3 Physiology of the Nose and Paranasal Sinuses
Davide Tomenzoli
D. Tomenzoli, MD
Department of Otorhinolaryngology, University of Brescia, Piazzale Spedali Civili 1, Brescia, BS, 25123, Italy
3.1
Introduction
Many papers and investigations on nasal physiol- ogy 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 con- clusive evidence of a functional relevance of the paranasal sinuses has yet to be found. Even though the existence of the paranasal sinuses may be unex- plained, their susceptibility to disease is a common
source of suffering for patients and a focus of atten- tion 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 signifi cant functions other than breathing: conditioning and moistening of the nasal air-fl ow, fi ltration of inspired noxious materials, specifi c and non-specifi c antibacterial and antiviral activities, refl ex action, collection of water from expired airfl ow, olfactory function.
3.2 Breathing
Every day 10,000 l of ambient air reach lower respi- ratory airways for pulmonary ventilation. Air enters the nose through the nostrils, as a consequence of a pressure gradient existing between external ambi- ent and pulmonary alveoli, and converges through the so-called nasal valve, positioned in the anterior part of the nasal fossa just behind the nasal vestibu- lum. 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 airfl ow. 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 airfl ow becomes tur- bulent. When airfl ow reaches nasal fossa it splits into three air streams, the largest of which fl ows over the superior edge of the inferior turbinate. A second smaller airfl ow (about 5%–10%) runs along the olfactory mucosa localized on the roof of the
CONTENTS
3.1 Introduction 29 3.2 Breathing 29
3.3 Mucociliary System 30 3.4 Filtration 30
3.5 Heating and Humidifi cation 31 3.6 Antimicrobial Defense 31 3.7 Refl ex Action 31 3.8 Recovery of Water 31 3.9 Resonance 32 3.10 Olfactory Function 32
3.11 The Role of Paranasal Sinuses 32
3.11.1 Lighten the Skull for Equipoise of the Head 32 3.11.2 Impart Resonance to the Voice 32
3.11.3 Increase the Olfactory Area 33 3.11.4 Thermal Insulation of Vital Parts 33
3.11.5 Secretion of Mucus to Moisten the Nasal Cavity 33 3.11.6 Humidify and Warm the Inspired Air 33
3.11.7 Absorption of Stress with Possible Avoidance of Concussion 33
3.11.8 Infl uence on Facial Growth and Architecture 33 References 34
nasal fossa, the medial surface of the upper and middle turbinates, and the opposed part of the sep- tum. Finally, a minimal fl ow runs on the fl oor of the nasal fossa (Fig. 3.1). The subdivision of the nasal airfl ow and the presence of a turbulent fl ow 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.
of mucus and an underlying layer of serous fl uid.
This fl uid is deep enough to avoid entanglement of the cilia with the viscoelastic mucus that fl oats on its surface enabling the mucus (which contains entrapped contaminants, microorganisms and de- bris) 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 fl uid and the thick viscoelastic mucus (Cole 1998; Nishihira and McCaffrey 1987).
3.3
Mucociliary System
Nasal mucosa presents a ciliated columnar pseu- dostratifi ed 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 epi- thelium 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 fl uid 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 ex- ogenous particles toward rhinopharynx. Cilia are plunged in a mucus blanket that is made up of a double liquid layer: a superfi cial viscous sheet
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 fl ows along the middle turbinate (2);
the second and third fl ow along the ethmoid roof (3) and nasal fossa fl oor (4)
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 me- atus (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 sus- pended exogenous particulate material. The upper respiratory tract, especially the nose, must act as the fi rst line of defense and plays a signifi cant role as a protective fi lter for particles as well as for irritant gases. Turbulence and impingement cause deposi- tion of particles just behind the constricted area of the nasal valve. Thus, the nose is normally the principal site of particle deposition, but the effi cacy of this nasal fi lter 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
are not fi ltered out, reaching the delicate structures of the alveoli. Deposited particles, between 10 and 1 µm in diameter, are removed from the nasal mu- cosa within 6–15 min depending on the effi cacy of the mucociliary system.
3.5
Heating and Humidifi cation
The blood vessels of the nasal mucosa are of paramount importance for the functions of heating and humidifi - cation. 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 con- tinuous 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 infl uenced by agents and drugs carried in the blood. Between the capil- laries 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 capil- laries adapted to some of the functional demands of the airway, i.e. moistening and heating of the inspired air. Nasal blood vessels can be classifi ed according to their principal function into capacitance, resistance and exchange vessels. The amount of sinonasal blood volume depends on the tone of the capacitance ves- sels (mainly venous vessels and cavernous sinusoids), while the blood fl ow 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 fac- tors other than nasal blood vessels such as seromucous glands, goblet cells, plasmatic transudate and lacri- mal secretion. Furthermore, the nose has additional properties that contribute to heating and humidifying inspired air such as: maximum wall contact for the mixed fl ow 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 fl owing rapidly through the arteriovenous anastomoses of the turbinates; the contribution to the inhaled air of atomized watery se- cretion from serous glands.
3.6
Antimicrobial Defense
In addition to physical removal of microorganisms and other noxious materials by mucociliary trans- port, an important line of defense is provided by the surface fl uids that contain macrophages, basophils and mast cells, leucocytes, eosinophils, and antibacte- rial/antiviral substances that include immunoglobu- lins, lactoferrin, lysozymes and interferon. These cells and substances discourage microbial colonization and enhance the protective properties of the sinona- sal mucosa against infections.
3.7
Refl ex Action
Nasal mucosa is supplied by nerves from the so- matic and autonomic systems. The sensory fi bers travel with the trigeminal nerve, while the parasym- pathetic fi bers are derived from the facial nerve and the sympathetic fi bers from the superior cervical ganglion.
Afferent impulses are transported via the sensory fi bers to the central nervous system giving rise to tickling or pain. Efferent impulses are propagated through autonomic, vasomotor and secretory-mo- tor nerve fi bers. The stimulation of nasal mucosa results in sneezing, watery rhinorrhea and changes in blood fl ow (Allison and Powis 1971).
Other than nasal effects, the stimulation of the nasal mucosa can produce systemic refl exes as the inhibition of respiration due to an increase in air- way 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-
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 contri- bution, 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 op- posed 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 epi- thelium: 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 defi ned 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 scientifi cally. There is some suggestion that different odors produce dif- ferent patterns of activity across the olfactory mu- cosa. 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, pyri- form cortex, amygdaloid nucleus, and hypothala- mus. 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 non- functional remnants of a common mammalian an- cestor. 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 fi rst objection came from Braune and Clasen (1877), who claimed that if the sinuses were fi lled with spongy bone the total weight of the head would be increased by only 1%. Despite statements that man’s musculature is ad- equate 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 signifi cant 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 sup- port from Howell (1917), when he stated that the peculiar quality or timbre of the individual voice arises from the accessory sinuses and the bony frame- work 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 cavi- ties (Negus 1958). Finally, Flottes et al. (1960) reported that the physical properties of paranasal
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 epithe- lium, but is lined by a thinner, less vascular mucosa which is more loosely fi xed 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).
3.11.5
Secretion of Mucus to Moisten the Nasal Cavity
This theory is also discounted on the basis of his- tology. First advocated by Haller (1763, reported by Wright 1914) it proposes that the sinuses are important for moistening the nasal olfactory mu- cosa. 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 humidifi cation and warming of inspired air. Aerated sinuses develop in large swiftly moving mammals with an active respiration, while slow moving mam- mals, especially those living in a humid medium like the hippopotamus, have small sinuses (Flottes et
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 insignifi cant (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 protec- tive function of endocranial structures.
3.11.8
Infl uence on Facial Growth and Architecture
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 ossifi ed. 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 consid- ered important elements. He stated that in the evolu- tion of mammals from primates to humans, sinuses originally acted as an aid to olfaction, but were infl u- enced by the retraction of the maxillofacial box and by the process of cerebral enlargement. The develop- ment 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
morphology. Further research into craniofacial form and development needs to be done before the exact role of the paranasal sinuses in humans can be de- fi nitively clarifi ed 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 tech- niques much accurate data about paranasal sinus size and morphology can be collected and further differ- ential growth studies performed.
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