Physiology Of Nose

Functions of the nose are classified as:

1. Respiration.
2. Air conditioning of inspired air.
3. Protection of lower airway.
4. Vocal resonance.
5. Nasal reflex functions.
6. Olfaction.
7. Respiration

Nse is the natural pathway for breathing. Mouth
breathing is acquired through learning. So natural is the
instinct to breath through the nose that a newborn
infant with choanal atresia may asphyxiate to death if
urgent measures are not taken to relieve it. The nose also
permits breathing and eating to go on simultaneously.
During quiet respiration, inspiratory air current passes
through middle part of nose between the turbinates and
nasal septum. Very little air passes through inferior meatus
or olfactory region of nose. Therefore, weak
odorous substances have to be sniffed before they can
reach olfactory area.
During expiration, air current follows the same course
as during inspiration, but the entire air current is not
expelled directly through the nares. Friction offered at
limen nasi converts it into eddies under cover of inferior
and middle turbinates and this ventilates the sinuses
through the ostia.
Anterior end of inferior turbinate undergoes swelling
and shrinkage thus regulating inflow of air.
Nasal cycle. Nasal mucosa undergoes rhythmic cyclical
congestion and decongestion, thus controlling the air
flow through nasal chambers. When one nasal chamber is
working, total nasal respiration, equal to that of both nasal
chambers, is carried out by it. Nasal cycle varies every
2.5-4 hours and may be characteristic of an individual.
Air-conditioning of Inspired Air
Nose is aptly called the "air-conditioner" for lungs. It filters
and purifies the inspired air and adjusts its temperature
and humidity before it passes it on to the lungs.
(a) Filtration and purification. Nasal vibrissae at the
entrance of nose act as filters to sift larger particles
like fluffs of cotton. Finer particles like dust, pollen
and bacteria adhere to the mucus which is spread
like a sheet all over the surface of the mucous membrane.
The front of the nose can filter particles up to
3 fLm, while nasal mucus traps particles 0.5-3.0 um.
Particles smaller than 0.5 um seem to pass through
the nose into lower airways without difficulty.
(b) Temperature control of the inspired air is regulated by
large surface of nasal mucosa which is structurally
adapted to pelform this function. This mucous
membrane, particularly in the region of middle and
inferior turbinates and adjacent parts of the septum
is highly vascular with cavernous venous spaces or
sinusoids which control the blood flow, and this
increases or decreases the size of turbinates. This
makes an efficient "radiator" mechanism to warm up
the cold air. Inspired air which may be at 20°C or
O°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 body
temperature.
(c) Humidification. This function goes on simultaneously
with the temperature comrol of inspired air. Relative
humidity of atmospheric a ir varies depending on climatic
conditions. Air is dry in winter and satura ted
with moisture in summer momhs. 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 functi on 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.
[n nasal obstruction, gaseous exchange is affected in the
lungs, leading to rise in pCO2, causing apnoeic spells during
sleep; it also decreases pO2.
Protection of Lower Airway
(a) Mucociliary mechanism. Nasal mucosa is rich in goblet
cells, secretory glands both mucous and serous.
Their secretion forms a continuo us sheet called
mucous blanket spread over the normal mucosa.
Muco us blanket consists of a superficial mucus layer
and a deeper serous layer, floating on the top of cilia
which are constantly beating to carry it like a "conveyer
belt" towards the nasopharynx. It
moves at a speed of 5-10' mm per minute and the
complete sh eet of mucus is cleared into the pharynx
every 10 to 20 minutes. The inspired bacteria ,
viruses and dust particles are entrapped on the viscous
mucous blanket and then carried to the
nasopharynx to be swallowed. Presence of turbina tes
almost doubles the surface area to perform this function.
A bout 600-700 ml of nasa l secre tions are produced
in 24 hours.
In mammals, cilia beat 10-20 times per second at room
temperature. They have a rapid "effective stroke" and a
slow "recovery stroke". In the former, the extended cilia
reach mucus layer while in the recovery stroke, they bend
and travel slowly in the reverse direction in the thin
serous layer, thus moving the mucous blanket in only one
direction. In immotile cilia syndrome, cilia are defective
and cannot beat effectively, leading to stagnation of
mucus in the nose and sinuses and bronchi causing
chronic rhinosinusitis and bronchiectasis. Movements
of cil ia a re affected by drying, drugs (adrenaline), excessive
heat or cold, smoking, infections and noxious fumes
like sulphur dioxide and carbon dioxide.
(b) Enzymes and immunoglobulins. Nasal secretions also
contain an enzyme called muramidase (lysozyme)
which kills bacteria and viruses. ImmunoglobulInS
IgA and IgE, and interferon are also present in nasal
sec retions and provide immunity against upper respiratory
tract infections.
(c) Sneezing. It is a protective reflex. Foreign particles
which irritate nasal mucosa a re expelled by sneezing.
Copious flow of nasal secretions that foltows initation
by noxious substance helps to wash them out.
The pH of nasal sec retion is nearly constant at 7. The
cilia and the lysozyme act best at this pH. Alteration in
nasal pH, due to infections or nasal drops, seriously
impair the functions of cilia and lysozyme.
So efficient are the functio ns of nose th~lt 500 cubic
feet of air, that we breathe every 24 hours, is filtered,
humidified, adjusted to proper temperature and cleared
of all the dust, bacteria and viruses befo re reaching the
lungs.
Vocal Resonance
Nose forms a resonating chamber for certain consonants
in speech. In phonating nasal consonants (MINING),
sound passes through the nasopharyngeal isthmus and
is emitted through the nose. When nose (or nasopharynx)
is blocked, speech becomes denasal, i.e. MINING
are uttered as B/D/G respectively.
Nasal Reflexes
Several reflexes are initiated in the nasa l mucosa. Smell
of a palatable food cause reflex secretion of sa liva and
gastric juice. Irritation of nasal mucosa causes sneezing .
Nasal function is closely related to pulmonary functions
th ro ugh nasobronchial and nasopulmonary reflexes. It
has been observed tha t nasal obstruction leads to
increased pulmona ry resistance and is re versed when
nasa l obstruction is surgically treated. Nasal packing in
cases of epistaxis or after nasal surgery leads to lowering
of p02 which returns to norma l after remova l of the
pack. Pulmonary hypertension or cor pulmonale can
develop in children with longstanding nasal obstruction
due to tonsil and adenoid hypertrophy and can be reversed
after removal of the tonsils and adenoids.
Olfaction
Sense of smell is well developed in lower animals to give
warning of the environmental dangers but it is comparatively
less important in man, still it is 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.
Olfactory pathways. Smell is perceived in the olfacrory
region of nose which is situated high up in the nasal
cavity. This area contains millions of olfactory, receptor
cells. Peripheral process of each olfactory cell reaches the
mucosal surface and is expanded into a ventricle with several
cilia on it. This acts as a sensory receptor to receive
odorous substances. Central processes of the olfactory cells
are grouped into olfactory nerves which pass through the
cribriform plate of ethmoid and end in the mitral cells of
the olfactory bulb. Axons of mitral cells form olfactory
tract and carry smell to the prepiriform cortex and the
amygdalOid nucleus where it reaches consciousness.
Olfactory system is also associated with autonomic system
at the hypothalamic level.
Disorders of smell: It is essential for the perception of
smell that the odorous substance be volatile and that it
should reach the olfactory area unimpeded. Also necessary
are the healthy state of olfactory mucosa and the
integrity of neural pathways, i.e . olfactory nerves, olfactory
bulb and tract and the cortical centre of olfaction.
Anosmia is total loss of sense of smell while hyposmia
is partial loss. They can result from nasal obstruction due
to nasal polyp i, enlarged turbinates or oedema of mucous
membrane as in common cold, allergic or vasomotor
rhinitis. Anosmia is also seen in atrophic rhinitis, a
degenerative disorder or nasa l mucosa; peripheral neuritis
(toxic or influenzal) ; injury to olfactory nerves or
olfactory bulb in fractu res of anterior cranial fossa; and
intracranial lesions lik e ahscess, tumour or meningitis
which cause pressure on olfactory tracts.
Parosmia is perversion of smell; the person interprets
the odours incorrectly. Often these persons complain of
disgusting odours. It is seen in the recovery phase of
postinfluenzal anosmi a and the probable explanation is
misdirected regeneration of nerve fibres. Intracranial
tumour should be excluded in all cases of parosmia.
Sense of smell can be tested by asking the patient to
smell common odours such as lemon, peppermint, rose,
garlic or cloves from each side of the nose separately,
with eyes closed. Quantitative estimation (quantitative
olfactometry) requires special equipment.