The nose is an olfactory and respiratory organ, consisting of a nasal skeleton that houses the nasal cavity. The nasal cavity has four functions: it warms and humidifies the inspired air; it removes and traps pathogens and particulate matter from the inspired air; it is responsible for the sense of smell; and it drains and clears the paranasal sinuses and lacrimal ducts. In this article, we shall look at the anatomy of the nasal cavity - its divisions, structure, and neurovascular supply.
Divisions
The nasal cavity is the most superior part of the respiratory tract, extending from the vestibule of the nose to the nasopharynx. It has three divisions:
Nasal Conchae
Projecting out of the lateral walls of the nasal cavity are curved shelves of bone, called conchae (or turbinates). There are three conchae - inferior, middle and superior. They project into the nasal cavity, creating four pathways for the air to flow. These pathways are called meatuses:
The function of the conchae is to increase the surface area of the nasal cavity. This increases the amount of inspired air that can come into contact with the cavity walls, and slow and turbulent air that spends longer in the nasal cavity and can be humidified.
Openings into the Nasal Cavity
In addition to the respiratory and olfactory functions, the nose drains a variety of structures. Thus, there are many openings into the nasal cavity.
The paranasal sinuses drain into the nasal cavity. The frontal, maxillary and anterior ethmoidal sinuses open into the middle meatus. The location of this opening is marked by the semilunar hiatus, a crescent-shaped groove on the lateral walls of the nasal cavity. The middle ethmoidal sinuses empty out onto a structure called the ethmoidal bulla. This is a bulge in the lateral wall formed by the middle ethmoidal sinus itself. The posterior ethmoidal sinuses open out at the level of the superior meatus. The only structure not to empty out onto the lateral walls of the nasal cavity is the sphenoid sinus. It drains onto the posterior roof.
In addition to the paranasal sinuses, other structures open into the nasal cavity:
The nose is a complex organ, and its anatomy and functions make it a vital part of both olfactory and respiratory systems. The divisions, structure, and neurovascular supply of the nasal cavity are essential for its proper functioning, and its ability to remove and trap pathogens, humidify inspired air, and enable drainage of vital structures.
The conchae are small curved structures located in the nasal cavity and consist of the superior, middle, and inferior conchae. Located between the middle and superior conchae is the superior meatus, and superiorly and posteriorly to the superior concha is the spheno-ethmoidal recess. The primary function of the conchae is to increase the surface area of the nasal cavity, allowing more inspired air to come in contact with the cavities walls. Additionally, the conchae disrupt the flow of air from the laminar state – a fast, uniform flow – to a slow, turbulent one. This slows down the speed of the air, allowing it to spend longer in the nasal cavity and become humidified.
Because the nose is responsible for draining a variety of structures, there are many openings that lead into the nasal cavity. The paranasal sinuses are drained through these openings, with the frontal, maxillary, and anterior ethmoidal sinuses emptying into the middle meatus – a crescent-shaped groove located on the lateral walls of the nasal cavity. The middle ethmoidal sinuses empty out onto the ethmoidal bulla, a bulge in the lateral wall, while the posterior ethmoidal sinuses open out at the level of the superior meatus. The only sinus to not empty out onto the lateral wall of the nasal cavity is the sphenoid sinus, which drains onto the posterior roof. Furthermore, the nasolacrimal duct, which is responsible for draining tears from the eye, opens into the inferior meatus, while the auditory (eustachian) tube, allowing the middle ear to equalize with the atmospheric air pressure, opens into the nasopharynx at the level of the inferior meatus.
In addition to openings for drainage, the nasal cavity also requires pathways to allow the entry of nerves, vasculature, and lymphatics. The cribriform plate of the ethmoid bone forms a portion of the roof of the nasal cavity and contains numerous small perforations that allow fibers of the olfactory nerve to enter and exit. The sphenopalatine foramen is located at the level of the superior meatus and serves as a gateway for the sphenopalatine artery, nasopalatine, and superior nasal nerves. Furthermore, the incisive canal, which allows for communication between the nasal cavity and the incisive fossa of the oral cavity, transmits the nasopalatine nerve and greater palatine artery.
The nose has a highly developed vascular supply that provides it with the capacity to effectively modify humidity and temperature of inspired air. The nose receives its blood supply from both the internal and external carotid arteries. Internal carotid branches include the anterior ethmoidal artery and the posterior ethmoidal artery – both of which descend into the nasal cavity through the cribriform plate. External carotid branches include the sphenopalatine artery, the greater palatine artery, the superior labial artery, and the lateral nasal arteries, which form anastomoses with each other in the anterior portion of the nose. The veins of the nose usually follow the same course as the arteries and drain into the pterygoid plexus, facial vein, or cavernous sinus. In some cases, a few nasal veins may join with the sagittal sinus and provide a potential route for infection to spread from the nose into the cranial cavity.
The nose receives both special and general innervation. Special sensory innervation is responsible for the ability of the nose to smell and is carried out by the olfactory nerves. These nerves run through the cribriform plate after leaving the olfactory bulb in the brain. General sensory innervation, on the other hand, is supplied by the nasopalatine nerve (a branch of the maxillary nerve) and the nasociliary nerve (a branch of the ophthalmic nerve). Furthermore, the trigeminal nerve is responsible for innervating the external skin of the nose.
