The bronchial tree is a complex network of passages within the respiratory system that transports air from the windpipe into the lungs. Visually, it resembles an upside-down tree, with a main trunk that progressively splits into smaller limbs, serving as the primary conduit for air to reach the lung’s deeper tissues.
Structure of the Bronchial Tree
The bronchial tree begins where the trachea, or windpipe, divides at a point called the carina into the right and left primary bronchi, each entering a lung. The right main bronchus is wider, shorter, and more vertical than the left, which is why foreign objects are more commonly lodged on this side. These main bronchi are supported by C-shaped rings of hyaline cartilage that keep the airways open.
Once inside the lungs, the primary bronchi branch into smaller secondary, or lobar, bronchi. The right lung has three lobes, so it has three lobar bronchi, while the left lung has two lobes and two lobar bronchi. These secondary bronchi then divide further into tertiary, or segmental, bronchi, each supplying air to a specific segment of the lung lobes. There are typically 10 segmental bronchi in the right lung and 8 to 10 in the left.
As branching continues, the airways become narrower and their structure changes. The cartilage providing support in the larger bronchi is replaced by patches and disappears in the smallest airways, known as bronchioles. The walls of the bronchioles contain a higher proportion of smooth muscle instead of cartilage. This allows the diameter of these airways to change, regulating airflow. The branching continues through about 23 generations, ending in the terminal bronchioles.
Primary Functions
The primary function of the bronchial tree is air conduction. It provides a pathway for air to travel to the alveoli, where gas exchange happens. The rigid cartilage in the larger bronchi ensures these main passages do not collapse, guaranteeing a clear route for inhalation and exhalation.
Beyond moving air, the bronchial tree also conditions it. The inner lining of the bronchi is covered by a mucous membrane with goblet cells and cilia. Goblet cells produce a sticky mucus that traps inhaled debris like dust and pathogens. The cilia, tiny hair-like structures, beat in an upward motion, sweeping this mucus toward the throat to be swallowed or coughed out. This mechanism, the mucociliary escalator, cleans, warms, and humidifies air before it reaches the lungs.
While the bronchial tree is not where gas exchange occurs, it makes the process possible by delivering air to the alveoli. The network ensures sufficient air reaches the respiratory zone. Terminal bronchioles open into respiratory bronchioles and alveolar ducts. These are the final segments of the pathway leading to the alveolar sacs where gas exchange with the bloodstream happens.
Associated Medical Conditions
Many medical conditions that impact the bronchial tree involve inflammation of the airway walls, which leads to breathing difficulties. The specific location of the inflammation often defines the condition and its symptoms.
Bronchitis is the inflammation of the larger airways—the primary and secondary bronchi. This inflammation causes the cells lining the bronchi to swell and produce excess mucus, leading to coughing and phlegm. Acute bronchitis is often caused by a viral infection affecting these larger tubes.
Asthma, in contrast, primarily affects the smaller airways. It is a chronic condition where the bronchioles become inflamed and hypersensitive. During an asthma attack, the smooth muscle in the walls of these small, cartilage-free airways constricts, causing them to narrow. This inflammation and bronchoconstriction obstruct airflow, resulting in wheezing, chest tightness, and shortness of breath.