The respiratory system begins with inhalation via the nostrils or mouth. The nostrils have microscopic hairs called cilia that sweep dust and germs away from the lungs as well as nose hairs to trap dirt and dust and other debris. There is mucous to also trap particles. Air is warmed to body temperature. It then passes to the pharynx or throat on its way to the trachea. The trachea is known commonly as the windpipe. Its cartilaginous rings keep the airway open. It branches into the bronchial tubes-one leading to each lung. These subdivide into tinier tubes called bronchioles, much like a tree trunck divides into smaller and smaller branches. At the end of the bronchioles are the alveoli. They resemble clusters of grapes. They are thin walled and moist and their function is exchange of respiratory gases. Because capillaries are wrapped around the alveoli there is a close association between the respiratory and circulatory systems. When someone inhales, the oxygen diffuses from the alveoli into the bloodstream through the walls of the capillaries. Likewise, the carbon dioxide that is transported by the circulating blood to the lungs diffuses out of the capillaries and enters the alveoli. When someone exhales, it is excreted along with water vapor.