The trachea is situated between the lower end of the larynx and the center of the chest, where it splits into the two bronchi at a ridge called the carina. The trachea is stabilized and kept open by rings made of cartilage that surround the front and sides of the structure; these rings are not closed and do not surround the back, which is made of membrane.[8] The bronchi split into smaller branches and then to bronchioles that supply air to the alveoli, the tiny air-filled sacs in the lungs responsible for absorbing oxygen. An arbitrary division can be made between the intrathoracic and cervical trachea at the thoracic inlet, an opening at the top of the thoracic cavity.[9] Anatomical structures that surround and protect the tracheobronchial tree include the lungs, the esophagus, large blood vessels, the rib cage, the thoracic spine, and the sternum.[9] Children have softer tracheas and a more elastic tracheobronchial trees than adults; this elasticity, which helps protect the structures from injury when they are compressed, may contribute to the lower incidence of TBI in children.[8]