Respiratory tract :
The respiratory tract cleans, warms, and moistens air during its trip to the lungs. The tract can be divided into an upper and a lower part. The upper part consists of the nose, nasal cavity, pharynx (throat), and larynx. The lower part consists of the trachea , bronchi, and bronchial tree.
The nose has openings to the outside that allow air to enter. Hairs inside the nose trap dirt and keep it out of the respiratory tract. The external nose leads to a large cavity within the skull, the nasal cavity. This cavity is lined with mucous membrane and fine hairs called cilia. Mucus moistens the incoming air and traps dust. The cilia move pieces of the mucus with its trapped particles to the throat, where it is spit out or swallowed. Stomach acids destroy bacteria in swallowed mucus. Blood vessels in the nose and nasal cavity release heat and warm the entering air.
Air leaves the nasal cavity and enters the pharynx. From there it passes into the larynx, which is supported by a framework of cartilage (tough, white connective tissue). The larynx is covered by the epiglottis, a flap of elastic cartilage that moves up and down like a trap door. The epiglottis stays open during breathing, but closes during swallowing. This valve mechanism keeps solid particles (food) and liquids out of the trachea. If something other than air enters the trachea, it is expelled through automatic coughing.
