blood vessels in the walls of the bronchi help to warm the air as it passes through, ensuring that the temperature of the inhaled air is close to body temperature by the time it reaches the deeper parts of the lungs. This prevents irritation and damage to the respiratory tissues from cold air.

Filtration of Foreign Particles

The bronchi act as a filter for inhaled air, trapping large particles before they can reach the alveoli.

• Mucus Trap: The bronchi produce mucus that traps larger particles such as dust, pollen, and bacteria. The sticky nature of mucus makes it an effective barrier to contaminants.
• Ciliary Action: The cilia in the bronchial epithelium constantly beat in a coordinated manner, moving mucus and trapped particles toward the throat. This mechanism helps to prevent harmful substances from reaching the alveoli, where gas exchange occurs.

Immunological Defense

The bronchi are part of the body’s immune defense system, providing a first line of defense against pathogens.

• Presence of Immune Cells: The bronchi contain immune cells, such as alveolar macrophages, that patrol the airways, identifying and engulfing foreign pathogens that have bypassed the upper respiratory defenses. These cells help to neutralize harmful bacteria, viruses, and other microbes before they can cause infection.
• Lymphatic Drainage: The bronchi are connected to an extensive network of lymphatic vessels, which transport immune cells and drain excess fluid from the bronchial tissue. This system helps filter and remove harmful substances from the respiratory tract, aiding in the body’s immune response.

Facilitation of Coughing and Expelling Mucus

The bronchi play a crucial role in expelling excess mucus and irritants through coughing.

• Cough Reflex: Irritation or blockage in the bronchi can trigger the cough reflex, an important defense mechanism that helps clear mucus, irritants, and pathogens from the airways. This helps maintain clear bronchial passages and ensures that airflow is unobstructed.
• Expectoration: During respiratory infections or allergies, the bronchi increase mucus production to trap pathogens. The body expels this mucus through coughing, a process called expectoration, preventing mucus buildup and promoting airway clearance.

Segmentation of Lung Function

The bronchi contribute to the functional segmentation of the lungs, dividing each lung into bronchopulmonary segments that can operate somewhat independently.

• Bronchopulmonary Segments: Each tertiary (segmental) bronchus supplies air to a specific bronchopulmonary segment, which is a distinct, functionally independent unit of the lung. This segmentation allows for localized lung function, where one segment can be affected by disease or damage without compromising the function of the other segments.
• Surgical Relevance: The ability to isolate bronchopulmonary segments is clinically significant, as it allows for surgical procedures such as segmentectomy (removal of a lung segment) to treat localized conditions like tumors or infections without affecting the entire lung.

Clinical Significance

The bronchi play a crucial role in respiratory health, and their dysfunction or damage can lead to several significant clinical conditions. Diseases affecting the bronchi include bronchitis, asthma, chronic obstructive pulmonary disease (COPD), and bronchiectasis.

• Bronchitis involves inflammation of the bronchi, often caused by infections or irritants, leading to symptoms like coughing, mucus production, and difficulty breathing.
• Asthma is characterized by hyper-reactivity of the bronchi, causing bronchoconstriction in response to allergens or irritants, which can result in wheezing and shortness of breath.
• In COPD, the bronchi can become obstructed due to long-term inflammation and mucus production, leading to persistent airflow limitation.
• Bronchiectasis is the abnormal widening and thickening of the bronchi due to chronic infections or inflammation, resulting in impaired mucus clearance and frequent lung infections.