Although the mechanism is not well understood, TBI due to blunt trauma is widely thought to be caused by any combination of three possible mechanisms: an increase in pressure within the airways, shearing, and pulling apart.[18] The first type of injury, sometimes called an "explosive rupture", may occur when the chest is violently compressed, for example when a driver strikes the steering wheel in a vehicle accident[4] or when the chest is crushed.[8] The pressure in the airways, especially the larger airways (the trachea and bronchi), quickly rises as a result of the compression,[15] because the glottis reflexively closes off the airways.[2] When this pressure exceeds the elasticity of the tissues, they burst; thus the membranous part of the trachea is more commonly affected by this mechanism of injury than cartilaginous portions.[15]