The trachea is also known as the windpipe is part of the respiratory system that is responsible for carrying oxygen-rich air into the lungs. They are also responsible for expelling carbon dioxide out of the lungs. The trachea contracts subtlely that happens involuntarily during the process of respiration. There is cartilage (U-shaped) that lines the trachea and it is flexible and is able to close and open when the trachealis (muscles in the trachea) contract and relaxes.