Breathing is so vital to life that it happens automatically. Each day, you breathe about 20,000 times, and by the time you're 70 years old, you'll have taken at least 600 million breaths.
All of this breathing couldn't happen without the respiratory system, which includes the nose, throat, voice box, windpipe, and lungs.
At the top of the respiratory system, the nostrils (also called nares) act as the air intake, bringing air into the nose, where it's warmed and humidified. Tiny hairs called cilia protect the nasal passageways and other parts of the respiratory tract, filtering out dust and other particles that enter the nose through the breathed air.
Air can also be taken in through the mouth. These two openings of the airway (the nasal cavity and the mouth) meet at the pharynx, or throat, at the back of the nose and mouth. The pharynx is part of the digestive system as well as the respiratory system because it carries both food and air. At the bottom of the pharynx, this pathway divides in two, one for food (the esophagus, which leads to the stomach) and the other for air. The epiglottis, a small flap of tissue, covers the air-only passage when we swallow, keeping food and liquid from going into the lungs.
The larynx, or voice box, is the uppermost part of the air-only pipe. This short tube contains a pair of vocal cords, which vibrate to make sounds.
The trachea, or windpipe, extends downward from the base of the larynx. It lies partly in the neck and partly in the chest cavity. The walls of the trachea are strengthened by stiff rings of cartilage to keep it open. The trachea is also lined with cilia, which sweep fluids and foreign particles out of the airway so that they stay out of the lungs.
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