Tube in the throat and upper thoracic cavity through which air passes in respiration. It begins at the larynx and splits just above heart level into the two main bronchi, which enter the lungs. In adults it is about 6 in. (15 cm) long and 1 in. (2.5 cm) in diameter. Its structure—a membrane strengthened by 16–20 cartilage rings open in the back, with their free ends connected by muscle bands—allows the trachea to stretch and contract in breathing. An inner mucous membrane has cilia (see cilium) that project inward to trap particles. Muscle fibres over and alongside the trachea contract in response to cold air or irritants in inhaled air; in coughing, the airway narrows to about one-sixth of its normal size to increase the speed and force of exhalation and to dislodge foreign bodies. Such diseases as diphtheria, syphilis, tuberculosis, and typhoid often involve the trachea.
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