Nostril

The nostril, also known as the anterior nare, is one of the two external openings of the nose that allow air to enter and exit the nasal cavity. The nostrils are lined with skin, tiny hairs (vibrissae), and a mucous membrane, providing the initial passageway for airflow into the respiratory system. The nostrils are separated by the nasal septum, which is composed of cartilage and bone.

Location

The nostrils are located on the external nose at the base, directly above the upper lip. They are positioned on either side of the columella (the soft tissue separating them) and surrounded by the soft, flexible alar cartilage, which forms the lateral boundary of each nostril.

Anatomy

The nostril, also known as the anterior nare, is the external opening of the nose that allows air to pass into the nasal cavity. Each nostril is part of the external nose and is anatomically designed to facilitate airflow while providing filtration and structural support. The nostrils are symmetrical openings separated by the nasal septum and surrounded by soft tissues, cartilage, and skin. Below is a detailed description of the anatomy of the nostril:

External Structure of the Nostril

The external nostril comprises various tissues, including skin, cartilage, and muscles, which together give it shape and flexibility:

Nasal Openings

• The nostrils are oval-shaped openings located at the base of the external nose.
• They serve as the entry and exit points for air into the nasal cavity.

Alar Cartilage

The nostrils are primarily supported by the lower lateral cartilage, also known as the alar cartilage.

Each alar cartilage has two components:

• Medial Crus: The portion of the alar cartilage closest to the midline, forming the boundary of the columella.
• Lateral Crus: The portion that forms the outer walls of the nostrils, providing structural stability and flexibility.

Columella

• The soft tissue that separates the two nostrils at the midline.
• It is composed of skin, connective tissue, and the medial crura of the alar cartilage.

Nasal Septum

• The nasal septum forms the medial boundary of each nostril.
• It is made up of the septal cartilage (anteriorly) and the vomer and perpendicular plate of the ethmoid bone (posteriorly).

Skin and Vibrissae

• The inner lining of the nostrils is covered with skin, which contains sebaceous glands and coarse hairs called vibrissae.
• Vibrissae are positioned near the nostril openings and help trap large particles like dust and debris.

Internal Structure of the Nostril

The nostrils lead directly into the nasal vestibule, which is the anterior-most part of the nasal cavity:

Nasal Vestibule

• The nasal vestibule is the region just inside the nostril.
• It is lined with stratified squamous epithelium (skin-like tissue) and contains vibrissae that act as a protective filter.
• The vestibule transitions posteriorly into the nasal cavity at the limen nasi (a ridge marking the boundary between the vestibule and the respiratory region).

Alar Rim

• The curved, soft edge of the nostril opening, which is part of the external nose.
• It provides shape and structural support to the nostril while maintaining flexibility.

Surrounding Cartilaginous Framework

The nostrils are shaped and supported by a cartilaginous framework:

Lower Lateral Cartilages (Alar Cartilages):

Provide structural support and define the contour of the nostrils.

Upper Lateral Cartilages:

Located superior to the nostrils, these contribute to the shape of the external nose and help maintain the patency of the nasal passage.

Septal Cartilage:

Forms the central support structure that separates the two nostrils.

Muscles Controlling the Nostril

Several small muscles surround the nostrils and control their movement. These muscles are part of the muscles of facial expression and are innervated by the facial nerve (cranial nerve VII):

Nasalis Muscle

Divided into two parts:

• Transverse Part: Compresses the nostrils.
• Alar Part: Dilates the nostrils, allowing for increased airflow.

Levator Labii Superioris Alaeque Nasi

Elevates the upper lip and dilates the nostrils.

Depressor Septi Nasi

Pulls the nasal septum downward, constricting the nostrils.

These muscles allow for dynamic movements of the nostrils, including dilation and compression.

Vascular Supply of the Nostril

The nostril is richly vascularized by branches of the external carotid artery and internal carotid artery:

Arterial Supply:

• Facial Artery: Provides the main blood supply to the external nose, including the nostrils.
• Lateral Nasal Artery: A branch of the facial artery supplying the nostril’s lateral walls.
• Sphenopalatine Artery: Supplies blood to the deeper regions of the nasal cavity.

Venous Drainage:

• Blood drains via the facial vein and pterygoid venous plexus.
• This rich vascular supply makes the nostrils prone to bleeding (epistaxis) when injured.

Nerve Supply of the Nostril

The nerve supply to the nostril includes both sensory and motor innervation:

Sensory Innervation:

Provided by the trigeminal nerve (CN V):

• Ophthalmic Division (V1): Supplies the skin and vestibule of the upper part of the nostril.
• Maxillary Division (V2): Supplies the lower portion of the nostril and nasal cavity.

Motor Innervation:

The facial nerve (CN VII) innervates the muscles controlling nostril movements (e.g., nasalis, levator labii superioris alaeque nasi).

Lymphatic Drainage

Lymphatic drainage of the nostrils occurs via:

• Submandibular Lymph Nodes: Drain lymph from the external nose and nostril region.
• Deep Cervical Lymph Nodes: Handle deeper drainage from the nasal cavity.

External Features of the Nostril

• Alar Region: The soft tissue and cartilage forming the outer curved wall of the nostrils.
• Columella: The central soft tissue separating the two nostrils.
• Nasal Tip: The anterior-most part of the nose, directly above the nostrils.

Function

The nostrils (anterior nares) serve as the primary external openings of the nose, playing a critical role in respiration, filtration, olfaction, and airflow regulation. They allow air to enter and exit the nasal cavity, ensuring that the respiratory system functions efficiently while protecting the body from environmental irritants. The nostrils’ structure, including the surrounding cartilage, skin, and muscles, facilitates their functions. Below is a detailed explanation of the nostrils’ functions:

Air Passage for Respiration

The primary function of the nostrils is to allow air entry into the respiratory system:

• During inhalation, air enters the nostrils and flows into the nasal cavity, where it is filtered, humidified, and warmed.
• During exhalation, the nostrils provide an exit point for carbon dioxide-rich air.
• The nostrils serve as the gateway for oxygen intake, which is vital for the body’s metabolic needs.

The paired nostrils ensure a continuous flow of air, optimizing respiration and minimizing resistance during breathing.

Filtration of Air

The nostrils play a key role in protecting the respiratory tract by filtering out particles and contaminants:

• Nasal Hairs (Vibrissae): Coarse hairs at the entrance of the nostrils trap large particles, such as dust, pollen, and debris, preventing them from entering the nasal cavity and lungs.
• Mucus Production: The mucosa lining the nostrils secretes mucus that traps smaller particles, including pathogens and allergens.
• Cilia: Tiny hair-like structures in the nasal mucosa move trapped particles toward the throat, where they can be swallowed or expelled.

This filtration process ensures clean air reaches the lungs, protecting the respiratory system from infections, irritation, and inflammation.

Humidification and Warming of Air

As air passes through the nostrils and into the nasal cavity, it is humidified and warmed to match body temperature:

• The nasal mucosa lining the nostrils and nasal cavity secretes moisture, which humidifies dry air.
• The rich vascular supply in the nasal mucosa transfers heat to the inhaled air, warming it as it passes through the nostrils.
• This conditioning prevents the lungs and lower respiratory tract from being exposed to cold or dry air, maintaining optimal respiratory function.

Regulation of Airflow

The nostrils contribute to the regulation of airflow during breathing:

The size of the nostrils can adjust airflow:

• During activities like exercise or heavy breathing, the muscles around the nostrils (e.g., nasalis and levator labii superioris alaeque nasi) dilate the nostrils to allow for increased airflow.
• During rest, the nostrils maintain a smaller opening to reduce unnecessary air loss.

The nasal cycle naturally regulates airflow between the two nostrils.

• One nostril experiences greater airflow while the other rests by slight swelling of the nasal mucosa.
• This cycle alternates every few hours, ensuring that the mucosa remains moist and functional.

This dynamic regulation of airflow ensures efficient oxygen intake and optimal respiratory function.

Olfaction (Sense of Smell)

The nostrils enable the flow of air toward the olfactory epithelium, located in the roof of the nasal cavity, facilitating the sense of smell:

• Airborne odor molecules enter through the nostrils and travel to the olfactory receptors in the nasal cavity.
• These receptors detect and transmit signals via the olfactory nerve (Cranial Nerve I) to the olfactory bulb in the brain.
• The brain interprets these signals as different smells.

This function is critical for identifying food, detecting danger (e.g., smoke, gas), and enhancing the sense of taste.

Sneezing Reflex

The nostrils play a role in initiating the sneezing reflex, which helps clear the nasal passages:

• When the mucosa of the nostrils or nasal cavity is irritated by dust, allergens, or pathogens, the sensory nerves in the nostrils trigger a reflexive sneeze.
• During sneezing, air is expelled forcefully through the nostrils, removing irritants and clearing the airways.

This protective mechanism prevents harmful substances from reaching the lower respiratory tract.

Smell-Taste Connection

The nostrils assist in enhancing the sense of taste by enabling olfaction:

• As air passes through the nostrils, odor molecules from food reach the olfactory receptors, contributing to the perception of flavor.
• Without olfaction, the ability to differentiate between complex tastes is significantly reduced, as taste buds alone can only detect basic tastes (e.g., sweet, salty, sour, bitter, umami).

The nostrils’ ability to direct airflow toward the olfactory system is crucial for a full sensory experience during eating.

Equalizing Air Pressure

The nostrils help maintain air pressure balance within the nasal cavity and sinuses:

• Air passing through the nostrils ensures that pressure remains equalized across the nasal cavity and sinuses, preventing discomfort or blockages.
• Blocked nostrils (e.g., due to congestion) can disrupt this balance, causing sinus pressure or pain.

Facial Aesthetics and Expression

The nostrils contribute to the appearance and symmetry of the nose, which is a defining feature of the face:

• The size, shape, and contour of the nostrils influence facial aesthetics.
• Muscles around the nostrils (e.g., nasalis) facilitate facial expressions, such as flaring the nostrils during anger, exertion, or surprise.

Clinical Significance

The nostrils (anterior nares) play a crucial role in respiration, filtration, and olfaction, making them clinically significant for a variety of conditions:

Nasal Obstruction

Blockage of the nostrils due to deviated nasal septum, nasal polyps, or enlarged turbinates can impair airflow, causing difficulty breathing.

Epistaxis (Nosebleeds)

The Kiesselbach’s plexus in the nostrils is prone to bleeding due to trauma, dry air, allergies, or hypertension.

Rhinitis

Inflammation of the nostrils and nasal mucosa due to infections (viral, bacterial) or allergens leads to congestion, sneezing, and discharge.

Foreign Body Obstruction

Common in children, where objects lodged in the nostrils cause pain, discharge, and infection.

Olfactory Disorders

Conditions such as anosmia (loss of smell) or hyposmia (reduced smell) can result from nostril or nasal cavity obstruction or infections.

Nasal Deformities

Trauma or congenital conditions like cleft lip and palate can affect the nostril shape, requiring surgical correction for function and aesthetics.