Physiology of the Nose and Paranasal Sinuses
3
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Physiology of the Nose and Paranasal Sinuses
Davide Tomenzoli
CONTENTS
3.1
3.2
3.3
3.4
3.5
3.6
3.7
3.8
3.9
3.10
3.11
3.11.1
3.11.2
3.11.3
3.11.4
3.11.5
3.11.6
3.11.7
Introduction 29
Breathing 29
Mucociliary System 30
Filtration 30
Heating and Humidification 31
Antimicrobial Defense 31
Reflex Action 31
Recovery of Water 31
Resonance 32
Olfactory Function 32
The Role of Paranasal Sinuses 32
Lighten the Skull for Equipoise of the Head 32
Impart Resonance to the Voice 32
Increase the Olfactory Area 33
Thermal Insulation of Vital Parts 33
Secretion of Mucus to Moisten the Nasal Cavity 33
Humidify and Warm the Inspired Air 33
Absorption of Stress with Possible Avoidance
of Concussion 33
3.11.8 Influence on Facial Growth and Architecture 33
References 34
3.1
Introduction
Many papers and investigations on nasal physiology have been published in the last 40 years; as a
consequence, knowledge of nasal functions has now
been well established. In contrast, however, the role
of the human paranasal sinuses remains as much
an enigma today as it was nearly two millennia ago
(Blaney 1990). According to Cole (1998), the conclusive evidence of a functional relevance of the
paranasal sinuses has yet to be found. Even though
the existence of the paranasal sinuses may be unexplained, their susceptibility to disease is a common
D. Tomenzoli, MD
Department of Otorhinolaryngology, University of Brescia,
Piazzale Spedali Civili 1, Brescia, BS, 25123, Italy
source of suffering for patients and a focus of attention for clinicians.
“Physiologic” breathing occurs through the nose;
it may be supplemented by oral respiration under
demanding conditions of exercise or of severe nasal
obstruction. Nasal fossae may not only be considered
the front door of the respiratory system, but are also
characterized by peculiar and significant functions
other than breathing: conditioning and moistening
of the nasal air-flow, filtration of inspired noxious
materials, specific and non-specific antibacterial and
antiviral activities, reflex action, collection of water
from expired airflow, olfactory function.
3.2
Breathing
Every day 10,000 l of ambient air reach lower respiratory airways for pulmonary ventilation. Air enters
the nose through the nostrils, as a consequence of a
pressure gradient existing between external ambient and pulmonary alveoli, and converges through
the so-called nasal valve, positioned in the anterior
part of the nasal fossa just behind the nasal vestibulum. The term “nasal valve” refers to an area lying
on a perpendicular plane to the anteroposterior axis
of the nasal fossa, which is bordered medially by the
nasal septum, laterally by the head of the inferior
turbinate and superiorly by the posterior margin of
the lateral crus of the alar cartilage. This restricted
area accounts for about 50% of the total resistance
of the respiratory system and gives rise to a laminar
airflow. As inspiratory air leaves the narrow valvular
area and enters the much larger cross-section of
the nasal fossa, its velocity decelerates from 18 m/
s to 4 m/s and the laminar airflow becomes turbulent. When airflow reaches nasal fossa it splits
into three air streams, the largest of which flows
over the superior edge of the inferior turbinate. A
second smaller airflow (about 5%–10%) runs along
the olfactory mucosa localized on the roof of the
D. Tomenzoli
30
nasal fossa, the medial surface of the upper and
middle turbinates, and the opposed part of the septum. Finally, a minimal flow runs on the floor of the
nasal fossa (Fig. 3.1). The subdivision of the nasal
airflow and the presence of a turbulent flow allows
the maximal distribution of inspired air throughout
the nasal cavity, enabling exchanges of heat, water
and contaminants between the inspired air and the
respiratory mucosa.
Fig. 3.1. Breathing at rest. Inspired air once it has passed
through the nasal valve (red area, 1) divides into three air
streams. The main one flows along the middle turbinate (2);
the second and third flow along the ethmoid roof (3) and nasal
fossa floor (4)
3.3
Mucociliary System
Nasal mucosa presents a ciliated columnar pseudostratified epithelium that lines the nose and the
paranasal sinuses and is bounded by squamous
epithelium at the level of the nasal vestibulum. The
area of the luminal surface of the sinonasal epithelium is greatly expanded by 300–400 microvilli
x cell. Also, columnar cells bear about a hundred
cilia x cell beating 1000 x/min in sequence with
those of neighboring ciliated cells (Mygind 1978).
The cilia beat in a serous periciliary fluid of low
viscosity. The beat of a single cilium consists of
a rapid forward beat and a slow return beat with
a time ratio of 1:3. Within a limited mucosal area
all cilia beat in the same direction; the cilia beat
synchronously in parallel ranks one after another
forming metachronous waves that transport the exogenous particles toward rhinopharynx. Cilia are
plunged in a mucus blanket that is made up of
a double liquid layer: a superficial viscous sheet
of mucus and an underlying layer of serous fluid.
This fluid is deep enough to avoid entanglement
of the cilia with the viscoelastic mucus that floats
on its surface enabling the mucus (which contains
entrapped contaminants, microorganisms and debris) to be propelled along well-established routes
to the pharynx, where it is swallowed (Fig. 3.2).
Serous and seromucinous glands localized in the
intermediate layer of the lamina propria, and the
intraepithelial goblet cells are the producers of the
periciliary fluid and the thick viscoelastic mucus
(Cole 1998; Nishihira and McCaffrey 1987).
Fig. 3.2. Prechambers and paths of normal mucous drainage.
Structures are demonstrated after subtotal removal of middle
turbinate. Frontal sinus, anterior ethmoid cells, and maxillary
sinus drain into the middle meatus (red arrows). The sphenoid
sinus and posterior ethmoid cells drain into the superior meatus (blue arrows). Arrowheads indicate the insertion of the
middle turbinate’s ground lamella on the lateral nasal wall. FS,
frontal sinus; B, bulla ethmoidalis; PEC, posterior ethmoid cells;
SS, sphenoid sinus; UP, uncinate process; IT, inferior turbinate
3.4
Filtration
The inspired air contains a great amount of suspended exogenous particulate material. The upper
respiratory tract, especially the nose, must act as the
first line of defense and plays a significant role as a
protective filter for particles as well as for irritant
gases. Turbulence and impingement cause deposition of particles just behind the constricted area
of the nasal valve. Thus, the nose is normally the
principal site of particle deposition, but the efficacy
of this nasal filter depends on the diameter of the
particles inhaled (Muir 1972). Few particles greater
than 10 µm are able to penetrate the nose during
breathing at rest, while particles smaller than 1 µm
Physiology of the Nose and Paranasal Sinuses
are not filtered out, reaching the delicate structures
of the alveoli. Deposited particles, between 10 and
1 µm in diameter, are removed from the nasal mucosa within 6–15 min depending on the efficacy of
the mucociliary system.
3.5
Heating and Humidification
The blood vessels of the nasal mucosa are of paramount
importance for the functions of heating and humidification. As reported by detailed studies (Cauna 1970)
the arterioles of the nasal mucosa are characterized
by the total absence of the internal elastic membrane
so that the endothelial basement membrane is continuous with the basement membrane of the smooth
muscle cells. In addition, nasal blood vessels are also
characterized by porosity of endothelial basement
membrane so that the subendothelial musculature
of these vessels may be rapidly influenced by agents
and drugs carried in the blood. Between the capillaries and the venules are interposed the cavernous
sinusoids; these are localized in the lower layer of the
lamina propria especially on the inferior turbinates.
Cavernous sinusoids are regarded as specialized capillaries adapted to some of the functional demands of
the airway, i.e. moistening and heating of the inspired
air. Nasal blood vessels can be classified according to
their principal function into capacitance, resistance
and exchange vessels. The amount of sinonasal blood
volume depends on the tone of the capacitance vessels (mainly venous vessels and cavernous sinusoids),
while the blood flow on the tone of resistance vessels
(mainly small arteries, arterioles and arteriovenous
anastomoses). Finally, transport through the walls of
vessels takes place in the exchange vessels (mainly
capillaries).
Nasal air condition also depends on a number of factors other than nasal blood vessels such as seromucous
glands, goblet cells, plasmatic transudate and lacrimal secretion. Furthermore, the nose has additional
properties that contribute to heating and humidifying
inspired air such as: maximum wall contact for the
mixed flow of air (laminar and turbulent, according to
the different areas of the nasal cavities); the ability to
change the turbinates cross-section depending on the
variation in temperature and humidity of the ambient
air; the large amount of blood flowing rapidly through
the arteriovenous anastomoses of the turbinates; the
contribution to the inhaled air of atomized watery secretion from serous glands.
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3.6
Antimicrobial Defense
In addition to physical removal of microorganisms
and other noxious materials by mucociliary transport, an important line of defense is provided by the
surface fluids that contain macrophages, basophils
and mast cells, leucocytes, eosinophils, and antibacterial/antiviral substances that include immunoglobulins, lactoferrin, lysozymes and interferon. These cells
and substances discourage microbial colonization
and enhance the protective properties of the sinonasal mucosa against infections.
3.7
Reflex Action
Nasal mucosa is supplied by nerves from the somatic and autonomic systems. The sensory fibers
travel with the trigeminal nerve, while the parasympathetic fibers are derived from the facial nerve and
the sympathetic fibers from the superior cervical
ganglion.
Afferent impulses are transported via the sensory
fibers to the central nervous system giving rise to
tickling or pain. Efferent impulses are propagated
through autonomic, vasomotor and secretory-motor nerve fibers. The stimulation of nasal mucosa
results in sneezing, watery rhinorrhea and changes
in blood flow (Allison and Powis 1971).
Other than nasal effects, the stimulation of the
nasal mucosa can produce systemic reflexes as the
inhibition of respiration due to an increase in airway resistance or laryngospasm. Furthermore, an
increase in resistance in vessels of the skin, muscle,
splanchnic and kidney circulation can be observed.
Finally, cardiac output is reduced during nasal
stimulation as a result of bradycardia (Angell and
Daly 1972).
3.8
Recovery of Water
During expiration warm air coming from the lower
airway condenses in the anterior part of the nose,
which has a temperature 4°C lower than that of the
lung. With this mechanism, called the “piggy bank”
function, the nose is able to recover about 100 ml of
water everyday. Nevertheless, during nasal breath-
D. Tomenzoli
32
ing at room temperature the daily total loss is about
500 ml of water and 300 kcal (Ingelstedt and
Toremalm 1961).
3.9
Resonance
Even though it can not be considered a vital function,
the nose acts as a resonance box which gives its contribution, together with paranasal sinuses and pharynx,
to the characterization of the tone of the voice.
3.10
Olfactory Function
The superior turbinate, the cribriform plate, the
upper surface of the middle turbinate and the opposed part of the nasal septum are covered by a
specialized epithelium containing receptors cells.
The sense of smell is mediated via stimulation
of these olfactory receptors by volatile chemicals.
Five different types of cells form the olfactory epithelium: the bipolar olfactory neuron, which is a
primary sensory neuron with an olfactory knob
from which several olfactory cilia extend; the basal
cell, which replaces the bipolar neuron cells every
7 weeks; sustentacular cell, which acts as a support
cell supplying nutrients for bipolar neuron cells;
microvillar cell, which have no clearly defined role
except to perhaps assist olfaction; Bowman’s glands,
which provide a serous component to the mucous
layer covering the olfactory epithelium (Rice and
Gluckman 1995).
The exact mechanism of olfaction is somewhat
vague. Multiple theories have been proposed but
none have really been supported scientifically. There
is some suggestion that different odors produce different patterns of activity across the olfactory mucosa. Whatever the explanation at the molecular
level, depolarization of the bipolar neurons occurs,
resulting in an action potential that is transmitted
along the olfactory nerve, and the information is
processed centrally in the olfactory tubercle, pyriform cortex, amygdaloid nucleus, and hypothalamus. Interestingly enough, olfactory receptor cells
are the only nerve cells capable of regeneration,
allowing for (at least theoretically) the possibility
of regeneration after severe injury (Laffort et al.
1974).
3.11
The Role of Paranasal Sinuses
No conclusive theory on the role of paranasal sinuses
has been accepted yet. Some authors have suggested
a functional role, while others have argued that the
paranasal sinuses in higher primates are merely nonfunctional remnants of a common mammalian ancestor. The following sections review the different
theories.
3.11.1
Lighten the Skull for Equipoise of the Head
This is the oldest of all theories. The first objection
came from Braune and Clasen (1877), who claimed
that if the sinuses were filled with spongy bone the
total weight of the head would be increased by only
1%. Despite statements that man’s musculature is adequate to maintain head poise regardless of the state
of paranasal sinuses (Flottes et al. 1960), it was not
until 1969 that an electromyographic investigation
was made of the activity of human neck muscles in
response to loading the anterior aspect of the head.
It was concluded that the human paranasal sinuses
are not significant as weight reducers of the skull for
maintenance of equipoise of the head (Biggs and
Blanton 1970).
3.11.2
Impart Resonance to the Voice
In the seventeenth century, Bartholinus asserted that
paranasal sinuses are important phonatory adjuncts
in that they aid resonance. This theory received support from Howell (1917), when he stated that the
peculiar quality or timbre of the individual voice
arises from the accessory sinuses and the bony framework of the face. This conclusion was related to the
observation that Maori – who have a small frontal
sinus – possess a peculiarly dead voice. Blanton
and Biggs (1969) also supported this theory on the
basis that the howling monkeys possess particularly
large paranasal sinuses. Nevertheless, a few authors
discounted the resonance theory by observing that
animals with loud voices such as the lion can have
small sinuses (Proetz 1953), or that other animals,
such as the giraffe and rabbit, have small or shrill,
non-resonant voices despite having large sinus cavities (Negus 1958). Finally, Flottes et al. (1960)
reported that the physical properties of paranasal
Physiology of the Nose and Paranasal Sinuses
sinuses make them poor resonators and added that
sinus surgery does not modify the voice.
3.11.3
Increase the Olfactory Area
This theory arose when Cloquet (1830) incorrectly
stated that the human maxillary sinus was lined with
olfactory epithelium such as in some mammals. On
the contrary, the mucous membrane of the human
paranasal sinuses is made up of non-olfactory epithelium, but is lined by a thinner, less vascular mucosa
which is more loosely fixed to the bony wall than that
of the respiratory region of the nasal cavity.
3.11.4
Thermal Insulation of Vital Parts
This theory was originally proposed by Proetz (1953)
who compared the paranasal sinuses to an air-jacket
enveloping the nasal fossae. Nevertheless, Eskimos
often possess no frontal sinus, while African Negroes
possess large frontal sinuses (Blaney 1990).
33
al. 1960). However, some authors demonstrated that
exchange of gases between the nose and paranasal
sinuses is negligible and thus also the contribution
of the sinuses to the conditioning of the inspired air
proves to be insignificant (Paulsson et al. 2001).
3.11.7
Absorption of Stress with Possible Avoidance of
Concussion
This theory originated from Negus’ work on horned
ungulates (Negus 1958). He noted that the air
spaces which extend over the cranial vault and into
the horns, such as the ox and goat, are sometimes
explained as stress distributors. However, in other
horned ungulates such as the moose, the horns are
attached directly to the cranium without air spaces.
Rui (1960) observed that the sinus complex could
be considered as a pyramidal buffer with the base
situated anteriorly and the apex at the sphenoid thus
forming an architectural structure suited to a protective function of endocranial structures.
3.11.8
Influence on Facial Growth and Architecture
3.11.5
Secretion of Mucus to Moisten the Nasal Cavity
This theory is also discounted on the basis of histology. First advocated by Haller (1763, reported
by Wright 1914) it proposes that the sinuses are
important for moistening the nasal olfactory mucosa. However, Skillen (1920) and Negus (1958)
observed that an adequate amount of mucus for this
purpose cannot be secreted by the human paranasal
sinuses lining. In contrast to the nose with its 100,000
submucosal glands, the sinuses have only 50–100
glands (Dahl and Mygind 1998).
3.11.6
Humidify and Warm the Inspired Air
It has long been known that air exchange takes place
in the sinuses during respiration. However, a debate
existed as to whether this exchange occurs to enable
humidification and warming of inspired air. Aerated
sinuses develop in large swiftly moving mammals
with an active respiration, while slow moving mammals, especially those living in a humid medium like
the hippopotamus, have small sinuses (Flottes et
According to Proetz (1953) the paranasal sinuses
are the result of a plastic rearrangement of the skull
as a consequence of a disproportionate growth of the
face and cranium and associated structures after they
are fully or partly ossified. However, Negus (1958)
documented that individuals with a single frontal
sinus do not show a defective facial growth. Eckel
(1963) attributed the presence of sinus cavities to
strains and stress of the skull created solely by the
pressure exerted by the chewing apparatus. However,
Takahashi (1984) emphasized that the shape of the
neurocranium and cranial base must also be considered important elements. He stated that in the evolution of mammals from primates to humans, sinuses
originally acted as an aid to olfaction, but were influenced by the retraction of the maxillofacial box and
by the process of cerebral enlargement. The development of human paranasal sinuses is thus the result
of an increase in the angle between the forehead and
frontal cranial base, and decrease in the angle of the
cranial base at the sella turcica.
In conclusion, according to Blaney (1990), it is
becoming apparent that an architectural theory is
far more likely in that it is known that craniofacial
form has an important bearing on paranasal sinus
34
morphology. Further research into craniofacial form
and development needs to be done before the exact
role of the paranasal sinuses in humans can be definitively clarified or established. It is encouraging
that the more recent studies have emphasized the
importance of differential sinuses (Takahashi 1984;
Blaney 1986). With the advent of new imaging techniques much accurate data about paranasal sinus size
and morphology can be collected and further differential growth studies performed.
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