Week 9 - L2 - Anatomy and Functions of the Nasal Cavity and Paranasal Sinuses
General Overview and Functions of the Nasal Cavity
Definition: The nasal cavity is the respiratory passage located between the external nose and the nasopharynx. Its primary role is to conduct air into the lungs.
Division: The cavity is bisected into left and right halves by the nasal septum, which is a midsagittal plate.
Airflow Pathway: - Entry: Air enters through two external openings known as nares or nostrils. - Exit: Air exits into the nasopharynx through the choanae (also termed internal nares).
Protective Barrier: The muscular soft palate serves as a barrier between the nasopharynx and the oral cavity. This structure prevents food from entering the nasal cavity during the process of swallowing, also known as deglutition.
Functional Regions and Zones: - Nasal Vestibules: The initial entry points of the nasal cavity. - Respiratory Region: The largest portion of the nasal cavity. Its primary functions include filtering, warming, and humidifying inhaled air. - Respiratory Mucosa: Consists of a ciliated epithelium that secretes mucus and watery fluid. - Filtration Mechanism: The fluid traps inhaled particles and pathogens. Ciliated cells then transport this mucus film to the pharynx to be swallowed and destroyed by stomach acid. - Thermoregulation: The mucosa contains a dense network of subepithelial capillaries that radiate heat to warm the inhaled air. - Olfactory Region: Located in the roof of the nasal cavity. - Olfactory Epithelium: Contains a high concentration of olfactory receptor cells. - Cribriform Plate: A portion of the ethmoid bone perforated by numerous tiny foramina that allow the passage of olfactory nerves for the sense of smell.
Anatomical Boundaries of the Nasal Cavity
The Floor: Formed essentially by the hard palate; it is characterized as wide.
The Roof: Formed mostly by the ethmoid bone; it is much narrower than the floor.
The Median Wall: Formed by the nasal septum. - Bony Components: The perpendicular plate of the ethmoid bone and the vomer. Additional contributions are made by the sphenoid, palatine, and maxillary crests. - Cartilaginous Component: The anterior portion is formed by the septal cartilage.
The Lateral Wall: A complex structure featuring three shell-shaped bony projections called conchae or turbinates. - Superior Concha: Part of the ethmoid bone. - Middle Concha: Part of the ethmoid bone. - Inferior Concha: A separate, independent bone.
Meatuses and Recesses: The conchae curve inferomedially to create convoluted spaces: - Superior Meatus: Space below the superior concha. - Middle Meatus: Space below the middle concha. - Inferior Meatus: Space below the inferior concha. - Sphenoethmoidal Recess: A small space located between the superior nasal concha and the ethmoid and sphenoid bones.
Function of Conchae: They significantly increase the surface area of the nasal cavity to facilitate the filtering, moistening, and warming of air.
Anatomy of the External Nose and Choanae
External Nose Structure: - Superior Part: Formed by bony elements including the nasal bone, the frontal process of the maxilla, and the nasal part of the frontal bone. - Anterior Part: Primarily composed of a cartilaginous skeleton for flexibility. - Cartilages (5 Main): - Two lateral cartilages. - Two major alar cartilages (the lateral sides also contain three to four minor alar cartilages). - One septal cartilage in the midline. - Morphological Variation: Differences in human nose size and shape are largely due to variations in these nasal cartilages.
Choanae Boundaries (Internal Nares): - Medial: The vomer (forming the posterior nasal septum). - Inferior: The horizontal plate of the palatine bone. - Lateral: The medial pterygoid plate. - Superior: The ala of the vomer and the body of the sphenoid bone.
Paranasal Sinuses and Drainage Pathways
Definition: Air-filled cavities surrounding the nasal cavity, named after the bones they occupy.
Types of Sinuses: - Frontal Sinuses: Located at the superior medial margin of the orbit, inferior to the forehead. - Maxillary Sinuses: The largest paranasal sinuses, located within the body of the maxilla. - Ethmoidal Cells: Small chambers located between the nasal cavity and the orbit, associated with the superior and middle conchae. - Sphenoidal Sinuses: Located in the body of the sphenoid bone (potentially extending into the wings); they are adjacent to the pituitary gland.
Drainage Routes into the Nasal Cavity: - Sphenoethmoidal Recess: Receives drainage from the sphenoidal sinus. - Superior Nasal Meatus: Receives drainage from the posterior ethmoidal cells. - Middle Nasal Meatus: - Ethmoid Bulla: A rounded elevation formed by middle ethmoidal cells; receives their drainage. - Semilunar Hiatus (Hiatus Semilunaris): A half-moon shaped depression inferior to the ethmoid bulla. - Ethmoidal Infundibulum: A passage within the semilunar hiatus. - Structures Draining Here: Maxillary sinus (via multiple openings/ostia), anterior ethmoidal cells, and the frontal sinus (via the frontonasal duct/canal). - Inferior Nasal Meatus: Receives the nasolacrimal duct, which drains tears from the ocular surface and lacrimal sacs.
Pharyngotympanic Tube: Connects the middle ear with the nasopharynx to regulate pressure across the cavities.
Clinical Foundations of the Maxillary Sinus
Infection Susceptibility: The maxillary sinuses are the most frequently infected paranasal sinuses.
Drainage Challenges: The openings (ostia) of the maxillary sinuses are located high on their superomedial walls. Due to bipedal locomotion and upright head posture, natural drainage is inefficient and only occurs when the sinus is completely full.
Postural Drainage: Drainage can be facilitated by tilting the head anteriorly to mimic a "quadrupedal" position.
Dental Relationship: The floors of the maxillary sinuses are near the roots of the three maxillary molars. The removal of these molars can create a communication between the oral cavity and the sinus, leading to potential infection.
Symptoms of Sinusitis: Nasal blockage, discharge, headache, loss of smell, and facial pain.
Comparative Anatomy: The Neanderthal Nose
Historical Theory: Large Neanderthal noses and sinuses were traditionally viewed as cold-climate adaptations to warm air before it reached the lungs.
Modern Research Findings: - Sinus Volume: Recent studies indicate that Neanderthal sinus volumes are similar to those of modern humans, challenging the specialized cold-adaptation theory. - Nasal Aperture: Eco-geographical patterns in modern humans would predict reduced nasal breadth for cold climates, which Neanderthals do not exhibit. - Airflow Efficiency: Neanderthal noses were not more efficient at conditioning flow than those of modern humans in various environments. - Volume and Metabolism: Neanderthal noses allowed for a higher volume of air intake. This supports the physiological needs of their large, barrel-shaped chests and demanding lifestyles, which required estimated energy intakes between and .