against positive pressure from within the thoracic cavity by the discontinous ring of cartilage
against shearing of the trachea by longitudinal forces as the elasticity of the tube permits stretching; essential when:
swallowing - the larynx moves superiorly relative to the tracheal bifurcation
inspiring - the diaphragm and hence the central mediastinal contents including the tracheal bifurcation move inferiorly relative to the larynx
protection:
mucus from the goblet cells and submucosal glands traps particles
'mucociliary escalator' of co-ordinated ciliary movement moves mucus to the larynx for swallowing
mucus humidifies the passing air to minimize dessication of mucosa
mucus warms the air
site of receptors for sensory nerves which run in vagus; involved with afferent limb of protective cough reflex
special reflexes e.g. trachealis contraction during cough decreases luminal calibre and elevates intratracheal pressure prior to release of vocal cords
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