Upper airways patenty physiological aspects and physiopathological implications

Interest in extrathoracic upper airways, i.e. from the thoracic outlet to the mouth and nostrils is of recent date. Several investigators have described their air-conditioning role as well as their implications in the defence mechanism of the respiratory tract. More recently, explosive accumulation of data on obstructive sleep apnea syndrome focalized the attention of investigators on upper airways, and especially pharyngeal airway.
Upper airways are composed of the nasal and mouth airways, the pharynx (divided in naso-, oro- and hypopharynx), larynx and extrathoracic trachea. They are all involved in a major role : conducting air from atmosphere to alveoli. Upper airways surfaces are the first exposed to the inspiratory tide of air. Their peculiar shape accounts for the capacity of upper airways to modify the temperature and humidity of air prior to its access to the lungs. Upper airways serve the initial part of digestion: biting and chewing for the mouth, and swallowing for pharynx and larynx. The coexistence into the upper airways of air on one hand and solids and liquids on the other hand may explain the complexity of their physiology. Upper airways have different morphological and structural characteristics. Trachea possesses a rigid cartilaginous structure, sufficient to maintain its calibre by itself. By contrast, pharyngeal structure is predominantly muscular: this means that its calibre depends on the tone of pharyngeal muscles. This is probably the weakest link in the chain, but not the only one. Indeed, larynx shares with trachea a cartilaginous supporting structure but also possesses rapidly moving part, the vocal cords, able to close completely the glottis orifice and therefore the airways. It can release the pressure accumulated into the lungs suddenly, during cough, or gradually, during speech.
To get air from atmosphere to alveoli, one must create a negative pressure inside the alveoli. This negative pressure may collapse a compliant airway like pharyns. A neuromuscular mechanism must therefore intervene to maintain upper airways permeability. Upper airways receptors play an important part in this control, as show in section 2.1.
Upper airways calibre may change their size when submitted to transmural pressure changes. This mechanical characteristic is responsible for some artefacts when measuring airways resistance and lung volume by body plethysmography and respiratory resistance by the oscillation method (sections 2.2. and 2.3). Conversely, one can take profit of this mechanical characteristic to measure cross-sectional area changes of upper airways by inductive plethysmography (section 2.4).
Glottis may be used to express a conflict in a dramatic way without verbalization (section 3.1). In this case it can mimick bronchial asthma.
Physiology and physiopathology of upper airways may intermingle, producing either snoring in otherwise healthy subjects and snoring accompanying obstructive sleep apnea (sections 3.2 and 3.3)