Structure and Functions of the Human Nose
Introduction
The human nose is an important sensory and respiratory organ. It performs two major functions: allowing air to enter the respiratory system and detecting different smells.
During breathing, the nose filters, warms and moistens the incoming air before it reaches the lower respiratory tract. It also contains specialized sensory cells that detect odours and help us recognize different smells.
For example, when you smell food, the nose detects chemical substances released by the food and sends signals to the brain, allowing you to identify its smell.
The nose is also closely associated with the sense of taste. This is why food may seem less flavourful when the nose is blocked during a common cold.
The human nose consists of the external nose, nasal cavity and specialized olfactory structures responsible for the sense of smell.
[IMG 1 — Horizontal labelled anatomical diagram of the human nose showing the external nose, nostrils, nasal cavity, nasal septum, nasal conchae, olfactory region, nasopharynx and paranasal sinuses.]
1. External Structure of the Nose
The external nose is the visible part of the nose present in the centre of the face. It provides an opening through which air enters and leaves the respiratory system.
It is supported by bones and cartilage and is covered by skin.
The main parts of the external nose include:
1. Root
The root is the uppermost part of the nose, located between the eyes. It connects the nose with the forehead.
2. Bridge
The bridge is the upper bony portion of the nose. It provides structural support and helps maintain the shape of the nose.
3. Apex (Tip)
The apex is the pointed lower part of the external nose.
4. Nostrils (External Nares)
The nostrils are the two external openings of the nose through which air enters and leaves.
5. Ala of the Nose
The alae are the rounded lateral portions surrounding the nostrils. They help form the shape of the external nasal openings.
6. Nasal Septum
The nasal septum is the partition that separates the nasal cavity into right and left portions. It is formed by bone and cartilage.
The anterior portion contains septal cartilage, while the posterior portion is formed mainly by the perpendicular plate of the ethmoid bone and the vomer.
[IMG 2 — Horizontal labelled diagram of the external nose showing root, bridge, apex, ala, nostrils and nasal septum, with bony and cartilaginous portions.]
2. Nasal Cavity
The nasal cavity is an air-filled space located behind the nostrils. It extends from the external nasal openings to the nasopharynx.
The nasal septum divides it into two chambers: right and left nasal cavities.
The nasal cavity plays an important role in filtering, warming and moistening the incoming air.
A. Nasal Mucous Membrane
The nasal cavity is lined by a mucous membrane that contains blood vessels, mucus-secreting cells and ciliated epithelial cells.
Its major functions include:
Producing mucus to trap dust and microorganisms.
Moistening incoming air.
Helping protect the respiratory tract from harmful particles.
The cilia are tiny hair-like structures that move mucus and trapped particles towards the throat, where they can be swallowed or expelled.
B. Nasal Conchae (Turbinates)
The nasal cavity contains three pairs of curved bony projections called nasal conchae.
They are:
Superior nasal concha
Middle nasal concha
Inferior nasal concha
These structures project into the nasal cavity and increase the surface area available for contact between air and the nasal mucous membrane.
They also create airflow patterns that help warm, humidify and filter incoming air.
Important: The superior and middle nasal conchae are parts of the ethmoid bone, whereas the inferior nasal concha is a separate bone.
C. Nasal Meatuses
The spaces located beneath the nasal conchae are called nasal meatuses.
They provide pathways for airflow and openings for the drainage of secretions from the paranasal sinuses and certain ducts.
The nasal cavity communicates with the paranasal sinuses through these openings.
[IMG 3 — Horizontal coronal section of the nasal cavity showing nasal septum, superior, middle and inferior nasal conchae, corresponding meatuses and mucous membrane.]
3. Paranasal Sinuses
Paranasal sinuses are air-filled cavities located within certain bones surrounding the nasal cavity.
They communicate with the nasal cavity through small openings.
There are four pairs of paranasal sinuses:
1. Frontal Sinuses
Located within the frontal bone, above the eyes.
2. Maxillary Sinuses
Located within the maxillary bones, on either side of the nasal cavity. They are the largest paranasal sinuses.
3. Ethmoidal Sinuses
Located within the ethmoid bone, between the nasal cavity and the orbits.
4. Sphenoidal Sinuses
Located within the sphenoid bone, towards the back of the nasal cavity.
Functions of Paranasal Sinuses
Help reduce the weight of the skull.
Produce mucus that drains into the nasal cavity.
Help condition the air passing through the nasal region.
Contribute to the resonance of the voice.
Clinical connection: Inflammation of the mucous membrane lining the paranasal sinuses is called sinusitis. It may cause facial pain or pressure, nasal blockage and nasal discharge.
[IMG 4 — Horizontal anatomical illustration of the skull showing the four pairs of paranasal sinuses: frontal, maxillary, ethmoidal and sphenoidal.]
4. Olfactory Region and Receptors
The olfactory region is the specialized area of the nasal cavity responsible for detecting smells.
It is located mainly in the upper part of the nasal cavity, near the roof, including the superior nasal concha and upper nasal septum.
It contains the olfactory epithelium, which consists of specialized sensory cells responsible for detecting odours.
The main components include:
1. Olfactory Receptor Cells
These are specialized sensory neurons that detect chemical substances present in inhaled air.
When odour molecules reach the olfactory epithelium, they interact with receptors on the sensory cells and initiate electrical signals.
2. Supporting Cells
Supporting cells provide structural and functional support to the olfactory receptor cells.
3. Basal Cells
Basal cells act as stem cells and help replace damaged or aged olfactory receptor cells.
4. Olfactory Glands (Bowman's Glands)
These glands produce secretions that help dissolve odour molecules and keep the olfactory surface suitable for detecting smells.
[IMG 5 — Horizontal labelled microscopic diagram of the olfactory epithelium showing olfactory receptor neurons, supporting cells, basal cells, cilia, mucus layer and Bowman's glands.]
5. Mechanism of Smell (Olfaction)
Olfaction is the process by which the nose detects and identifies different smells.
It occurs when airborne chemical substances stimulate olfactory receptor cells in the nasal cavity.
Steps Involved in the Mechanism of Smell
Step 1: Entry of Odour Molecules
When we breathe in, air containing odour molecules enters the nasal cavity.
Step 2: Reaching the Olfactory Region
Some of these molecules reach the upper part of the nasal cavity, where the olfactory epithelium is located.
Step 3: Dissolving in Mucus
Odour molecules dissolve in the mucus covering the olfactory epithelium.
Step 4: Stimulation of Olfactory Receptors
The dissolved molecules interact with specific receptors present on the cilia of olfactory receptor cells.
This stimulation produces electrical signals in the sensory neurons.
Step 5: Transmission of Signals
The signals travel through the olfactory nerve fibres, which pass through small openings in the cribriform plate of the ethmoid bone.
They reach the olfactory bulb, where the information is processed.
Step 6: Interpretation by the Brain
Signals are transmitted from the olfactory bulb through the olfactory pathways to brain regions involved in smell perception.
The brain interprets these signals, allowing us to recognize different odours.
[IMG 6 — Horizontal flowchart showing the mechanism of smell: Odour molecules → Nasal cavity → Olfactory mucus → Olfactory receptors → Olfactory nerve fibres → Olfactory bulb → Olfactory cortex → Recognition of smell.]
6. Functions of the Human Nose
The human nose performs several important functions.
A. Respiration
The nose provides a passage for air to enter and leave the respiratory system.
It is the preferred route for normal breathing because it helps condition the incoming air.
B. Filtration of Air
Nasal hairs, mucus and cilia help trap dust, pollen and other airborne particles.
This reduces the entry of unwanted particles into the lower respiratory tract.
C. Warming of Air
The rich blood supply of the nasal mucous membrane helps warm the incoming air before it reaches the lungs.
D. Humidification of Air
The mucus and moist surface of the nasal cavity add moisture to inhaled air, preventing excessive drying of the respiratory passages.
E. Sense of Smell
Olfactory receptors detect different odour molecules and transmit sensory information to the brain.
This helps us recognize food, fragrances, smoke and potentially harmful substances.
F. Contribution to Taste and Flavour
The sense of smell contributes significantly to the perception of food flavour.
During eating, odour molecules can travel from the back of the mouth towards the nasal cavity. This is called retronasal olfaction.
This explains why food may taste less flavourful when the nose is blocked due to a cold.
G. Voice Resonance
The nasal cavity and paranasal sinuses contribute to the resonance and quality of the voice.
Changes in nasal airflow, such as during nasal congestion, can alter the sound of speech.
7. Common Disorders of the Nose
The nose can be affected by infections, allergies, inflammation and structural abnormalities.
Some common nasal disorders are described below.
A. Rhinitis
Rhinitis is inflammation of the nasal mucous membrane.
It may occur due to allergies, infections or exposure to irritants.
Common symptoms:
Sneezing
Runny nose
Nasal itching
Nasal congestion
Allergic rhinitis commonly occurs when the immune system reacts to allergens such as pollen, dust mites or animal dander.
B. Sinusitis
Sinusitis is inflammation of the mucous membrane lining the paranasal sinuses.
It may develop due to viral infections, bacterial infections or allergies.
Common symptoms:
Facial pain or pressure
Nasal blockage
Thick nasal discharge
Reduced sense of smell
Headache in some cases
C. Epistaxis (Nosebleed)
Epistaxis refers to bleeding from the nose.
It commonly occurs due to damage to small blood vessels in the nasal mucous membrane.
Possible causes include:
Dry nasal mucosa
Nose picking or injury
Nasal infections
Allergies
Certain medicines that affect blood clotting
D. Deviated Nasal Septum
A deviated nasal septum occurs when the nasal septum is displaced towards one side rather than remaining near the midline.
It may be present from birth or develop following an injury.
Common symptoms:
Difficulty breathing through one or both nostrils
Nasal blockage
Recurrent nasal congestion
Nosebleeds in some individuals
E. Anosmia and Hyposmia
These conditions affect the sense of smell.
Anosmia: Complete loss of the sense of smell.
Hyposmia: Reduced ability to detect smells.
They may occur due to respiratory infections, nasal inflammation, head injuries or damage to olfactory pathways.
Clinical importance: A reduced sense of smell can affect flavour perception and may also make it difficult to detect warning odours such as smoke or spoiled food.
8. Clinical and Pharmaceutical Importance
Understanding the anatomy and physiology of the nose is important in pharmacy because several medicines are administered through the nasal route or used to manage nasal disorders.
A. Nasal Drug Delivery
The nasal route is used to administer certain medicines through sprays, drops and other formulations.
The nasal mucosa has a rich blood supply, which allows some drugs to be absorbed into the circulation.
Examples include:
Certain nasal corticosteroid sprays used for allergic rhinitis.
Nasal decongestants used for temporary relief of nasal blockage.
Certain medicines administered through the nasal route for specific therapeutic purposes.
B. Antihistamines
Antihistamines such as cetirizine and loratadine are used to relieve allergic symptoms, including sneezing, itching and a runny nose.
C. Nasal Corticosteroids
Medicines such as fluticasone and mometasone reduce inflammation in the nasal mucosa and are commonly used in allergic rhinitis.
D. Nasal Decongestants
Medicines such as oxymetazoline reduce nasal congestion by constricting blood vessels in the nasal mucosa.
Important: Topical nasal decongestants should generally be used only for a short period as directed. Prolonged use may cause rebound nasal congestion, also known as rhinitis medicamentosa.
Note: Nasal medicines should be used according to their instructions. Incorrect or excessive use may cause irritation or other unwanted effects.
Key Takeaways
The human nose is responsible for respiration and the sense of smell.
The external nose includes the root, bridge, apex, alae and nostrils.
The nasal septum divides the nasal cavity into right and left portions.
Nasal conchae increase the surface area and help condition incoming air.
Paranasal sinuses include frontal, maxillary, ethmoidal and sphenoidal sinuses.
The olfactory epithelium contains specialized sensory cells responsible for detecting odours.
The olfactory nerve carries smell-related information towards the olfactory bulb and brain.
The nose filters, warms and humidifies incoming air.
Common nasal disorders include rhinitis, sinusitis, epistaxis, deviated nasal septum and loss of smell.
Remember: The respiratory region of the nose helps prepare inhaled air for the lungs, while the olfactory region detects smells. Both functions make the nose an important respiratory and sensory organ.