Physiology of the Nose

Physiology of the Nose

The nose is an essential organ that performs a variety of physiological functions crucial for respiration, protection, and sensory perception. Its role extends beyond a mere passage for air to the lungs, encompassing air conditioning, protection of lower airways, vocal resonance, reflex actions, and olfaction. These functions work harmoniously to ensure optimal respiratory and sensory performance.

Functions of the Nose

The primary functions of the nose are classified as follows:

1. Respiration
2. Air-conditioning of Inspired Air
3. Protection of the Lower Airway
4. Vocal Resonance
5. Nasal Reflex Functions
6. Olfaction

Respiration. The nose serves as the natural pathway for breathing, an instinctive function critical for life. Mouth breathing is an acquired act that can compromise respiratory efficiency. For instance, a newborn with choanal atresia – a blockage of the nasal airway – can experience life-threatening asphyxiation if not treated promptly. The nose also facilitates simultaneous breathing and eating.

1. Mechanism of Respiration

• Inspiration: During quiet breathing, air flows through the middle part of the nose, passing between the turbinates and the nasal septum. Minimal air passes through the inferior meatus or olfactory region, requiring substances with weak odours to be actively sniffed to reach the olfactory area.
• Expiration: Air exits through the same pathway but forms eddies due to resistance at the limen nasi. This eddy formation ventilates the paranasal sinuses through their openings, ensuring proper aeration. The anterior end of the inferior turbinate undergoes swelling and shrinkage thus regulating the inflow of air.

2. Nasal Cycle: The nasal mucosa undergoes rhythmic congestion and decongestion, known as the nasal cycle, which alternates airflow between the two nasal chambers. This cycle typically spans 2.5–4 hours and ensures efficient respiratory function even if one nasal passage is partially blocked.

Air-Conditioning of Inspired Air. The nose is aptly described as an “air-conditioner” for the lungs, regulating the air’s temperature, humidity, and cleanliness before it enters the lower airways.

1. Filtration and Purification

• Nasal Vibrissae: The small hairs at the entrance of the nose trap large particles up to 3 μm, such as cotton fibres.
• Mucus Layer: A mucus blanket on the nasal mucosa traps finer particles like dust, pollen, and bacteria. This filtering system is highly efficient, trapping particles as small as 0.5 microns in diameter. Particles smaller than 0.5 μm pass easily through the nose into lower airways.

2. Temperature Regulation: The nasal mucosa, especially near the middle and inferior turbinates and adjacent parts of the septum is highly vascular with cavernous venous spaces or sinusoids. It controls the blood flow, and this increases or decreases the size of turbinates. Blood flow in the venous sinusoids of the mucosa acts as a radiator system, warming cold air to near body temperature within milliseconds. This makes an efficient radiator system, warming cold air to near body temperature within milliseconds, from 20°C or 0°C or even at subzero temperature to near body temperature (37°C). Similarly, hot air is cooled before reaching the lungs.

3. Humidification: Inspired air is humidified by water that evaporates from the nasal mucosa and raises the relative humidity of the inspired air to 75% or more. This ensures the air reaches the lungs at optimal moisture levels, preserving ciliary function and preventing respiratory infections. Moisture is crucial for keeping the ciliary epithelium healthy and functional. If the relative humidity drops to 50%, the ciliary function can stop within 8 to 10 minutes. This means that breathing dry air can increase the likelihood of respiratory tract infections. Each day, about 1 litre of water evaporates from the nasal mucosa to maintain these humidity levels.

Protection of the Lower Airway. The nasal cavity employs several mechanisms to safeguard the lower respiratory tract from harmful dust particles, bacteria and viruses.

1. Mucociliary Mechanism: The mucociliary mechanism in the nasal mucosa plays a vital role in trapping and clearing pathogens and particles from the air we breathe. The mucosa contains goblet cells and secretory glands that produce a mucous blanket, consisting of a superficial mucus layer and a deeper serous layers. Cilia beat continuously, moving this blanket toward the nasopharynx at a speed of 5–10 mm per minute, clearing it every 10–20 minutes. This process traps bacteria, viruses, and dust particles, preventing respiratory infections. The presence of turbinates increases the surface area for trapping particles. In mammals, cilia beat 10–20 times per second, with an effective stroke that pushes mucus forward and a recovery stroke that returns them to their original position. Conditions like immotile cilia syndrome (Kartagener Syndrome) can impair this function, leading to mucus buildup and chronic issues such as rhinosinusitis and bronchiectasis. Factors like drying, drugs, and pollutants can also affect ciliary movement.

2. Enzymes and immunoglobulins: Nasal secretions contain lysozyme (muramidase), an enzyme that destroys bacteria and viruses, and immunoglobulins (IgA and IgE) that provide immunity against infections. Interferons also contribute to the nose’s protective functions.

3. Protective Reflexes: Sneezing expels foreign particles that irritate the nasal mucosa. This reflex, combined with a large flow of nasal secretions efficiently washes them out. The pH of these nasal secretions remains nearly constant at 7, which is optimal for the action of cilia and lysozyme. Changes in nasal pH, caused by infections or nasal drops, can significantly impair the functions of cilia and lysozyme. The nose performs its functions so efficiently that it filters, humidifies, and warms about 500 cubic feet of air every 24 hours. This process clears the air of dust, bacteria, and viruses before it reaches the lungs.

Vocal Resonance. The nasal cavity serves as a resonating chamber for certain consonants during speech, such as “M,” “N,” and “NG.” When the nasal passage is blocked, these sounds become denasalized, i.e. M/N/NG are uttered as B/D/G, respectively. Conversely, velopharyngeal insufficiency may lead to hypernasal speech.

Nasal Reflexes. The nasal mucosa initiates various reflex actions:

• Sneezing: Sneezing expels foreign particles that irritate the nasal mucosa.
• Salivation and Gastric Secretion: Smelling palatable food stimulates the secretion of saliva and gastric juices.
• Pulmonary Reflexes: Long-standing nasal obstruction (i.e., tonsil and adenoid hypertrophy) can increase pulmonary resistance leading to pulmonary hypertension or cor pulmonale. Surgical correction of nasal blockage often improves pulmonary function.

Olfaction. Smell, or olfaction, plays a vital role in sensory perception and quality of life. While its importance is reduced in humans compared to animals, it remains crucial for detecting environmental dangers and is essential for enjoying food. Ammonia vapours are not used to test smell because they stimulate trigeminal nerve fibres and cause irritation in the nose instead of stimulating the olfactory receptors.

1. Olfactory Pathway. We perceive smell in the olfactory region, which is located in the upper third of the nasal cavity. This area contains millions of olfactory receptor cells, which are special bipolar neurons responsible for our sense of smell. Each olfactory receptor cell has a peripheral process (also known as dendrites) and a central process (also known as axons). The dendrites extend into the olfactory mucous membrane and expand into a ventricle with cilia that detect odours. The axons form olfactory nerves that pass through the cribriform plate of the ethmoid bone and synapse with mitral and tuft cells (at the glomerulus) in the olfactory bulb. The axons then form the olfactory tract, transmitting signals to the prepyriform cortex and amygdaloid nucleus, where we consciously perceive smells.

2. Disorders of Smell. For proper smell perception, the odorous substances must be volatile and reach the olfactory area without obstruction. Healthy olfactory mucosa and intact neural pathways are also necessary. To test the sense of smell, a patient can sniff common odours like lemon or peppermint with their eyes closed, one nostril at a time. More precise measurements require special equipment. Olfactory disorders encompass a range of conditions that affect the sense of smell. The following terms are commonly used to describe various olfactory dysfunctions:

• Anosmia is a total absence of smell function and Hyposmia (or Microsmia) is a decreased sensitivity to odours, resulting in a diminished ability to perceive smell. These can occur due to nasal obstructions like polyps or swelling from conditions such as colds or allergies. Anosmia can also result from atrophic rhinitis, nerve injuries from head trauma, pressure from tumours or infections affecting the olfactory tract.
• Partial Anosmia: The patient can perceive some odours while being unable to detect others.
• Hyperosmia: The patient has an increased sensitivity to common odours, leading to an enhanced olfactory perception.
• Dysosmia (cacosmia or parosmia) is a distortion of smell where patients misinterpret odours, often finding them unpleasant. This can occur during recovery from anosmia and may indicate nerve fibre issues. Intracranial tumours should be ruled out in cases of parosmia.
• Phantosmia: This phenomenon involves the perception of a dysosmic sensation in the absence of an actual odour stimulus, commonly known as an olfactory hallucination.
• Olfactory Agnosia: Olfactory agnosia is characterized by an inability to recognize odour sensations despite intact olfactory processing, language capabilities, and general intellectual functions. This condition may be observed in certain stroke patients.

Additionally, several less commonly used terms describe specific olfactory phenomena:

• Heterosmia: A condition in which all odours are perceived as having the same scent.
• Presbyosmia: This term refers to a decline in the sense of smell associated with ageing. It is important to note that presbyosmia is less specific than other terms, as it does not differentiate between anosmia and hyposmia. Furthermore, it implies that age itself is the primary factor contributing to the age-related olfactory deficit.
• Osmophobia: Osmophobia denotes a dislike or fear of certain smells, which can significantly impact an individual’s quality of life.

——– End of the chapter ——–

Reference Textbooks.

• Scott-Brown, Textbook of Otorhinolaryngology-Head and Neck Surgery.
• Cummings, Otolaryngology-Head and Neck Surgery.
• Stell and Maran’s, Textbook of Head and Neck Surgery and Oncology.
• Ballenger’s, Otorhinolaryngology Head And Neck Surgery
• Susan Standring, Gray’s Anatomy.
• Frank H. Netter, Atlas of Human Anatomy.
• B.D. Chaurasiya, Human Anatomy.
• P L Dhingra, Textbook of Diseases of Ear, Nose and Throat.
• Hazarika P, Textbook of Ear Nose Throat And Head Neck Surgery Clinical Practical.
• Mohan Bansal, Textbook of Diseases of Ear, Nose and Throat Head and Neck Surgery.
• Hans Behrbohm, Textbook of Ear, Nose, and Throat Diseases With Head and Neck Surgery.
• Logan Turner, Textbook of Diseases of The Nose, Throat and Ear Head And Neck Surgery.
• Arnold, U. Ganzer, Textbook of  Otorhinolaryngology, Head and Neck Surgery.
• Ganong’s Review of Medical Physiology.
• Guyton & Hall Textbook of Medical Physiology.

Author: