Physiology and anatomy of the upper airway (UA)

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The upper airway is a complex and multifunctional structure that regulates different functions of the human organism in a contradictional way. This complexity is worth us analyzing the anatomy of the upper airway in healthy subjects for the better understanding of the complexity in patients with OSAHS. The upper airway alternates the respiratory and ventilatory functions with gastric ventilation and phonation (speech). On it, the responsible mechanisms for olfactory and digestif sense are based, which allows the articulation of speech and includes the respiratory system and ventilation. The UA is divided into fossa nasalis, pharynx and larynx.

The malfunctioning at some point in this complex causes intermittent occlusion of the upper airway, that predisposes the development of snoring and sleep apnea. The exact causes of this pharyngeal collapse resulting in the cessation of airflow, are still being determined, although many anatomic and functional factors related to this obstruction have been analized, the root reasons are still unknown to modern medicine.

The fosas nasales
It is the beginning of the upper airway and the input channel for the more common air flow. It consists of rigid and collapsible walls and in the absence of pathological factors it has no transcendece in the processes of snoring and apnea. They form cavities separated by an osteo-cartilaginous septum (septum), with walls, in which there are bone reliefs covered with a erectile mucous and nasal cornets, whose size varies depending on the amount of blood, that they involve. They are divided into: nostrils, previous openings of the fossa nasalis and choanae, the posterior openings.

Pharynx
This is the area of greatest importance, because here most of the episodes of obstruction that result in OSAHS are located. It is the portion of the aerodigestive tract, that runs from the back of the nose portion (nasopharynx, nasopharyngeal nasopharynx) to the entrance of the esophagus (hypopharynx). It is a membranous collapsible canal of soft walls, that extends from the skull base to the sixth cervical vertebra. It has the peculiarity of being a common segment of the respiratory and digestive system. Its approximate size is 13 inches long and is divided into different anatomical sections: Nose section or nasopharynx, oropharynx or oral section and larynx hypopharynx section. The anatomical region between the posterior pharynx to the larynx is devoid of cartilage therefore it is susceptible to the obstruction.

The rinopharynx or nasopharynx
It is the top of the larynx just behind the nasal cavity through the choanae. It is composed of rigid walls without movement except for the soft palate, which is membranous and collapsible. The rhinopharynx is not collapsible, so its involvement in the OSAHS only happens, if there is an occupation of the same, as it is typical in children affected by adenoidal hypertrophy.

Oropharynx
The name is given to the middle of the pharynx, between the nasopharynx and hypopharynx. The oropharynx is the portion of the pharynx that lies between an imaginary line to continue the hard palate and the other prolonging the top edge of the hyoid bone. The oropharynx is the portion of the pharynx more involved in the pathophysiology of OSAS, because it is collapsible, and anatomically very variable. It is bounded by the palate and the upper edge of the epiglottis. The top opening of the throat is the epiglottis, which is formed by the hyoid bone and cartilage. In the rear edge of the glottis there is a tab called epiglottis (cartilage subject to the back of the tongue), which automatically closes the larynx in the act of swallowing, to prevent it from entering food in the airway. It contains the palatine tonsils, lymphoid tissue organs located in the side walls. Tonsils vary in size and often enrolled in inflammation, what is known as tonsillitis. The oropharynx can be divided into two distinct parts:

• Velopalatal oropharynx is the part of the oropharynx, that can be seen by opening the mouth. It includes the soft palate or the velum of the soft palate, formed by the uvula and tonsillar pillars and back on each side, and the palatine tonsils.
• Basilingual Oropharynx: includes the base of the tongue,that means, the lingual portion that remains after the "v" lingual, and that stays outside the oral cavity, and is not accessible to direct visual examination.

Hypopharynx or laryngopharynx
It is the bottom part of this canal, and extends from the top edge of the epiglottis to the lower border of the cricoid cartilage. The hypopharynx is because of the digestive tract, unrelated to the passage of air.

Larynx
It is part of the airway, that acts as a sphincter control, through the vocal cords. In the upper part of the larynx lies the epiglottis, mobile cartilage, which can occasionally collapse the air passage. It is a rigid and collapsible tube, that connects the pharynx to the trachea. It measures approximately 4 cm long and about 3 cm in diameter. It is the principal organ of phonation. It is a moving organ, which adapts its morphology to phonation and swallowing. Through this there exist more mobility. It is carried upward and forward in its entirety, removing the glottis of the passage of food, which is drained by the sides of the epiglottis. The non-collapsible structure makes it irrelevant in the obstructive processes except in pathological anomalies like tumor presences.

Trachea
It is a tube about 12 cm long and 2.5 cm in diameter. Normally it is not a collapsible tube formed by cartiloaginosos incomplete rings. The permeability of the trachea is held by these 20 horseshoe-shaped cartilages.