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Anatomy of the Upper Respiratory Tract
The upper airway can be considered as extending from the nose, through the larynx to the trachea. The sinuses and middle ear are connected to the upper airway, and these air spaces can also be considered as part of the airway even though they are not in the main airstream.
The upper airway and lungs are relatively recent developments in the process of evolution from a fish-like ancestor to an air-breathing mammal. The nose developed from olfactory pits which grew to meet the pharynx and the lungs developed as an outgrowth from the fore gut.
At birth the nose and lungs are fully developed but smaller versions of the adult organs. In contrast the paranasal sinuses are hardly developed and slowly grow in size during childhood to reach near maximum size at puberty.
Nose:
In the process of evolution the nose developed as a chemosensor which was later adapted as part of the upper airway. The olfactory area of the nose provides us with our sense of smell which in modern man appears to be relegated to the appreciation of food but which is very well developed in other mammals where it also helps in finding food and partners of the opposite sex. The nose acts as an air conditioner at the entrance to the airway and performs the functions of air filter, heat exchanger and humidifier.
The trigeminal sensory nerves supplying the nasal vestibule and nasal mucosa provide physical and chemical sensors to detect inspired particles and irritants. Stimulation of the nasal trigeminal sensory nerves causes sneezing and protective reflex secretions.
The nasal cavity is divided into two separate airways by the nasal septum. Each half of the nose has its own nerve supply and the two halves of the nose can operate quite separately.
The nasal vestibules are trumpet shaped orifices and each narrows from a cross sectional area of around 90 square mm to a slit of 30 square mm which separates the nasal vestibules from the main nasal cavity.
Sinuses:
The nasal airway is surrounded by the paranasal sinuses that consist of the maxillary, frontal, sphenoidal, and ethmoidal sinuses. The paranasal sinuses may act as insulating space to prevent the cold inspired air from cooling the brain which sits just above the nasal airway. The sinuses communicate with the nasal cavity via small openings or ostia which are only 2-6 mm in diameter.
Larynx:
The larynx acts as a protective valve at the entrance of the lower airways and it consists of two sphincters: the aryepiglottic fold or false vocal cords and the vocal fold or true vocal cords. In protecting the airway both sphincters close together to form an air tight and water tight seal. The surface of the larynx is very sensitive to mechanical and chemical stimuli.
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