[Physiology of the upper airways].

Patency of the upper airways is maintained by an adjustable balance between the level of tonic and phasic upper airway dilator muscle activity and closing forces generated by inspiratory negative pressure, gravity and surface tension. During sleep, dilator muscle tone is decreased, producing elevated respiratory resistances. During exercise, increased ventilation is normally associated with combined oronasal breathing. Various receptors are implicated to maintain an adequate level of respiratory resistances.