Bronchioli
Smaller branches of the bronchi in the lungs. Up to a diameter of 1 mm there are further divisions and only up to this point does the bronchial wall contain cartilage in order to ensure that the bronchial tubes remain open and that the entire lung is ventilated. With progressive division cup cells and ciliated epithelium decrease and a ring-shaped muscle system forms under the mucosa. Its contraction can lead to spastic asthma bronchiale. The bronchioles are divided into 4-5 end bronchioles each, called bronchioli terminales, and these again into bronchioli respiratorii, which are about 1-3.5 mm long and 0.4 mm wide. In places their walls are already formed by air bubbles, called alveoli, which are only separated from the pulmonary circulation of the blood by a thin two-layer membrane. Here the transfer of oxygen to erythrocytes and the release of carbon dioxide to the respiratory air takes place.
https://de.wikipedia.org/wiki/Bronchialsystem