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2.2.2 Physiology of the Nose and Nasal Cavity
The nose and its internal nasal cavity provides a passageway for the air to pass
through to the lungs, warms and moistens (humidifies) the inhaled air, filters and
cleans the inhaled air from any foreign particles, resonates sounds for speech, and
houses the olfactory receptors for smell. At the entrance of the nasal cavity in the
vestibule region, the surface wall is made up of stratified squamous epithelium (same
as the external skin) which contains sebaceous glands, and nose hairs (vibrissae),
serving to filter out inhaled particulates. In the main nasal passage the walls are
lined with respiratory mucosa. This is made up of a pseudo stratified ciliated columnar epithelium surface containing interspersed goblet cells that sits atop a
lamina propia. The serous glands produce and deliver to the surface a watery fluid
containing anti-bacterial enzymes while the mucous glands and goblet cells secrete
a slimy, semi-sticky liquid called mucous. Approximately 125 mL of respiratory
mucous (sputum) is produced daily which forms a continuous sheet called a ‘mu-
cous blanket’. The mucous traps any inhaled particulates such as dust, and bacteria,
while the antibacterial enzymes destroy the particulates. The ciliated epithelium
cells have cilia on their surface which are fine microscopic hair-like projections.
These cilia move back and forth in a rhythmical movement (mucociliary action)
which transport the secreted mucous blanket from the nasal cavity to the throat
where it is swallowed into the digestive system. This movement occurs at a rate
of about 1–2 cm/h. The respiratory mucosa in the turbinates has a thick, vascular
and erectile glandular tissue layer which is subject to tremendous erectile capabil-
ities of nasal congestion and decongestion, in response to the climatic conditions
and changing needs of the body. This affects the flow resistance due to the airway
passages narrowing or expanding. Near the roof of the nasal cavity in the region
from the superior nasal concha and the opposed part of the septum at the olfac-
tory region, the mucosa changes, having a yellowish colour and the epithelial cells
are columnar and non-ciliated. This surface is referred to as the olfactory mucosa
and it contains the sensory receptor cells for smell detection. The cells form the
nerves that then pass through the cribiform plate to the olfactory centres within the
brain.
Heating and conditioning of the inspired air occurs through a network of
thin-walled veins that sits under the nasal epithelium. The superficial location and
abundance of the blood vessels causes a natural heat transfer process to the colder
inspired air. The turbinates that protrude into the main passage increases the mu-
cosal surface area to enhance the heating and conditioning of the inspired air. The
mucous walls are also supplied with sensory nerve endings, which trigger a sneeze
reflex when it comes into contact with inhaled particles. The nose is also supplied by
nerves capable of detecting pain, temperature and pressure. The surrounding sinuses
lighten the skull, and also act as resonating chambers for speech. Each paranasal
sinus is lined with the same respiratory mucosa found in the main nasal passage and
therefore has the same heating and air conditioning capabilities. Particles can also
be trapped by the mucous secretions produced in the sinuses which continually flow
into the nose by the ciliated surface. In addition blowing of the nose helps to drain
the sinuses. Smell is another function of the nose. Sensory activity is transmitted via branches
of the olfactory nerve, which cross the roof of the nasal cavity through the cribiform
plate of the ethmoid bone. During the course of breathing the nasal cavity geom-
etry can be affected by the nasal cycle. This physiologic phenomenon which has
been reported in more than 80 % of normal individuals (Keay et al. 1987), is an
important consideration when a patient undergoes a CT or MRI scan since the scan
is an instant snapshot in time of the nasal cavity’s physiological state. It also has a
significant effect on the airflow through the nasal passage. The nasal cycle is defined as a cyclic fluctuation in congestion and decongestion of the nasal venous sinusoids
ranging over a period of 30 min to 6 h. Airflow through the nasal cavity is normally
asymmetrical, where one nasal passage (left or right) is dominant. This asymmetry
is referred to as the nasal cycle which is a result of congestion (swelling) of the
erectile tissue (cavernous tissues of the mucosa) in one nasal cavity while at the
same time decongestion (shrinking) occurs to the erectile tissue in the other cav-
ity. The airflow through the each nasal cavity is then governed by the resistance
caused by the cross-sectional area of each airway. The changes in nasal resis-
tance associated with the nasal cycle are not always regular, and the term nasal
cycle may be a misnomer, as there is little evidence to indicate a regular peri-
odicity to the changes in nasal resistance (Eccles 1996). The functional role of
the nasal cycle is not exactly known but some hypothesis include: a contribution
towards respiratory defence during nasal infection (Eccles 1996); and increased
contact of inspired air with the mucosa since there is increased airflow through
a decongested airway which provides increased levels of turbulence.