Tasks of the respiratory system
The respiratory tract is not only used to supply fresh air during inspiration (= inhalation) and to exhaust "alveolar air" during expiration (= exhalation), but it also fulfils a number of auxiliary functions to aid breathing. These include cleansing, warming up and humidifying the inhaled air.
The inhaled air is partly purified in the nose, where smaller particles, dust and bacteria are trapped by the mucous membranes (this is why chronic breathing through the mouth increases susceptibility to respiratory diseases).
Other inhaled particles are deposited on the layer of mucus covering the walls of the incoming airways.
(Inhaled air is routed through the nasopharynx, the larynx and its narrowest point, the glottis, into the trachea (windpipe). The trachea is a large-lumen tube of connective tissue, which forks, at the level of the 5th thoracic vertebra, into the two main bronchial tubes (stem bronchi), which in turn each branch into one of the lobes of the lung below.)
The mucus secreted by goblet cells and subepithelial gland cells is constantly transported towards the mouth via rhythmic movement of the cilia of the respiratory epithelium and then swallowed. Mucus transport therefore ensures that foreign particles and bacteria are removed from the respiratory tract. If the cilia are damaged, as is the case with chronic bronchitis, for example, mucus accumulates in the respiratory tract, leading to increased respiratory resistance.