19.2: Upper Respiratory Tract
- Page ID
- 53808
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When you breathe in air flows through the nostrils and enters the nasal cavity which is lined by goblet cells that release mucus. That mucus is salty sticky and has lysozyme, enzymes that help kill bacteria. Nose hairs at the entrance of the nasal cavity get coated with that mucus and are able to trap large particles of dust and pollen as well as bacteria forming tiny clumps in the nose. The nasal cavity is connected to four sinuses, which are air-filled spaces inside the bones that surround the nose called the paranasal sinuses. Maxillary sinus helps the inspired air to circulate for a bit so it has time to get warm and moist. They also act like tiny echo chambers that help amplify the sound of your voice, this is why you sound so different when you plug your nose.
Above: Paranasal sinuses.
That relatively clean warm and moist air goes from the nasal cavity into the pharynx, composed of three regions (listed from superior to inferior): nasopharynx (posterior to the nasal cavity), oropharynx (posterior to the oral cavity), and laryngopharynx (at the pharynx-larynx transition). The soft palate which is the softer portion of the roof of your mouth behind the hard palate which is the part you can feel with your tongue and the pendulum-like uvula hanging at its end move together to form a flap or valve that closes the nasal pharynx off when you eat to prevent food from going up into the nasal pharynx. Finally there's the laryngopharynx the part of the pharynx that's continuous with the larynx or the voice box up to this point.
Above: Structures of the upper respiratory tract, lateral view.
Food and air share a common path but at the top of the larynx, a spoon shaped flap of cartilage called the epiglottis acts like a lid that seals off the airway when you are eating so that food can only go one way down the esophagus and towards the stomach. If anything other than air gets into the larynx then you have a cough reflex that kicks it right back out.
Above: Cadaver images of upper respiratory structures (left) lateral view from the left side of the nasal and oral cavities and(right) lateral view from the right side of the larynx.
Above: External structure of the larynx, anterolateral view of the left anterior of the larynx and superior trachea.