Bronchus
Anatomy
The bronchus is a passage of airway in the respiratory system that conducts air into the lungs. The first bronchi to branch from the trachea are the right main bronchus and the left main bronchus, also known as the primary bronchi. These are the widest and enter the lungs at each hilum, where they branch into narrower secondary bronchi or lobar bronchi, and these branch into smaller tertiary bronchi or segmental bronchi. Further divisions of the segmental bronchi are known as 4th order, 5th order, and 6th order segmental bronchi, or grouped together as subsegmental bronchi. The bronchi when too narrow to be supported by cartilage are known as bronchioles. No gas exchange takes place in the bronchi.
Histology
In the histological perspective, the bronchus is lined by pseudostratified ciliated columnar epithelium. The lamina propria contains seromucous glands, which secrete both mucus and serous fluid. The serous fluid is produced by serous cells and aids in moistening the incoming air, while the mucus traps dust and other particles preventing them from reaching the lungs. The epithelium may be infiltrated by lymphocytes and is supported by a lamina propria. Layers of muscle and connective tissue, containing elastic fibers, are located beneath the lamina propria. The cartilage in the bronchus forms a series of discontinuous rings, in contrast to the continuous ring seen in the trachea. Smooth muscle cells, in the muscularis mucosae, form a spiral pattern around the bronchus. There is a thin layer of smooth muscles between the cartilage and the lamina propria. This muscle allows the diameter of the bronchus to change to regulate the amount of air entering the lungs.
Function
The primary function of the bronchus, like other parts of the respiratory tree, is to carry air from the trachea into the lungs. It is lined with cilia (tiny hair-like structures), which trap foreign particles and prevent them from entering the lungs. Additionally, the bronchus also plays a crucial role in the body's immune response. The cells lining the bronchus produce a small amount of mucus that traps dust and other particles that have been inhaled. This mucus is then moved upwards towards the throat by the action of the cilia, where it can be swallowed or coughed up. This is known as the mucociliary escalator, an important defense mechanism against air-borne infection.
Clinical Significance
The bronchus is prone to several diseases, including bronchitis and bronchial carcinoma. Bronchitis is inflammation of the bronchi. This condition can be acute or chronic. Acute bronchitis is usually caused by viral or bacterial infections. Chronic bronchitis, a type of chronic obstructive pulmonary disease, is usually caused by prolonged exposure to irritants such as tobacco smoke, dust, and air pollution.
Bronchial carcinoma, also known as bronchogenic carcinoma, is a type of lung cancer originating from the epithelium of the bronchial tree. It is a common type of lung cancer and has a high mortality rate. Symptoms include cough, hemoptysis (coughing up blood), wheezing, shortness of breath, and weight loss. Diagnosis is usually made by chest X-ray, CT scan, and bronchoscopy.