The trachea, also called the windpipe.

The trachea, also called the windpipe, is a cartilaginous tube that connects the larynx to the bronchi of the lungs, allowing the passage of air, and so is present in almost all air-breathing animals with lungs. The trachea extends from the larynx and branches into the two primary bronchi. At the top of the trachea the cricoid cartilage attaches it to the larynx. The trachea is formed by a number of horseshoe-shaped rings, joined together vertically by overlying ligaments, and by the trachealis muscle at their ends. The epiglottis closes the opening to the larynx during swallowing.

The trachea begins to form in the second month of embryo development, becoming longer and more fixed in its position over time. It is epithelium lined with column-shaped cells that have hair-like extensions called cilia, with scattered goblet cells that produce protective mucins. The trachea can be affected by inflammation or infection, usually as a result of a viral illness affecting other parts of the respiratory tract, such as the larynx and bronchi, called croup, that can result in a barking cough. Infection with bacteria usually affects the trachea only and can cause narrowing or even obstruction. As a major part of the respiratory tract, when obstructed the trachea prevents air entering the lungs and so a tracheostomy may be required if the trachea is obstructed. Additionally, during surgery if mechanical ventilation is required when a person is sedated, a tube is inserted into the trachea, called intubation.

Structure-

An adult's trachea has an inner diameter of about 1.5 to 2 centimetres (0.59 to 0.79 in) and a length of about 10 to 11 centimetres (3.9 to 4.3 in); wider in males than females.[2] It begins at the bottom of the larynx, and ends at the carina, the point where the trachea branches into left and right main bronchi.[2] The trachea is surrounded by 16 - 20 rings of hyaline cartilage; these 'rings' are 4mm high in the adult, incomplete and C-shaped.[2] Ligaments connect the rings.[3] The trachealis muscle connects the ends of the incomplete rings and runs along the back wall of the trachea.

Function-The trachea is one part of the respiratory tree, that is a conduit for air to pass through on its way to or from the alveoli of the lungs. This transmits oxygen to the body and removes carbon dioxide