The trachea, also known as the windpipe, is a tube-like structure that connects the larynx (voice box) to the bronchi of the lungs, allowing the passage of air. It is a key part of the respiratory system. The trachea is a U-shaped structure that is composed of hyaline cartilage on the anterior and lateral walls, with the trachealis smooth muscle forming the posterior border of the trachea. The trachea is lined with a moist mucous-membrane layer composed of cells containing small hairlike projections called cilia, which trap particles. The trachea is about 4 inches (10 centimeters) long and an inch (2.5 centimeters) wide in most people. The trachea is part of the conducting airway system that begins immediately inferior to the larynx connected to the cricothyroid cartilage (C6 level) by the cricotracheal ligament. The trachea helps defend against disease by capturing microorganisms such as viruses and harmful bacteria before they enter the lungs.