The Human Respiratory System - Computational Fluid and Particle Dynamics in the Human Respiratory System - page 25

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 (Lang et al.
2003).

2.2.3 Nasal Cavity Variations and Diseases

Variations and diseases in the human anatomy are vast and this section introduces the
reader to some of the possible variations to the nasal cavity that has an effect on the
air and particle respiration. This theme is carried throughout the subsequent sections
on variations and diseases of the other parts of the respiratory pathway (pharynx,
larynx, and tracheobronchial and lung airway).
There have been a large number of studies indicating that morphological variation
of the human nose is found among populations from different eco-geographical lo-
cations through adaptation to climate (Franciscus and Long 1991a). For example, in
cold or dry environments the nose has a large external protrusion, small constricted
nostrils, and is tall and narrow ( leptorrhine nose ). The nasal cross-sectional area is
smaller to facilitate heat and moisture exchange (Carey and Steegmann 1981). For
hot or moist environments, the nose has a small external protrusion, large flaring nos-
trils, and is short and broad ( platyrrhine nose ) in comparison to leptorrhine noses.
The cross-sectional area is greater which reduces the heat transfer during exhalation
(Seren and Seren 2009).The nasal index, which compares the width of the base of the
nose with the height of the nose, (e.g. Index = (width 100)/height) is used as a way

to determine the nose type. A low index (< 70) indicates a narrow nose and is consid-
ered as leptorrhine, and a high index > 85 is considered platyrrhine. In between 70
and 85 the nose is considered messorhine. Other morphological differences include
differences between males and females, and also one's age (child, adult, elderly)
(Fig. 2.6 ).
Nasal obstruction is the term used for any increased resistance to the airflow
that is experienced by a person. The sensation is subjective and can occur from the

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