The respiratory system starts working when air is pulled into the nasal passages (nose) by the expansion of the rib cage and relaxing of the diaphragm.The air that passes through the nose must then be filtered, heated and moistened by the sinuses, before it goes down through the throat (pharnyx), where it meets the trachea (wind pipe). The air travels down the trachea, which then splits into two parts called the bronchi tubes. Each one of these tubes lead into either the left or right lung, where the bronchi splits many times into passageways or tubes called bronchioles. The bronchioles look like an upside down tree with the bronchi being the base of the tree. At the end of the these small tubes are little sacs called the alveoli,where the exchange of oxygen for carbon dioxide occurs. The diaphragm then contracts, causing the rib cage to pull in and force the air from the alveoli back up the bronchi to the trachea and out through the mouth. This whole cycle completes in under five seconds and will be repeated millions of times over a lifetime, all without thought as it is involuntary.