The lower respiratory tract consists of the air passages and lungs. The main passage, the trachea, runs from the level of the lower cricoid cartilage, down into the chest (thorax) and ends behind the sternum at the level of thoracic vertebrae four to five. Here it divides into two main bronchi, right and left, this division is called the carina. Stiffened with U-shaped rings of cartilage, open ends pointing backwards, it is lined with a ciliated epithelium well supplied with mucus secreting cells. The cilia beat keeping a lubricating flow of mucus moving and sweeping upwards any foreign matter.
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