bronchus, any of the large airway passages of the lowerrespiratory tract thatdiverge from thetrachea (windpipe) and connect to thelungs. The trachea divides in an inverted Y into the two stem (or main) bronchi, one each for the left and rightlung. The right main bronchus has a larger diameter, is oriented more vertically, and is shorter than the left main bronchus. As the bronchi extend into the lungs, they become smaller and narrower and ultimately subdivide into increasingly narrow airways known as bronchioles. The bronchi function to warm, moisten, and clean inspired air and distribute it to the gas-exchanging zone of the lung.

The bronchi are lined by the typical respiratory epithelium with ciliated cells and numerous interspersed mucus-secreting goblet cells. Ciliated cells are present far down in the airway tree, their height decreasing with the narrowing of the tubes. The frequency of goblet cells also decreases as the airway tree progresses.Cartilage rings provide support for the stem bronchi; as the bronchi extend deeper into the lungs, the rings are replaced by irregular cartilage plates. The bronchi also containsmooth muscle between themucosa and the fibrocartilaginous layers.Contraction of the smooth muscle layers controls bronchi diameter and length and functions in the expiration of air from the lungs.

The bronchi also are ensheathed by looseconnective tissue that is continuous with the other connective tissue elements of the lung and hence is part of the fibrous skeleton spanning the lung from the hilum to the pleural sac. This outer fibrous layer contains, besides lymphatics and nerves, small bronchial vessels to supply the bronchial wall withblood from thesystemic circulation.

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respiratory disease: Acute diseases of the bronchi

The major bronchi can become the seat of chronicinflammation, such as in chronicbronchitis or bronchiectasis; the latter disease may be caused bycystic fibrosis. The major bronchi may also be the site of development of malignant disease. Inflammation and increased sensitivity of the smaller bronchi, which can cause the smooth muscle in the bronchi walls to contract and obstruct airflow, contribute toasthma.