The<unk> s are those parts of the respiratory system through which air flows, to get from the external environment to the alveoli.
The<unk>  begins at the mouth or nose, and accesses the trachea via the pharynx. The trachea branches into the left and right main bronchi at the carina, situated at the level of the second thoracic vertebra. The bronchi branch into large bronchioles, one for each lobe of the lung. Within the lobes, the bronchi further subdivide some 20 times, ending in clusters of alveoli.
The epithelial surfaces of the<unk> s contain cilia. Inhaled particles stick to mucus (secreted by goblet cells) which is continuously removed from the<unk> s by these cilia. The<unk>  epithelium also secretes a watery fluid upon which the mucus can ride freely. The production of this fluid is impaired by the disease cystic fibrosis. Macrophages are present in the<unk> s. These cells protect the<unk> s from infection by engulfing inhaled particles and bacteria.
Certain conditions require tracheal intubation to secure the<unk> .<unk>  devices are used to assist intubation.
Airway
Heart
Throat
Upper respiratory tract
Pulmonary circulation
Central nervous system
Pharynx
Circulatory system