The windpipe is held open by C-shaped rings, not full circles
Your trachea is a tube about 10 to 11 centimetres long, braced by 16 to 20 rings of cartilage that are deliberately left open at the back. A band of muscle closes each gap and tightens during a cough, narrowing the tube so that expelled air rushes out faster.
The trachea runs from the bottom of the larynx, level with the sixth neck vertebra, to a fork called the carina at about the fourth thoracic vertebra, where it splits into the two main bronchi. In adults its inner width is roughly 1.5 to 2 cm, a little wider in men. Each cartilage ring stands about 4 millimetres tall, and ligaments tie them together so the tube can bend and stretch with movement. The open ends face backwards, where the trachealis muscle bridges them and the oesophagus lies right behind. The rings are springy but can calcify with age, and neighbouring ones sometimes fuse.
Its job goes beyond ducting air. The lining carries columnar cells topped with cilia, interspersed with goblet cells that make mucins, the main ingredient of mucus. Dust and microbes stick to the mucus, and the beating cilia sweep it up to the throat to be swallowed or coughed out, a self-cleaning system called mucociliary clearance. The trachea also warms and moistens incoming air.
It starts forming early. In the fourth week of embryonic life, ridges pinch the future windpipe off from the gut tube, and by the start of the fifth week the main bronchi are budding from its end. During the first year of life it is no more than 4 mm wide, reaching adult size of about 2 cm by late childhood, and in children it is rounder and more upright.
Things go wrong in several ways. Viral croup, most often caused by parainfluenza viruses, inflames the larynx, trachea and bronchi together. Bacterial tracheitis occurs almost entirely in children, and up to 80 percent of those affected need mechanical ventilation. Narrowing from scarring, tumours or a big thyroid goitre produces a harsh noise called stridor. In tracheomalacia the walls are floppy and collapse under pressure; toy and miniature dogs with the condition often give a honking cough like a goose.
Source: Trachea
