The respiratory system provides the energy needed by cells of the body. Air is breathed in through the nasal cavity and/or mouth and down through the throat (the pharynx).
Upper respiratory tract
The major passages and structures of the upper respiratory tract include the nose or nostrils, nasal cavity, mouth, throat (pharynx), and voice box (larynx).
When you breathe in through your nose or mouth, the air is "filtered" through natural lines of defense that protect against illness and irritation of the respiratory tract. Nasal hairs at the opening of the nostrils trap large particles of dust that might otherwise be inhaled. The entire respiratory system, as with the reproductive, digestive, and urinary systems, is lined with a mucous membrane that secretes mucus. The mucus traps smaller particles like pollen or smoke. Hair-like structures called cilia line the mucous membrane and move the particles trapped in the mucus out of the nose.
The pharynx is a muscular, funnel-shaped tube about 5 inches long that connects the nasal and oral cavities to the larynx. The pharynx houses the tonsils and the adenoids, which are lymphatic tissues that guard against infection by releasing white blood cells.
The larynx forms the entrance to the lower respiratory system. With the help of the epiglottis (a leaf-shaped flap), the larynx prevents food or liquid from entering the lower respiratory tract while swallowing. Two pairs of strong connective tissue bands that are stretched across the larynx vibrate to produce sounds while talking or singing.