The trachea or windpipe is the part of the respiratory system that allows the passage of air between the larynx and the first bronchial branching in the chest.  In humans, it is found at the front of the neck, just behind the skin.  It is lined with cells that produce mucous that trap particles that are not filtered by the hairs of the nose and the sinuses.  It is largely made up of several rings of cartilage together with muscle that holds the rings together.  In adults, it is about 20-25 mm in diameter and 10-16 cm long. 

The muscle in the trachea contracts when a person is coughing an allows more air to flow through the passage.  The rings of cartilage are incomplete as the esophagus runs right next to the trachea for much of its length, and the cartilage can then stretch to allow food to be swallowed.  The epiglottis is a flap of tissue that covers the trachea opening when a person is eating or drinking to prevent food from passing into the trachea. 

Trachea at Wikipedia

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