PHYSIOLOGY OF NOSE

AND PNS

DR. KRISHNA CHANDRA RIJAL

Functions of Nose

1. Respiration

2. Air conditioning of inspired air

3. Protection of lower airway

4. Ventilation & drainage of P.N.S.

5. Olfaction

6. Nasal resistance

7. Vocal resonance

8. Nasal reflexes

1. Respiration

Inspiration: air current passes along mid-

portion of nasal cavity in lamellar flow.

Expiration: resistance of nasal valve &

turbinates leads to formation of eddy current

in expired air.

This results in awareness of breathing &

ventilation of paranasal sinus.

2. Air conditioning of inspired air a) Filtration and purification: The front of the nose can filter particles up to

3 μm Nasal mucus traps particles as fine as 0.5-3.0

μm. Particles smaller than 0.5 μm seem to pass

through the nose into lower airways without difficulty.

(b) Temperature : Inspired air which may be at 20°C or 0°C or

even at subzero temperature is heated to near body temperature (37°C) in one-fourth of second that the air takes to pass from the nostril to the nasopharynx. Similarly, hot air is cooled to the level of body temperature.

(c) Humidification: Relative humidity of atmospheric air varies depending on

climatic conditions. Air is dry in winter and saturated with moisture in summer

months. Nasal mucous membrane adjusts the relative humidity of the

inspired air to 75% or more. Water, to saturate the inspired air, is provided by the nasal

mucous membrane which is rich in mucous and serous secreting glands.

About 1000 ml of water is evaporated from the surface of nasal mucosa in 24 hours.

Moisture is essential for integrity and function of the ciliary epithelium.

At 50% relative humidity, ciliary function stops in 8-10 minutes.

Thus, dry air predisposes to infections of the respiratory tract. Humidification also has a significant effect on gas exchange in

the lower airways. In nasal obstruction, gaseous exchange is affected in the

lungs, leading to rise in pCO2, causing apnoeic spells during sleep; it also decreases pO2.

3. Protection of lower airway

Muco-ciliary blanket: traps pathogens in

inspired air > 0.5 μm & transports them to

nasopharynx for swallowing

Sneezing: protects against irritants

Lysozyme: kills bacteria & viruses

Immunoglobulins A & E: for bacteria

Interferon: for protection against virus

Factors decreasing mucociliary function

Dry atmosphere (absence of humidity)

Smoking, air pollutants & nasal irritants

Infection

Extremes of temperature

Hypoxia

Drugs: anaesthetics, sedatives, topical

nasal decongestants, beta blockers

4. Ventilation of P.N.S.

Inspiration: negative pressure created in

nasal cavity sucks out air from paranasal

sinuses via their ostium.

Expiration: eddies within nasal cavity

create positive pressure ventilates

paranasal sinuses via their ostium.

5. Nasal resistance Nasal resistance to

expired air keeps positive pressure in respiratory tract & prevents alveolar collapse

Nose provides 50-60 % of total airway

resistance

Nasal resistance is provided by 3 areas:

Nasal vestibule

Nasal valve (maximum contribution)

Nasal turbinates

Nasal valve Anterior nasal valve (ostium internum) is located at limen nasi, 1.5 cm posterior to anterior nares.

Consists of inferior margin of upper lateral nasal cartilage + anterior end of inferior turbinate + adjacent nasal septum.

Cross-section of airway here is 20-40 mm2 on each side.

It is narrowest part of upper respiratory tract.

Cottle’s test checks valve patency.

6. Vocal Resonance Nose forms a resonating chamber for

certain consonants in speech. In phonating nasal consonants (M/N/NG),

sound passes through the nasopharyngeal isthmus and is emitted through the nose.

When nose (or nasopharynx) is blocked, speech becomes denasal, i.e. M/N/NG are uttered as B/D/G respectively.

7. Nasal reflexes

Smell reflex: increases secretions of saliva &

gastric juice

Naso-pulmonary reflex: Chronic, severe nasal

obstruction increased pulmonary resistance

pulmonary hypertension

Sneeze reflex: protection against F.B.

8. Olfaction Important for pleasure and for enjoying

the taste of food. When nose is blocked, food tastes bland

and unpalatable. Vapours of ammonia are never used to

test the sense of smell as they stimulate fibres of the trigeminal nerve and cause irritation in the nose rather than stimulate the olfactory receptors.

Disorder of sense of smell:

1. Anosmia: absence of olfactory sensation

2. Hyposmia: decreased olfactory sensation

3. Parosmia / cachosmia: perception of a

pleasant odour as unpleasant odour

4. Phantosmia: perception of odour in absence

of olfactory stimulus

5. Hyperosmia: increased olfactory sensation

6. Olfactory agnosia: unable to identify odour

7. Presbyosmia: decreased olfactory sensation

due to ageing process

PHYSIOLOGY OF PARANASAL SINUSES Ventilation of Sinuses: Ventilation of paranasal sinuses takes place

through their ostia. During inspiration, air current causes negative

pressure in the nose. This varies from -6 mm to -200 mm of H2O,

depending on the force of inspiration. During expiration, positive pressure is created

in the nose and this sets up eddies which ventilate the sinuses.

Thus, ventilation of sinuses is paradoxical; they are emptied of air during inspiration and filled with air during expiration.

Mucus Drainage of Sinuses:Mucus secreted in the paranasal sinuses

travels to the ostium in a spiral manner. Here the cilia are very active and propel mucus

into the meatuses from where it is carried to the pharynx.

Functions of Paranasal Sinuses: 1. Air-conditioning of the inspired air by

providing large surface area over which the air is humidified and warmed.

2. To provide resonance to voice. 3. To act as thermal insulators to protect

the delicate structures in the orbit and the cranium from variations of intranasal temperature.

4. To lighten the skull bones.