Nasal Cavity and Pharynx

The term respiration has three meanings: (1) ventilation of the lungs (breathing), (2) the exchange of gases between air and blood and between blood and tissue fluid, and (3) the use of oxygen in cellular metabolism. 

The principal organs of the respiratory system are the nose, pharynx, larynx, trachea, bronchi, and lungs.

These organs serve to receive fresh air, exchange gases with the blood, and expel the modified air. Within the lungs, air flows along a dead-end pathway consisting essentially of bronchi → bronchioles → alveoli (with some refinements to be introduced later). Incoming air stops in the alveoli (millions of thin-walled, microscopic air sacs in the lungs), exchanges gases with the bloodstream across the alveolar wall, and then flows back out.

The conducting division of the respiratory system consists of those passages that serve only for airflow, essentially from the nostrils through the bronchioles. The respiratory division consists of the alveoli and other dis- tal gas-exchange regions. The airway from the nose through the larynx is often called the upper respiratory tract (that is, the respiratory organs in the head and neck), and the regions from the trachea through the lungs compose the lower respiratory tract (the respiratory organs of the thorax).

The Nose:

The nose has several functions:

• it warms, cleanses, and humidifies inhaled air
• it detects odors in the airstream
• it serves as a resonating chamber that amplifies the voice.

The  nasal  cavity  extends  from the  anterior  (external)  nares  or nostrils,  to the  posterior (internal) nares, or choanae . The dilated chamber inside the ala nasi is called the vestibule. It is lined with stratified squamous epithelium and has stiff vibrissae  or guard hairs, that block the inhalation of large particles.

The nasal septum divides the nasal cavity into right and left chambers called nasal fossae. The vomer forms the inferior part of the septum, the perpendi- cular plate of the ethmoid bone forms its superior part, and the septal cartilage forms its anterior part. The ethmoid and sphenoid bones compose the roof of the nasal cavity and the palate forms its floor. The palate separates the nasal cavity from the oral cavity and allows you to breathe while there is food in your mouth. The paranasal sinuses and the nasolacrimal ducts of the orbits drain into the nasal cavity.

The pharynx:

The pharynx is a muscular funnel extending about 13 cm (5 in.) from the choanae to the larynx. It has three regions:

1. Nasopharynx
2. Oropharynx
3. Laryngopharynx

The nasopharynx : which lies posterior to the choanae and dorsal to the soft palate, receives the auditory (eustachian) tubes from the middle ears and houses the pharyngeal tonsil. Inhaled air turns 90 downward as it passes through the nasopharynx. Dust particles larger than 10 ’m generally cannot make the turn because of their inertia. They collide with the posterior wall of the nasopharynx and stick to the mucosa near the tonsil, which is well positioned to respond to airborne pathogens.

The oropharynx: is a space between the soft palate and root of the tongue that extends inferiorly as far as the hyoid bone. It contains the palatine and lingual tonsils. Its anterior border is formed by the base of the tongue and the fauces , the opening of the oral cavity into the pharynx.

The laryngopharynx:  begins with the union of the nasopharynx and oropharynx at the level of the hyoid bone. It passes inferiorly and dorsal to the larynx and ends at the level of the cricoid cartilage at the inferior end of the larynx. The esophagus begins at that point. The nasopharynx passes only air and is lined by pseudostratified columnar epithelium, whereas the oropharynx and laryngopharynx pass air, food, and drink and are lined by stratified squamous epithelium.