physiology of nose & pns
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physiolofyTRANSCRIPT
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PHYSIOLOGY OF NOSE
AND PNS
DR. KRISHNA CHANDRA RIJAL
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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
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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.
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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.
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(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.
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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
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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
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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.
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5. Nasal resistance Nasal resistance to
expired air keeps positive pressure in respiratory tract & prevents alveolar collapse
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Nose provides 50-60 % of total airway
resistance
Nasal resistance is provided by 3 areas:
Nasal vestibule
Nasal valve (maximum contribution)
Nasal turbinates
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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.
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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.
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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.
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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.
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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
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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.
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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.
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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.