The role of the nasal cavity is to humidify and warm the inspired air. Also, as the air passes through, the nasal cavity removes minute airborne particles and other debris before the air reaches the lower airways. Columnar epithelium lines the nasal cavity. This type of epithelial lining also secretes mucus that coats the lining and helps with the mucociliary clearance of minute aerosolized particles that become trapped in the nasal mucosa. The nasal cavity also functions to facilitate drainage for the secretions from the adjacent paranasal sinuses. It also captures the odor bearing particles and transmits them to the olfactory recesses, that are in the superior portion of the nasal cavity, just medial to the superior turbinates. Air containing mucosal lined sinuses surround the nasal cavity, which includes the frontal, paired maxillary, sphenoid, and ethmoid sinuses. These cavities directly communicate with the nasal cavity. The secretions from these sinuses drain into the nasal cavity via the thin-walled ostia. Like the nasal cavity, the wall lining of the sinuses also secretes mucus. The cilia on the surface sweep the mucus in a carpet like fashion and move them towards the nasal ostia. The hard palate lines the floor of the nasal cavity. The lateral walls are spiral shaped mucosal folds that overlie the turbinates and sinus ducts draining into the ostia. The spiral shape of the turbinates is designed to increase the surface area for the inspired air.

The nasal cavity is the most cephalic part of the respiratory tract. It communicates with the external environment via the anterior apertures, nares, and the nasopharynx via the posterior apertures, choanae. This cavity is divided into two separate cavities by the septum and kept patent by a bone and cartilaginous framework. Each cavity consists of a roof, floor, medial wall, and lateral wall. Within each cavity are three regions; nasal vestibule, respiratory region, and olfactory region.

Surrounding the nasal cavities are air-containing mucosal lined sinuses, which include the frontal sinuses (superior anterior), ethmoid sinuses (superior), paired maxillary sinuses (lateral), and sphenoid sinuses (posterior). All of these paranasal sinuses, except the sphenoid, communicate with the nasal cavity via ducts that drain through ostia, which empty into spaces located on the lateral wall. The sphenoid sinus empties into the posterior roof. Having a fundamental knowledge of the anatomy of the nasal cavity is vital in understanding its functions.

Respiratory Region

The respiratory region functions to humidify, warm, filter, protect, and eliminate debris. Covered in respiratory epithelium and mucous cells, this is the most substantial part of the nasal cavity. As air traverses through the nasal cavity, it warms to body temperature and reaches near one hundred percent humidity. The neurovascular supply of this region aids this. It regulates the nasal airflow by controlling the blood volume in the erectile tissue on the inferior turbinate and anterior septum.  Under normal conditions, this tissue is continuously stimulated by sympathetic signals via the superior cervical ganglia to keep the nasal cavity uncongested.

Particles that get past the nasal vestibule then become trapped in the mucosa of the nasal cavity. When this occurs, the mucociliary system helps get rid of these particles. The ciliated pseudostratified columnar epithelium sweeps particles at a rate of one centimeter per minute into the nasopharynx for further expulsion.

The mucus of the nasal cavity forms a protective barrier to inhaled pathogens. The components of the mucus that actively protect the host are immunoglobulin A, lysozymes, and lactoferrin.[1][2][3]

Olfactory Region

Olfaction requires orthonasal or retronasal airflow to transport odor-bearing particles up to the olfactory epithelium located at the apex of the nasal cavity. As odorants become trapped in the mucus, it binds to odorant binding proteins that concentrate and help solubilize the particles. The particles are then attached to olfactory receptors on cilia that transmit specific signals up through the cribriform plate to synapse with neurons of the olfactory bulb, which then sends signals through the olfactory nerve (CNI) into the secondary neurons for higher processing before entering the brain. A unique feature of the olfactory receptors is that a single receptor cell can detect only one odorant type and cannot regenerate.[1][2][3]

Nasal Vestibule

The nasal vestibule is the first area encountered as you move posteriorly through the anterior nares, also known as the nostrils or external nasal valve. The first half of the vestibule has a covering of keratinized stratified squamous epithelium that contains coarse hairs called vibrissae. These hairs filter inhaled particles. The covering of the second half of the vestibule is in respiratory epithelium, pseudostratified ciliated columnar epithelium.[1][2][3]

• Lateral: lateral crus of the lower lateral cartilage (LLC) and fibrofatty alar tissue
• Medial: medial crus of the LLC and septal cartilage
• Posterior: limen naris

The roof of the Nasal Cavity

The mucosa of the roof of the nasal cavity contains perforations that communicate with the cribriform plate. Within these perforations are the olfactory axons.[1]

• Anterior: nasal spine of the frontal bone and nasal bone
• Posterior: cribriform plate of the ethmoid and the body of the sphenoid

The floor of the Nasal Cavity

The floor of the nasal cavity is broader than that of the roof.[1]

• Anterior: the palatine process of the maxilla
• Posterior: horizontal plate of the palatine bone

Incisive Canal

This canal is located in the floor of the nasal cavity, posterior to the central incisor, and lateral to the nasal septum. This structure transmits the nasopalatine nerve into the oral cavity and the greater palatine artery into the nasal cavity.[1]

Nasal Septum

The nasal septum partitions the nasal cavity into two equal but separate compartments. Cartilage and bone comprise the nasal septum. It is covered by squamous epithelium, which differs from the lateral walls of the nasal cavity. A portion of the anterior septum is covered in erectile tissue. It also contributes to lateral projections called the upper lateral cartilages, which makes up the middle third of the nose. The bony segment of the septum is pneumatized, and when it over expands, it has the potential to obstruct airflow. Below are the components of the septum.[1]

Quadrangular (septal) cartilage: This is the most anterior portion of the septum. It contains the Kiesselbach plexus (see blood supply).[4]

Attachments:

• Superior: nasal bone
• Inferior: anterior nasal spine of the maxilla
• Posterior-Superior: perpendicular plate of the ethmoid
• Posterior-Inferior: vomer and maxillary crest

Perpendicular Plate of the Ethmoid: This is a vertical projection from the cribriform plate of the ethmoid inferiorly to the septal cartilage.[5]

Vomer: Located inferior and slightly posterior to the perpendicular plate of the ethmoid. It is attached inferiorly to the nasal crest of the maxilla and palatine bone.[5]

Nasal Crest of the Maxilla and Palatine Bone: Together these bones form the inferior support for the septal cartilage.[5]

Anterior Nasal Spine of the Maxilla: This is a bony projection formed by the paired maxillary bones. It located anterior to the piriform aperture and are palpable at the superior portion of the philtrum of the upper lip.[5]

Lateral Wall of the Nasal Cavity

The nasal cavity's lateral wall has three medially projecting inferiorly curved bones called conchae. The middle and superior conchae are part of the ethmoid bone, whereas the inferior concha is a separate bone altogether. There is a normal variant called the supreme conchae. These conchae, when covered by mucosa, are termed turbinates. The turbinates augment the surface area of the nasal cavity to aid in its functions of humidifying, warming, and humidifying the air. The turbinates create four channels.  Three of these channels are termed meatuses, and the fourth is the sphenoethmoidal recess.[1][6][5][7]

Bones of the lateral wall:

• Ethmoid bone
• Perpendicular plate of the palatine bone
• The medial plate of the pterygoid process of the sphenoid bone
• Medial surface of the lacrimal and maxillary bones
• Inferior concha

Sphenoethmoidal Recess: Located superior to the superior turbinate and inferior to the nasal cavity roof, which is the drainage site of the sphenoid sinus.[1][6][5][7]

Meatuses

• Superior Meatus: located inferior to the superior turbinate and superior to the middle turbinate; this is the drainage site of the posterior ethmoid sinus.
• Middle Meatus: located inferior to the middle turbinate and superior to the inferior turbinate - there are several structures within this meatus. This is the drainage site of the frontal, anterior ethmoid, and maxillary sinuses.
• Inferior Meatus: Located inferior to the inferior turbinate and superior to the floor of the nasal cavity. The nasolacrimal duct drains tears from the lacrimal sac at the medial aspect of the eye into the anterior portion of this meatus via Hasner's valve.[1][6][5][7]

Limen Naris: The limen naris is a mucosal ridge that signifies the posterior boundary of the nasal vestibule and the anterior boundary of the nasal cavity proper.[1][6][5][7]

Agger Nasi Cells: These cells are the most anterior portion of the anterior ethmoid air cells. They are located anterior and superior to the basal lamella, most anterior attachment to the lateral wall, of the middle turbinate to create the anterior aspect of the frontal recess.[1][6][5][7]

Frontal Recess: Located between the posterior wall of the agger nasi cells and the middle turbinate.[1][6][5][7]

Uncinate Process of the Ethmoid: This is a thin crescent-shaped bone that is part of the ethmoid bone. It is attached to the lacrimal bone anteriorly, the inferior turbinate inferiorly and superiorly to the lamina papyracea. This structure protects the sinuses of the infundibulum from inhaled foreign particles.[1][6][5][7]

Lamina Papyracea: This thin bone is the separation between the orbit and the ethmoid air cells.[1][6][5][7]

Ethmoid Infundibulum: This is a pyramidal shaped channel located at the anterior portion of the semilunar hiatus that drains the maxillary, anterior ethmoid, and frontal sinuses.[1][6][5][7]

Semilunar Hiatus: Located between the uncinate process anteriorly and the ethmoid bulla posteriorly, this is a space that empties the ethmoid infundibulum.[1][6][5][7]

Ethmoid Bulla: Located just anterior to the semilunar hiatus and superior to the ethmoid infundibulum, which is where the middle ethmoidal air cells open into the nasal cavity.[1][6][5][7]

Ostiomeatal Complex (OMC): This is an area located lateral to the middle turbinate that houses the ostia of the lateral wall sinuses; frontal, maxillary, and anterior/middle ethmoid sinuses.[1][6][5][7]

Sphenopalatine Foramen: This foramen connects the nasal cavity to the pterygopalatine fossa and is posterior to the middle turbinate in the posterior portion of the superior meatus. The significant content of this foramen is[1][6][5][7]:

• Sphenopalatine artery of the maxillary artery
• Nasopalatine branch of the maxillary nerve of the trigeminal nerve (CNV2)
• Posterior superior lateral nasal nerves of CNV2

Choanae

The choanae are also known as posterior nasal apertures. It is the posterior boundary of the nasal cavity proper. It opens into the nasopharynx.[1][6][5][7]

• Superior: the body of the sphenoid bone
• Inferior: horizontal plate of the palatine bone
• Lateral: the medial pterygoid process of the sphenoid bone
• Medial: vomer

Internal Nasal Valve (INV)

The INV is the narrowest portion of the nasal cavity and constitutes the area of highest resistance to airflow, which causes an increase in the acceleration of airflow. Without proper support, this increased airflow causes a decrease in intraluminal pressure, which ultimately causes the INV to collapse; this is Bernoulli's principle of flow. The average cross-sectional area of the INV in adults is around 0.73 square centimeters. At the apex of the valve the ULC and, the nasal septum come together at an angle of 10 to 15 degrees.[8]

• Superior: upper lateral cartilage (ULC/caudal edge)
• Inferior: nasal floor or hard palate
• Lateral: the anterior portion of the inferior turbinate
• Medial: nasal septum

The nasal placodes, oval thickenings of surface ectoderm, develop inferior and lateral to the frontonasal prominence at the end of the fourth week of the embryonic period. They contain mesenchyme near the outer edges that begin to proliferate to form the medial and lateral nasal prominences (MNP, LNP). As a result, the nasal placodes reside in deep nasal pits, which are the primitive anterior nares and nasal cavities. The LNP form the alae of the nose. The MNP forms the nasal septum, ethmoid bone, and cribriform plate.[9][10]

As the maxillary prominences proliferate the cause the nasal structures to move medially and by the end of the sixth week, the maxillary prominences begin to merge with the LNP at the nasolacrimal groove. During the sixth to eighth weeks, the nasal cavity and the oral cavity begin to communicate through the choanae. As this is occurring, the lateral wall of the nasal cavity begins forming five ethmoturbinals. The ethmoturbinals eventually form the turbinates and other structures found within each meatus of the nasal cavity.[9][10][11]

• One: agger nasi cells and uncinate process
• Two: ethmoid bulla
• Three: basal lamella of the middle turbinate
• Four: superior turbinate
• Five: supreme turbinate (a normal variant that undergoes

The olfactory epithelium is developing from the ectoderm in the roof of the nasal cavity while the conchae are forming.

Arterial Supply

The nasal cavity has an abundant supply of vasculature to aid in functions of warming and humidifying inhaled air. It allows the mucosa to enlarge and shrink, under the influence of sympathetic innervation. The arterial supply to the nose and nasal cavity originates from the internal and external carotid arteries.[12]

Internal Carotid Artery (ICA)

The primary branch off of the ICA that supplies the nasal cavity is the ophthalmic artery. Coming off of the ophthalmic artery are the anterior and posterior ethmoid arteries, as well as the dorsal nasal artery. The anterior ethmoid artery supplies the lateral nasal wall and the nasal septum. The posterior ethmoid artery supplies the superior turbinate and the nasal septum. The dorsal nasal artery supplies the dorsal aspect of the external nose.[1][2]

External Carotid Artery (ECA)

The ECA gives rise to the maxillary artery and the facial artery. These two significant arteries then branch into smaller vessels.

Maxillary Artery

The maxillary branches into the descending palatine artery that then travels through the pterygopalatine fossa down the palatine canal and then branches into the greater and lesser palatine arteries. The greater palatine artery then enters the greater palatine foramen on the posterior aspect of the palate before traversing the palate anteriorly to enter the nasal cavity via the incisive canal. It supplies the septum and the floor of the nasal cavity.

Like the descending palatine artery, the sphenopalatine artery is a branch of the maxillary artery. It branches off of the maxillary artery near the pterygopalatine fossa where it then enters the lateral wall of the nasal cavity through the sphenopalatine foramen, located just posterior to the medial turbinate. The sphenopalatine artery then branches into the posterior lateral nasal branches and the posterior septal branch. The posterior lateral branches supply the middle and inferior turbinates, while the posterior septal branch supplies the posterior septum.[1][2][13]

Facial Artery

The facial artery gives rise to the superior labial artery, the lateral nasal artery, and the angular artery. The superior labial artery gives off an alar branch and a septal branch that supply the same structures as their name. The lateral nasal artery supplies the alar cartilage on the external nose and also supplies the nasal vestibule. The angular artery supplies the external nasal tip, dorsum, and lateral wall.[1][2][13]

Kiesselbach's Plexus (Little's Area)

Kiesselbach's plexus is a vascular anastomosis between the anterior ethmoid artery, superior labial artery, greater palatine artery, and the terminal branch of the posterior septal branch of the sphenopalatine artery. This vascular plexus is located in the anterior nasal septum and is the most common site of epistaxis.[14] 

Woodruff's Plexus

Woodruff's plexus is a vascular anastomosis between the sphenopalatine artery and the ascending pharyngeal artery. Located on the lateral wall of the nasal cavity in the area posterior to the middle and inferior turbinates.[15] 

Venous Drainage

The names of the veins that drain the nose and nasal cavity follow that of the arteries with which they pair. The maxillary branches drain either into the cavernous sinus or the pterygoid plexus located in the infratemporal fossa. The veins of the anterior nasal cavity drain into the facial vein. Of note, infections located between the oral commissure and nasal bridge, have the potential to become intracranial infections. These must be treated promptly to prevent the extension of infection.

Lymphatics

The anterior nasal cavity drains anteriorly to the face that then makes its way to the submandibular lymph nodes in level IB. The lymphatics of the posterior nasal cavity and paranasal sinuses drain into the upper cervical lymph nodes and retropharyngeal lymph nodes.[16][17]

Olfactory Nerve (CNI)

The olfactory nerve transmits signals from the nasal cavity to the brain to give the sense of olfaction. The olfactory epithelium is in the superior portion of the nasal cavity. Within this epithelium are sensory cilia that project up through the cribriform plate to the olfactory bulb. From the olfactory bulb, signals are sent through the olfactory nerve proper to a network of secondary neurons for processing before ending up in the brain.[18]

Trigeminal Nerve (CNV)

The trigeminal nerve is the sensory innervation to the external and internal nose. The branches are the ophthalmic (V1), maxillary (V2), and mandibular (V3).  Sympathetic and parasympathetic fibers run with these branches to supply their target tissues. The ophthalmic and maxillary branches innervate the nose and nasal cavity.[19][20]

• Ophthalmic Branches (V1): As the ophthalmic nerve begins to branch, it gives off a nasociliary branch, which then provides the anterior and posterior ethmoid nerves. The anterior ethmoid gives off an external branch that supplies the nasal tip, an internal branch that supplies the anterosuperior nasal cavity and a septal branch that supplies the anterior superior nasal septum. The posterior ethmoid supplies the posterosuperior nasal cavity. Two other branches of the ophthalmic branch of the trigeminal nerve are the supratrochlear and infratrochlear nerves that supply the nasal dorsum.[19][20]

• Maxillary Branches (V2): The maxillary branches of the trigeminal nerve that innervate the nose and nasal cavity branch in or near the pterygopalatine fossa then enter the nasal cavity. The only external nasal branch is the infraorbital nerve, which supplies the malar and lateral nose. The nasopalatine nerve traverses the nasal septum from posterior to anterior in a downward projection to enter the incisive canal. It supplies the posterior and inferior nasal septum as well as mucosa just posterior to the incisors. The greater palatine nerve follows the greater palatine artery down the palatine canal, giving off posterior inferior lateral nasal nerves that supply the posterior lateral wall of the nasal cavity. Three other nerves come off the maxillary branch (V2). Two of these are the posterior superior lateral nasal nerve and posterior superior medial nasal nerve, both of which pass through the sphenopalatine foramen to supply the lateral and medial walls of the nasal cavity, respectively. The superior alveolar nerve is the last branch of V2, and it supplies the anterior septum and the area near the nasal vestibule.[19][20]