bronchi

The trachea bifurcates at the carina (approximately T4-T5 vertebral level) into the right and left main (primary) bronchi, which enter the lungs at the hilum. Each main bronchus divides into lobar (secondary) bronchi (3 on the right, 2 on the left), which further divide into segmental (tertiary) bronchi supplying bronchopulmonary segments—clinically important for localizing disease and surgical resection. Bronchi continue to branch into smaller bronchi until they lose cartilage and become bronchioles (<1mm diameter). Histologically, bronchi are lined by pseudostratified ciliated columnar epithelium with goblet cells, contain submucosal glands, smooth muscle (Reissner/muscularis mucosae), and hyaline cartilage rings/plates that provide structural support and prevent collapse during expiration. As airways progress distally, cartilage decreases and smooth muscle proportion increases. Clinical relevance includes: (1) foreign body aspiration—more common in the right main bronchus due to its more vertical orientation, wider diameter, and shorter length; (2) aspiration pneumonia often affects the right lower lobe due to bronchial anatomy; (3) bronchitis—inflammation of bronchi, classically defined clinically as productive cough for 3+ months in 2 consecutive years (chronic bronchitis, a component of COPD); (4) bronchiectasis—irreversible dilation of bronchi due to chronic infection/inflammation, associated with cystic fibrosis, ciliary dyskinesia, and recurrent infections; (5) bronchogenic carcinoma arises from bronchial epithelium, with squamous cell carcinoma classically arising centrally near bronchi. Understanding bronchial anatomy is essential for interpreting chest imaging, bronchoscopy findings, and surgical planning.