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The bronchioles (/ ˈ b r ɑː ŋ k i oʊ l s / BRONG-kee-ohls) are the smaller branches of the bronchial airways in the lower respiratory tract.They include the terminal bronchioles, and finally the respiratory bronchioles that mark the start of the respiratory zone delivering air to the gas exchanging units of the alveoli.
The respiratory bronchioles represent the transition from the conducting portion to the respiratory portion of the respiratory system. The narrow channels are usually less than 2 mm in diameter and they are lined by a simple cuboidal epithelium, consisting of ciliated cells and non-ciliated club cells, which are unique to bronchioles.
These airways carry oxygen to the bronchioles inside the lungs. Inflammation of the bronchii and bronchioles can cause them to swell up, which could lead to an asthma attack. This results in wheezing, tightness of the chest, and severe difficulty in breathing. There are different types of asthma that affect the functions of the bronchial tubes.
These enter the lungs where they branch into progressively narrower secondary and tertiary bronchi that branch into numerous smaller tubes, the bronchioles. In birds, the bronchioles are termed parabronchi. It is the bronchioles, or parabronchi that generally open into the microscopic alveoli in mammals and atria in birds.
From week 16 to week 26, the bronchi enlarge and lung tissue becomes highly vascularised. Bronchioles and alveolar ducts also develop. By week 26, the terminal bronchioles have formed which branch into two respiratory bronchioles. [59] During the period covering the 26th week until birth the important blood–air barrier is established.
Bronchial anatomy showing terminal bronchioles (BT) leading to respiratory bronchioles (BR) and alveolar ducts (DA) that open into alveolar sacs containing out pockets of alveoli (A) separated by alveolar septa (AS) The alveoli are first located in the respiratory bronchioles as scattered outpockets, extending from their lumens.
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The cells in the respiratory epithelium are of five main types: a) ciliated cells, b) goblet cells, c) brush cells, d) airway basal cells, and e) small granule cells (NDES) [6] Goblet cells become increasingly fewer further down the respiratory tree until they are absent in the terminal bronchioles; club cells take over their role to some extent here. [7]