Trachea

The trachea, or windpipe, is a tube that connects the pharynx and larynx to the lungs, allowing the passage of air.

The human trachea has an inner diameter of about 25 millimeters (1 in) and a length of about 10 to 16 centimeters (4 to 6 in). It commences at the lower border of the larynx, level with the sixth cervical vertebra, and bifurcates into the primary bronchi at the level of thoracic vertebra T5. The level of the bifurcation may be as high as the third thoracic vertebra or as low as the seventh thoracic vertebra, depending on breathing. 

A ring of hyaline cartilage called the cricoid cartilage forms the inferior boundary of the larynx and is attached to the top of trachea. The cricoid cartilage is the only complete ring of cartilage around the airway.