Anatomical Details of the Rabbit Nasal Passages and Their Implications in Breathing, Air Conditioning, and Olfaction.

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Anat Rec (Hoboken). Author manuscript; available in PMC 2017 July 01. Published in final edited form as: Anat Rec (Hoboken). 2016 July ; 299(7): 853–868. doi:10.1002/ar.23367.

Anatomical Details of the Rabbit Nasal Passages and Their Implications in Breathing, Air Conditioning, and Olfaction Jinxiang Xi1, Xiuhua April Si2, JongWon Kim3, Yu Zhang1, Richard E. Jacob4, Senthil Kabilan4, and Richard A. Corley4 Jinxiang Xi: [email protected] 1School

of Engineering and Technology, Central Michigan University, Mount Pleasant, MI, U.S.A

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2Department 3College

of Mechanical Engineering, California Baptist University, Riverside, CA, U.S.A

of Engineering, University of Georgia, Athens, GA, U.S.A

4Systems

Toxicology & Exposure Science, Pacific Northwest National Laboratory, Richland, WA,

U.S.A

Abstract

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The rabbit is commonly used as a laboratory animal for inhalation toxicology tests and detail knowledge of the rabbit airway morphometry is needed for outcome analysis or theoretical modeling. The objective of this study is to quantify the morphometric dimension of the nasal airway of a New Zealand white rabbit and to relate the morphology and functions through analytical and computational methods. Images of high-resolution MRI scans of the rabbit were processed to measure the axial distribution of the cross-sectional areas, perimeter, and complexity level. The lateral recess, which has functions other than respiration or olfaction, was isolated from the nasal airway and its dimension was quantified separately. A low Reynolds number turbulence model was implemented to simulate the airflow, heat transfer, vapor transport, and wall shear stress. Results of this study provide detailed morphological information of the rabbit that can be used in the studies of olfaction, inhalation toxicology, drug delivery, and physiology-based pharmacokinetics modeling. For the first time, we reported a spiral nasal vestibule that splits into three paths leading to the dorsal meatus, maxilloturbinate, and ventral meatus, respectively. Both non-dimensional functional analysis and CFD simulations suggested that the airflow in the rabbit nose is laminar and the unsteady effect is only significantly during sniffing. Due to the large surface-to-volume ratio, the maxilloturbinate is highly effective in warming and moistening the inhaled air to body conditions. The unique anatomical structure and respiratory airflow pattern may have important implications for designing new odorant detectors or electronic noses.

Keywords New Zealand white rabbit; nasal morphology; respiration; olfaction; lateral recess

Correspondence to: Jinxiang Xi, [email protected]. Present address: School of Engineering and Technology, Central Michigan University, 1200 South Franklin Street, Mount Pleasant, MI 48858, Phone: (989) 774-2456, Fax: (989) 774-4900

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Introduction Rabbits (Oryctolagus cuniculus) are often ultilized as human surrogates in olfaction and inhalation toxicology tests, both of which require the knowledge of the nasal airway structure and airflow dynamics within it. However, reports of detailed morphological dimensions of the rabbit airway are still scarce. Such information cannot be obtained from traditional methods such as gross dissection or histologic analysis and is becoming feasible only recently with high-resolution imaging techniques. Due to the small size of rabbit airways, in vitro tests to quantify the airflow field are still impractical.


The biological functions of the rabbit nose are closely related with its morphology. By comparing many species, Negus suggested that the complexity of the nose structure is an indicator of the olfactory acuity, with macrosmatic species (like rabbits, mice, dogs) having a very complex nasal structure that is missing in microsmatic species like humans (Negus, 1954). Rabbits are found in every continent except Antarctica, and their superior survival skills can be partially attributed to their keen sense of smell. Endowed with an exquisitely tuned sensory organ, rabbits communicate in most parts of their life through their noses, including locating food, perceiving danger, attracting mates, or demarking territories. A good sniff of the environment or between each other is comparable to a long human conversation. Studies show that rabbits can smell food below ground and sense predictors miles away (Schalken, 1976). A good sense of smell is even present in newborn rabbits, allowing them to find the nipples of its mother with closed eyes (Schaal et al., 2003). Rabbits wiggle their noses often, which not only helps to draw air in to fill the lungs, but also help them to differentiate the tiny trace of chemical molecules or pheromones in the air to detect danger, identify friends or potential mates (Sobel et al., 1998; Kromin and Ignatova, 2014).

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The nasal anatomy of different mammals has been examined by means of various methods, such as fixed tissue resections, airway casting, or medical imaging scans (magnetic resonance imaging, MRI, or computed tomography, CT) (Patra et al., 1986; Salazar et al., 1995; Deleon and Smith, 2014; Ruf, 2014). Nasal morphometry has been reported for the mouse (Gross et al., 1982), rat (Schreider and Raabe, 1981; Kimbell et al., 1997; Subramaniam et al., 1998; Corley et al., 2012), beagle dog (Schreider and Raabe, 1981), monkey (Yeh and Schum, 1980; Harris et al., 2003; Corley et al., 2012), and human (Subramaniam et al., 1998; Kimbell and Subramaniam, 2001; Shi et al., 2006; Xi et al., 2012). However, descriptive, rather than quantitative, characterizations of the nasal morphology were presented in these studies. Only recently, the high-resolution magnetic imaging technique was adopted to quantify the intricate nasal airway anatomy of macrosmatic animals (i.e., with keen sense of smell). Earlier endeavors using low resolution imaging techniques were unable to resolve the intricate concha structure. Moreover, MRI performs better than CT in resolving the interface of air and soft-tissues (De Rycke et al., 2003). To date, only a few studies reported detailed regional morphometric data for dogs (Craven et al., 2007), deer (Ranslow et al., 2014), marmosets (Smith et al., 2014), and humans (Orhan et al., 2014; Xi et al., 2014). Moreover, few studies considered the morphometry of the rabbit nasal airway. Using high-resolution MRI, Corley et al. (2009) divided the rabbit nasal airway into five zones and quantified the surface area and volume

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for each zone. Recently, with high-resolution CT, Ruf (2014) compared the turbinal skeleton in different Lagomorphs (hares, rabbits, and pikas). It was observed that the maxilloturbinate of the Lagomorphs is more complicated than the corresponding regions in rodents, monkeys, and humans. This sophisticated structure gives rise to a higher surface-volume ratio, which is ideal for effective air-conditioning of inhaled air, as well as for scrubbing of reactive gasses or toxic particles (Corley et al., 2009). However, a detailed quantitative study of the rabbit nasal morphology lacks in the above study. Moreover, the lateral recesses and the main nasal chamber were not isolated. The surface area and volume of each region and the associated airflow distribution per unit area or unit volume could be smaller than the actual values. As a commonly used laboratory animal, detailed nasal morphometric data of the rabbit are needed in theoretical or computational models of respiration, air-conditioning, olfaction, inhalation toxicology, or intranasal drug delivery (Zhou et al., 2013; Zhou et al., 2014; El Taoum et al., 2015).

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A significant issue in evaluating the morphology-function relation of the rabbit nose in respiration and olfaction is to accurately determine the detailed airflow dynamics. An accurate knowledge of airflow will also help determine the behavior and fate of inhaled vapors and particles, which convey a certain amount of odorant chemicals to specific areas in the olfactory mucosa. Several features of the rabbit nose anatomy have been claimed in regulating the inhaled air flows, thus adjusting the exchanges of air, heat, moisture, and inspired chemicals to the nasal mucosa (Mlynski et al., 2001; Churchill et al., 2004). These features include the alar fold that partially occlude the nose inlet and forms a comma-shape naris, the spiral (or helical) nasal vestibule that splits the inhaled air into different flow streams, the swell body that cause the nasal cycle, and the dorsal meatus that leads directly to the olfactory region, among others (Churchill et al., 2004).

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The objective of this study is to quantify the geometry of the nasal airway and the lateral recess of a New Zealand white (NZW) rabbit and to relate the nasal morphometry to its functions in respiration and olfaction. Specifically, we aim to (1) separate the lateral recess from the nasal airway of the NZW rabbit, (2) measure separately the morphometric data of the rabbit nasal airway and paranasal recess in terms of the cross-sectional area, perimeter, surface area, and internal volume, (3) analytically evaluate the functional parameters of Reynolds number and Womersley number, and (4) computationally predict the airflow, heat and mass transfer, and wall shear in the rabbit nasal airway. The nasal morphology of the rabbit will also be compared with those of other species reported in the literature. Results of this study will provide detailed morphometric data for the modeling of rabbit respiration or olfaction and will have implications for the design of scent detectors or electronic noses.

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Materials and Methods Rabbit Nasal Airway Data Acquisition, Segmentation, and Reconstruction An anatomically accurate rabbit nasal airway model was developed from high-resolution MRI scans of a female New Zealand white rabbit (weight 4.27 kg and six months of age). Detailed procedures for the data acquisition, image segmentation, and 3D model reconstruction can be found in Corley et al. (2009) and is briefly described as follows. The protocol of the rabbit data acquisition was approved by the Institutional Animal Use Anat Rec (Hoboken). Author manuscript; available in PMC 2017 July 01.

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Committee at Pacific Northwest National Laboratory. In this study, the weight and age of the rabbit were similar as those of the three rabbits used in Corley et al. (2009). However, more detailed nasal morphological data would be measured in this study that were lacking in the previous study, such as the regional dimensions in the right and left passages, those along the axial direction, as well as morphological data with and without the lateral recess. The cadaver of the rabbit was scanned at ∼4°C in a 2.0-tesla MRI scanner. High-resolution MRI images were acquired on a 512 × 512 × 256 matrix covering an 8 × 8 × 16 cm field of view with 0.625 mm increment (Fig. 1b). The iso-surface of the nasal airway was then extracted from the segmented image data using the Digital Data Viewer (DDV; CGC Consulting, Placitas, NM). For the regions that were convoluted or under-resolved, the surface was checked visually for apparent artifacts or poorly resolved regions and was rectified where necessary using the pixel-editing tool of DDV (Corley et al., 2009).


To facilitate the examination of anatomical details, a solid nasal airway model was manufactured using an in-house 3D printer (Object30 Pro, Stratasys, Northville, MI) with a translucent material (VeroClear, Northville, MI) (Fig. 1c). The solid model was then broken into different functional pieces for better examination of the anatomical details that were blocked by other structures and were not easy to observe in the 3D computer model. The ability to view, touch, and disassemble/assemble a complex structure such as the rabbit nasal airway three-dimensionally can greatly help to obtain a visual-textural-spatial understanding of its anatomy and functions. Likewise, in vitro hollow casts with anatomical details can be fabricated from the computer model using rapid-prototyping techniques for deposition tests. The model geometry was then imported into 3-matic (Materialise, Ann Arbor, MI) as a STL file for surface separation and computational mesh generation. The surface of the rabbit nasal airway model was separated into different functional regions, such as the nasal vestibule (NV), maxilloturbinate (MT), nasomaxillary (NM) region, ethmoturbinate (ET), and nasopharynx (NP). Airway Morphological and Functional Quantification

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Dimension quantification of a complex 3D geometry can be laborious. Even though the total volume and surface area of the rabbit nasal airway can be readily quantified, finding the regional distribution of geometrical parameters is more challenging. Morphometric analysis of the rabbit nose was carried out in two steps: (1) the axial distribution of the crosssectional area (Ac) and perimeter (P) were quantified from the MRI scan images, and (2) the regional surface area and volume were quantified from the 3D nasal airway model geometry. In the first step, the MRI scan images were processed in Adobe Photoshop to extract the airway lumina from the tissues and converted the images into binary format. The lateral recess in each image was then separated from the main airway. To find the cross-sectional area (Ac) and perimeter (P), the Image Regional Analyzer in the Matlab Image Processing and Computer Vision Toolbox was used. The hydraulic diameter was calculated as dh = 4Ac/P. The complexity level in each cross-section was computed from the area-perimeter relation.


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(1)

For a circle, the complexity is one. The more complex the surface is, the larger perimeter per unit area, and the higher value of the parameter Cx. Physically, Cx represents the level of complexity or irregularity of the airway borderlines (Lovejoy, 1982).

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In the second step, to quantify the surface area and internal volume versus the axial distance from the naris, the 3D nasal airway model was evenly cut into 50 parts in Magics (Materialise, Ann Arbor, MI). In each part, the total surface area, the area of the front and back, and the total volume can be found directly. The segmental airway surface area was then calculated by subtracting the front and back areas from the total area. The cumulative airway surface area (As) and volume (Vol) were obtained by summing all segments up to the current axial position. The functions of the nasal airway of the NZW rabbit in regulating breathing, airconditioning, and olfaction can be theoretically correlated to the airway anatomy through non-dimensional parameters, such as Reynolds number (Re) and Womersley number (Wo).

(2)

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where Vavg is the average velocity at each cross-section, f is the respiration frequency (Hz), and ν is the air viscosity. Reynold number Re is the ratio of the fluid inertia (destabilizing force) to the viscous force (stabilizing force) and represents the probability of turbulent flows. Generally, a flow in a pipe is considered as turbulent when Re is larger than 2,000. In complex geometries such as the nasal airways, the Re threshold of turbulent flows is lower, even though no specific values are given (Xi and Longest, 2007; Xi et al., 2008). Womersley number (Wo) denotes the ratio of the flow transient effect over the viscous effect. When Wo > 1, the flow is considered unsteady; when Wo < 1, the flow is quasi-steady or steady. Fluid, Heat, and Vapor Transport Equations

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Incompressible airflow was assumed for all simulations with steady breathing conditions. Considering that biological flows in small animals are largely laminar or transitional, a wellvalidated low-Reynolds-number k-ω turbulence model was implemented to resolve the flow field. This model had been shown to be able to model laminar-to-turbulent transitions (Xi et al., 2011). The trajectories of inhaled particle transport within the flow field were computed using the Lagrangian discrete phase model (Xi et al., 2011). The distribution of temperature (T) and vapor concentrations (Yv) in the rabbit nasal airway are solved using Eqs. (1) and (2), respectively (Bird et al., 1960).


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(3)

(4)

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here Cp is the specific heat, κg is the thermal conductivity of the air, Yv is the mass fraction of water vapor, D̃v is the diffusion coefficient of water vapor in the air, ScT is the turbulent Schmidt number (ScT = 0.9), and PrT is the turbulent Prandtl number (PrT = 0.9). The enthalpy of the air and water vapor is represented as hs. The relative humidity (RH) of the moist air is evaluated as

(5)

where Rv is the gas constant of water vapor and ρm is the mixture density. The Antoine equation was used to calculate the saturation vapor pressure Pv,sat (Green, 1997).


Boundary condition—The quiet breathing inhalation rate is around 0.68 L/min for a typical adult New Zealand white rabbit and the respiration frequency is 83±15 Hz at the normal body temperature (38.5oC) (Kasa and Thwaites, 1990; Thrall et al., 2009). The sniffing volume flow rate was chosen to be 1.91 L/min (a factor of 2.81 higher) based on the allometric relationship provided by Ranslow et al. (2014). The sniffing frequency was chosen to be 8 Hz from the allometric estimation that the sniffing frequency of the rat, dog, and human is 10, 5, and 2 Hz, respectively (Settles, 2005). To study the respiration rate effect, a spectrum of inhalation flow rates (i.e., 0.68 L/min ×0.5, 1.0, 2.0, 3.0, 4.0) was considered in computational simulations. Zero ambient pressure was specified at the naris and a vacuum pressure was specified at the nasopharynx. To define the pressure drops that correspond to specific inhalation flow rates, two steps were undertaken. First, the inlet velocity boundary condition and outflow condition were used, and the pressure drop between inlet and outlet were obtained once a converged solution was achieved. Second, the atmospheric pressure was specified at the two nostrils (i.e., inlets) and the computed vacuum pressure drop was applied at the outlet to match the specified inhalation rate. A rigid and non-slip condition was assumed for the airway surface. The surface temperature and relative humidity (RH) was specified as the mean body conditions (38.5°C and 99.5%) (Kasa and Thwaites, 1990). Numerical method and convergence analysis—ANSYS Fluent (Ansys, Inc) was used to simulate the flow dynamics and heat/mass transfer. Particular care was given to the mesh design to ensure sufficient resolution in the shear layer, cell isotropy, and limited grid


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stretching. A grid independence test was performed using mesh densities around 1.1 million, 2.3 million, 3.6 million, 4.9 million. The variations of velocity at selected points were less than 0.5% when increasing mesh density from 3.6 to 4.9 million. As a result, the final mesh consisted of 3.6 million cells.

Results Nasal Airway Anatomy


Figure 2a shows the sagittal images of the rabbit nasal airway at varying axial locations, with the lateral recess having already been removed. The nasal cavity is divided by the nasal septum into two passages, each comprising four anatomical regions: nasal vestibule (NV), maxilloturbinate (MT) region, nasomaxillary (NM) region, and ethmoturbinate (ET) region. According to Negus (1954), four types of turbinate (or concha) were recorded in mammals: folded, single scroll, double scroll, and branching, among which the branching-type turbinate is most sophisticated and provides the largest contact area. From Fig. 2a, the maxilloturbinate of the rabbit has branching structures and the ethmoturbinate is scroll-like in architecture. Between these two turbinate regions (MT and ET) locates the nasomaxillary region, which serves as a morphological transition from the branching-type MT to the scrolllike ET and houses the lateral recess. This is further demonstrated in Fig. 2b. The lateral recess (pink color) is fitted compactly between the MT and ET, in contrast to the outwardly protruding recess in deer (Ranslow et al., 2014), Marmosets (Smith et al., 2014), and humans (Xi et al., 2015c). The nasal septum ends at the NM region and the two nasal passages merge at the anterior point (approximately 43.0 mm from the naris) of the ET region. The NM splits into the ET region and the nasopharynx, which are separated by a horizontal plate of bone (Fig. 2a). The ET ends at a distance of 64.5 mm from the naris, while the nasopharynx gradually transits into an almond-shaped duct leading to the larynx and lungs.

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Inspections of both the 2D sagittal images and 3D airway geometry in Fig. 2a and 2b reveal more details of the MT. In the slices at locations from 14.3 mm to 28.7 mm in Fig. 2a, a thin meatus is observed to extend from the median nasal passage (solid arrow in Fig. 2a), which folds downward and forms an incomplete cover over the maxilloturbinate recess. This folding meatus will be refered to as the MT cover hereafter (Fig. 2a). Branch-like meatuses further ramify from this folding meatus (dashed arrow in Fig. 2a) and grow into a complex folding-branching pattern. The MT cover and the branch-like structure underneath it can also be seen in the solid nasal airway model (Fig. 1c), with the yellowish part being the maxilloturbinate. The yellowish color is due to the yellow support material that fills the interstitial space of the ventral concha and cannot be cleared due to the outside cover (Fig. 1c). The reconstructed surface model of the rabbit nasal airway at shown in Fig. 2b viewed from the lateral, top, and back, respectively. The lateral and top views are presented with 50% transparent of the surface to reveal the complex structures under the MT cover. The back view exhibits the ethmoturbinate with double-scrolls, as categorized in Negus (1954). Similar structure has been observed in rats and dogs, but is clearly distinct from the more complex folded ethmoturbinate observed in the white-tailed deer (Ranslow et al., 2014). Anat Rec (Hoboken). Author manuscript; available in PMC 2017 July 01.

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A more in-depth presentation of the reconstructed 3D rabbit nasal airway is shown in Fig. 3, where the nasal airway is dissembled into sub-regions according to their anatomies or functions. For instance, the lateral recess was separated from the main nasal airway (Fig. 3a). Besides the four major anatomical regions along the axial direction (NV, MT, NM, and ET), the MT is further divided into three zones: dorsal respiratory (DR), ventral respiratory I (VR I) with the folded cover and Ventral respiratory II (VR II). Functionally, the DR zone directly leads to the ethmoturbinate and acts as a short to the olfactory epithelium. The VR I, which has the most complex structure in the rabbit cavity, air-conditions and distributes the inhaled air either into the ethmoturbinate for olfaction or the nasopharynx for respiration. The VR II, which is located at the most ventral part of the MT and is in line with the nasopharynx, warms and moistens the inhaled airflow before directing them into the lungs. The surface areas of these functional regions in the left and right passages are listed in Table 1.

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The lateral recess consists of two parts, as shown in Fig. 3a. Based on the nasal fossa terminology in Maier (1993) and Smith and Rossie (2008), these two parts are the frontal recess (upper) and maxillary recess (lower), respectively. The lateral recess is weakly attached to the main passage through an ostium located in the frontal recess (Fig. 3a, right panel), while the frontal and maxillary recesses are solidly connected via a much larger slit between them. However, it is still premature to term a frontal recess with an ostium as a sinus because there are clear criteria that distinguish a sinus from a paranasal recess (Cave, 1967; Witmer, 1999; Rossie, 2006). For simplicity, the term “lateral recess” has been applied to the paranasal spaces observed in Fig. 3a (pink) with no differentiation between frontal and maxillary recesses throughout the text in this study.


The detailed shape of the maxilloturbinate is displayed in Fig. 3b by removing the lateral recess and the MT cover. Viewed laterally, the maxilloturbinate has an “accordion-like” or “radiator-fin-like” structure, which provides a large mucus-air contact area and effectively warms, moistens, and cleans the inhaled air. The nasal septum is revealed in Fig. 3c after removing the fin-like surface of the maxilloturbinate (or ventral concha). There are also multiple corrugated folds (red dashed ellipse, Fig. 3c) in the ventral region of the nasal septum, which may be the swell bodies (Bojsen-Moller and Fahrenkrug, 1971). The septal swell body is formed by vascular space beneath the epithelium and can regulate airflow ventilation by vascular dilation or constriction. During vascular dilation, the VR II was blocked and more air goes to the VR II for better air-conditioning and cleaning. During vascular constriction, the reduced resistance in the VR II allows the inhaled air passes freely through the ventral region before entering the nasopharynx and lungs. The latter mechanism may occur in high physical activity conditions, where oxygen needs are more important than air conditioning. The nasal vestibule has a spiral shape (Fig. 3b) and looks (or possibly acts) much like the swirling fuel injection vane in front of a combustion chamber (Cheng et al., 2000; Schefer et al., 2002). The spiral NV downstream of the naris splits into three paths (Fig. 3b), with one leading to the dorsal meatus, one to the VR I (or maxilloturbinate), and one to the VR II (or ventral meatus). It is believed that rabbit regulates the partition of inhaled flow by varying the nostril shape, respiration frequency, and tidal volume through sniffing (nose-wiggling).


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The difference in frequency and duration of sniffing helps the rabbit to pick different amounts of chemical molecules, which in turn helps to discriminate between objects with miniature variations of scent traces. Two needle structures were also observed in at the bottom of the ventral concha (Fig. 3c), which connect to the mouth and are presumably the vomeronasal organ (VNO) (Hornberg et al., 2009; Kang et al., 2009). As opposed to the olfactory region that senses the air-borne, volatile chemicals, the VNO can detect water-borne pheromones, which further induces hormonal and behavioral responses. The VNO and olfactory region differ in three main aspects: (1) receptor proteins and their genes, (2) specificity of receptors to stimuli, and (3) connection patterns between sensory cells, olfactory bulbs, and cortex (processing centers). Nasal Morphometrical and Functional Parameters

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Airway morphometric parameters of the NZW rabbit, such as cross-sectional area (Ac), perimeter (P), surface area (As) and airway volume (Vol), are shown in Fig 4 as a function of the axial distance from the naris. It is noted that the lateral recess has been removed and its dimension was not included in Fig. 4a, while in Fig. 4b the morphometric data were presented both with and without the lateral recess. The rationale for excluding the lateral recess was based on two factors: (1) its physical connection to the main nasal passage, and (2) negligible airflow into this space, which will be shown later in Fig. 8. By disassembling a 3D printed rabbit nasal airway model, it was observed that the lateral recess appeared to be weakly connected to the ethmoturbinate through a small slit-shaped ostium. The disassembling process was also repeated using the computer model of the rabbit nasal airway, as shown in Fig. 3. Except for the ostium, the lateral recess is an empty cavity with no outlet, making it virtually a stagnation zone for airflow. Furthermore, the ostium in the lateral recess opens caudally, which makes it more difficult for inhaled airflow in the ethmoturbinate to veer sharply in order to enter the lateral recess. Considering the particularly low flow partition into the ethmoturbinate, the airflow entering the lateral recess should be negligible. In addition, the lateral recess found in rabbits bears some similarities to the recesses observed in dogs (Craven et al., 2007). Craven et al. (2010) suggested a relatively lower airflow into the frontal recess than the olfactory recess in a dog nasal airway model. The findings of Craven et al. (2010) concur with the rabbit nasal modeling employed during this study in terms of both flow distributions (Fig. 8) and particle transport (Fig. 9), where no particle enters the lateral recess. As inhalation toxicology analysis requires both flow distributions and the corresponding surface areas, an accurate quantification of the regional surface is necessary. Considering that the lateral recess is a non-ventilated cavity, including it in the inhalation toxicology analysis will lead to a smaller particle distribution per unit area. The smaller distribution would underestimate the toxic effects of the exposure considered here. As a result, the rabbit nasal fossa was measured in two scenarios, with one including the lateral recess and the other excluding it, as presented in Fig. 4. More detailed morphological data of the lateral recess were also quantified separately in Fig. 6. Two peaks of cross-sectional dimension (Ac and P) exist in Fig. 4a, which represent the maxilloturbinate (MT) and ethmoturbinate (ET), respectively. In contrast, the dimensions (Ac and P) are relatively small in the nasal vestibule. It is also observed that the P–Ac ratio is


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larger in the MT than in the ET, indicating a higher level of geometry complexity and smaller effective flow diameter (hydraulic diameter, dh) in the MT (Fig. 4a, lower panel). Interestingly, the MT and ET, in spite of their much larger cross-sectional areas, have the smallest hydraulic diameters (Fig. 4a). From Fig. 4b, the total surface area covering the ET is about the same as that of the MT. The internal volume of the ET is around two times that of the MT.

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The complexity of the rabbit nasal airway was characterized in two steps. First, by following Craven et al. (2007), the log(Ac)–log(P) relation was plotted for the MT and ET (Fig. 5a). However, linear clusters of points were not observed as in the dog nasal airway (Craven et al., 2007), indicating a more irregular variation of the nose morphology in the axial direction in the rabbit than the dog. Instead, a new parameter was calculated using the Eq. 1, which is still based on the log(Ac)–log(P) relation, but will give the complexity magnitude of a circle as one. Figure 5b shows the distribution of the airway complexity versus the axial location. The highest complexities occur in the maxilloturbinate (MT), followed by the ethmoturbinate (ET). The nasomaxillary region, which is basically a narrow channel, has a much lower level of complexity in comparison to the neighboring MT and ET. The nasopharynx (NP), which has an approximate circle shape, has the lowest complexity levels (close to one).

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The morphological dimensions of the left lateral recess were also quantified in terms of the cross-sectional area (Ac), perimeter (P), airway surface area (As), and internal volume (Vol), as illustrated in Fig. 6. The mismatch between Ac and P in Fig. 6a suggests a highly irregular morphology of the lateral recess. The largest perimeter of the left recess is around 32 mm and the largest cross-section area is around 23 mm2. The area that connects the maxilloturbinate and the recess is found to be 2.79 mm2, which is about one-tenth of the largest Ac. As a result, very slow airflow is expected inside the recess. From Fig. 6b, the total surface area of the left recess is 1058 mm2 and the total volume is 1029 mm3. Due to the accumulative nature in calculating the surface area and volume, these two lines run almost linearly (Fig. 6b).

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Analytical distribution of functional parameters of the nasal airway versus the axial distance from the naris of the NZW rabbit is shown in Fig. 7 in terms of Reynolds number (Re) and Womersley number (Wo). Two typical breathing conditions were considered: quiet respiration (0.68 L/min, 1.38 Hz) and sniffing (1.91 L/min, 8 Hz). For both breathing conditions, Re is less than 300 throughout the rabbit nasal airway, indicating predominant laminar flow regime within the rabbit nose (the critical Re for turbulent pipe flow is around 2,000). From Fig. 7b, the local Womersley number, Wo, is oscillating around one at sniffing and falls below one at quiet respiration, indicating an appreciable transient effect for sniffing, while a slight or negligible transient effect under quiet breathing. Computational Simulations Airflow field and particle transport—The distribution of inhaled airflow can be highly heterogeneous within the rabbit nasal cavity. Moreover, the pattern of the flow distribution varies under different breathing conditions (Fig. 8). At slow respirations (0.34 L/min), the inhaled airflow is nearly evenly distributed (Fig. 8a). At a normal respiration rate of 0.68 L/ Anat Rec (Hoboken). Author manuscript; available in PMC 2017 July 01.

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min, the flow starts to exhibit patterns of heterogeneity, with high-speed flow in the dorsal meatus (upper region of Slice 1-1′), which ventilates more inhaled air to the ET and the olfactory epithelium (Fig. 8b). The high-speed flow zone shifts downward as the respiration rate increases to 1.36 L/min, and even further downward at a higher respiration rate of 2.04 L/min (Figs, 8c&d). This is because only a limited amount of flow can be ventilated to the olfactory region (or the ET region). Once the ET ventilation is saturated, all other flows will be directed to the ventral respiration zone, which causes the main flow to shift down. It is noted that the flow simulations in Fig. 8 retained the lateral recess (i.e., both the frontal and maxillary recesses). The frontal recess can be viewed in slice 2-2′ (black arrow in Fig. 8d) at the top on either side of the main airway, while the maxillary recess can be viewed in slice 3-3′ (red solid arrow in Fig. 8d) hanging from both sides of the main airway. The blue color in both the frontal and maxillary recesses indicated negligible flow ventilations into these airspaces. It is also noted that the functional parameters, Re and Wo, were calculated from averaged flow velocities (Eq. 2), and is useful only for rough estimates of the flow regimes. The heterogeneous velocity distribution in the same cross-section indicates a corresponding heterogeneous distribution of local Reynolds numbers. Similarly, the unsteady effects are dictated by, besides the breathing frequency, many other factors, such as the wake flow, eddy shedding, and boundary development. High-fidelity CFD simulations are necessary to study the local and instantaneous flow features.

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In Fig. 8a, the airstream traces were released from the same points in the nostrils for all cases considered to highlight the inhalation effects on flow distributions. Due to a limited number of seeded points (30 in this study), no stream trace was shown to have deeply penetrated into the ethmoturbinate airway passage. To visualize the transport of small particles and vapors, 3,000 1-μm particles were released into the nostrils to track their trajectories. Figure 9 shows the snapshots of particle locations at different instants after being released into the nostrils. A small fraction of small particles was observed to penetrate into the posterior ethmoturbinate for both inhalation rates listed in Fig. 9. The time required by the particles to reach the posterior ethmoturbinate was different for these two flow rates, with 0.4 seconds for 0.68 L/min and 0.1 seconds for 2.72 L/min (Figs. 9a vs. 9b). Furthermore, no particles were observed to penetrate into the lateral recess.

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Breathing Resistance—The breathing resistance in the rabbit nasal cavity is shown in Fig. 10 for a large spectrum of inhalation rates ranging from 0.34 L/min to 2.72 L/min. The pressure-drop across the entire nasal airway (naris - nasopharynx), as well as across individual anatomical regions, were quantified using CFD simulations. Figure 10a shows the pressure drop throughout the nasal cavity, which exhibits a nonlinear increase with the inhalation rates. For instance, when the flow rate increases from 0.68 L/min (normal) by a factor of 4 to 2.72 L/min (panting), the pressure drop increases by a factor of 7.3 (Fig. 10a). The pressure-drop across individual anatomical regions is shown in Fig. 10b. As expected, the maximum pressure-drop occurs in the maxilloturbinate region (Slice 1-1′ to 2-2′ in Fig. 10b), where narrow passage and complex meatus structure provide the maximum contact area with the inhaled air. Surprisingly, the pressure-drop across the ethmoturbinate region (ET) has the minimum magnitudes among all anatomical regions, even though it has a sophisticated architecture. Instead, the ethmoidal-nasopharyngeal transition region


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constitutes the second largest pressure-drop within the rabbit nasal cavity. These two seemingly counter-intuitive observations can be explained if the other two anatomical factors were taken into account. First, the ET is a dead zone (with no outlet), where the airflow speed is extremely low compared with other regions inside the nose (except the lateral recess); according to the Bernoulli equation, this causes a very small pressure difference. Second, the transition from the ET to NP involves a dramatic change of the cross-sectional areas. All inhaled flows converge at the NP and press into this small flow area, which necessitates a large pressure difference as the driving force.

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Air-conditioning—Figure 11 shows the distribution of temperature (T) and relative humidity (RH) inside the nasal airway by inhaling ambient air at 23°C and RH = 30% under quiet breathing conditions (Qin = 0.68 L/min). Even though rabbits are known for their superior adaptability to environments, it is still striking to observe how quick the rabbit nose warms and moistens the inhaled ambient air to the body conditions. The air temperature and relative humidity approaches 38°C and 99.5% shortly after the nasal vestibule. It is in contrast with the observation that, in humans, inhaled air can only be adjusted to the body condition until the nasopharynx (Longest and Xi, 2008; Kim et al., 2013). This remarkable air-conditioning capacity is credited to the unique architecture of the maxilloturbinate, whose narrow channel and branching structure provide a large contact area lining with capillary-rich mucus.

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Wall shear stress—Figure 12 shows the predicted wall shear stress under different flow rates. At slow inhalations (0.34 L/min), the distribution of the shear stress appears relatively uniform throughout the nasal cavity except the nasal vestibule, where larger shear stress occurs. At the normal inhalation rate (0.68L/min), heterogeneity of wall shear stress starts to show up in the ventral concha and the nasopharynx, while the highest shear stress still remains in the NV. The heterogeneity of the shear stress on the airway wall intensifies as the inhalation rate continues to increase (1.36 L/min, 2.04 L/min). New hot spots of shear stress emerge in the ventral meatus at 2.72 L/min, while the shear stress hot spots in the NP extend to an even larger scale. This escalated shear stress is consistent with the large pressure drops across the nasopharynx and maxilloturbinate region, as observed in Fig. 10.

Discussion

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The rabbit nose has an exquisite system in regulating the distribution of inhaled airflow within the nose to accomplish its functions. Anatomically, this system includes the slit-shape naris, spiral nasal vestibule, swell body in the ventral respiration region, and dorsal meatus directly connecting to the olfactory region. Results of this study demonstrated that air flows that are inhaled from different parts of the naris go to specific regions of the nasal airway. Depending on its initial position at the nostril, a streamline follows a specific path for a given inhalation flow rate. As shown in Fig. 13a, a streamline originated from the nose tip enters the olfactory region (red), while those from middle nostril enter the maxilloturbinate region (blue, green, pink), and that from the bottom of the nostril enters the inferior meatus (black). According to Corley et al. (2009), approximately 1.1% of inhaled airflow enters the posterior ethmoturbinate region. Considering that olfactory nerves are both highly sensitive and delicate, such a small fraction of airflow might be high enough for olfaction purposes Anat Rec (Hoboken). Author manuscript; available in PMC 2017 July 01.

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and low enough to protect the nerves from environmental damages. Second, the nasal vestibule has a unique structure for distributing inhaled air flows. There are apparently two spiral curves formed by the ala fold. The second curve is further split into two passageways by the dorsal concha. One leads to the middle meatus upper fold, the other leads to the dorsal meatus, which subsequently lead to the olfactory recess (Fig. 3). It is the first time that a spiral nasal vestibule was reported in the literature. Third, the dorsal meatus, which is connected to the tip of the slit naris, follows the third curve of the nasal vestibule, and goes swiftly to the ethmoturbinate lining with olfactory epithelium without meandering through the labyrinthine maxilloturbinate region. This shortcut endows two advantages to the olfaction function: (1) inhaled airflow entraining chemical molecules or pheromones can reach the olfactory neurons within shortest period of time, and (2) inhaled scent can be maximally preserved without experiencing the scrubbing and absorption by the epithelium of the maxilloturbinate.


Compared to the nasal airways of a human or monkey, the nasal airway architecture of the NZW rabbit is much more complex and bears more similarities to other macrosmatic animals such as dogs, rats, and deer (Craven et al., 2007; Corley et al., 2012; Richter et al., 2013; Ranslow et al., 2014; Xi et al., 2015b). However, appreciable disparities also exist among these non-primate species. For dogs (Canis familiaris), the maxilloturbinate and ethmoturbinate are branching-type and scroll-type, respectively (Craven et al., 2007; Craven et al., 2009). The canine maxilloturbinate region has the most complex structure among the three. The deer (Odocoileus virginianus) nose has a double-scroll-like maxilloturbinate and branching-type ethmoturbinate, the latter is the most complex among the three species (Ranslow et al., 2014). The rabbit (Oryctolagus cuniculus) nose has a branching-type maxilloturbinate and scroll-like ethmoturbinate (Fig. 2), which is similar as the dog. However, the ethmoturbinate in the rabbit apprears much less compalex than that in the dog. An explanation of these anatomical differences may still be premature, and is possibly attributed to their evolutional adaptions to external environments considering the different functions of maxilloturbinate (respiration, air-conditioning, and cleaning) and ethmoturbinate (olfaction).

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Results of this study show that the airflow in the rabbit nose is predominantly laminar even under sniffing breathing conditions. As a result, the regulating functions of the respiration parameters (inhalation rate and frequency) cannot be explained by means of their moderation on the core flow turbulence of inhaled airflow as suggested by Churchill et al. (2004); instead, an alternative mechanism may exist in the regulation of nose functions. In our previous study, we demonstrated that the near-wall vortex formation in human nose has a significant effect on the mucosa-air heat exchange and wall shear stress (Xi et al., 2015a). It is speculated that the near-wall vortex formation in the rabbit nose may also exert a similar effect in regulating airflow distribution, heat transfer, humidity distribution, and wall shear stress distribution. Considering that momentum and heat exchanges are inversely correlated with boundary layer thicknesses, near-wall vortices can greatly enhance the wall shear stress and air-mucosa heat transfer by disturbing the velocity and thermal boundary layers, as shown in Fig. 13b. This enhancement could be most pronounced on the surface of the rounded-shape vascular-rich turbinate, where the narrow meatal passages induce significant formation of near-wall vortices. This hypothesis, which holds valid for the human nose, Anat Rec (Hoboken). Author manuscript; available in PMC 2017 July 01.

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needs to be tested in the rabbit nose in future studies. Nevertheless, the air-conditioning capacity of the nose is crucial to lung health because the lung epitheliums are vulnerable to cold or dry air, which if not warmed and moisten to the body condition, can envoke injury and adverse symptoms such as inflammation.


Results of this study may help explain the observations of structural changes in the nasal epithelium when exposed cold air (Hilding, 1932; Constantinidis et al., 2000; Sorensen et al., 2006). Studies showed that there was an increase in the thickness of the membrane and an increase in the number of glands by exposing rabbits to different environments (Hilding, 1932). There is a thin layer (5-30 μm) of mucus lining the airway surface secreted from the glands in the underneath tissues. Without the moist mucus, the cilia will dry out and won't survive. Regarding its air-conditioning function, the mucus, as well as the vascular-rich tissue below, helps to augment heat transfer, humidify inhaled air, and recover the water content during exhalation. Under quiet breathing conditions (0.68 L/min) in an ambient environment of 23°C and 30% RH, it is estimated that the heat transfer rate is 0.20 W (0.048 calorie/s), and moisture exchange rate between mucus and air is 0.53 mg/s. From Table 1, the surface area of the ventral concha region (VR-I) is 2837.7 mm2 (= 1388.6 + 1449.1 mm2), which gives a heat flux of 70.48 W/m2 and a vapor (mass) flux of 186.77 mg/(s·m2). The convective heat transfer coefficient, h, is 4.55 W/m2·K based on the surface area of the ventral concha (VR-I), which is twice that of the ambient air (2.0 W/m2·K). Considering that the warming and moistening process occurs only in part of the ventral concha, the above theoretical heat and mass flux, as well as the heat transfer coefficient h, should be even larger locally, increasing the burden of the mucus in that region. The basal heat production for a 4.78 kg rabbit is around 210.6 calorie per day using the equation (heat = 39.35·body weight + 22.5) recommended by Lee (1939). In this study, the heat exchange between the mucosa and inhaled air within the nose is estimated to be 4,147 calories per day (0.048 calorie/s ×3,600 s/hour × 24 hours/day = 4,147 calories/day), which is even larger than the basal heat production rate (210.6 calories). This seemingly counter-intuitive observation can be reconciled by the efficient heat recovery during exhalation, when the mucosa reclaims the majority of the thermal energy in the exhaled airflow. It is, therefore, a reasonable conjecture that the rabbit nose plays an important role in regulating its body temperature. This may explain the observation by Kasa et al. that the respiration rate of rabbit increases quickly with increasing environmental temperatures so that the rabbit can stay at a relatively constant body temperature (Kasa and Thwaites, 1990).

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In addition to thermal regulation and water conservation, the mucus also acts as a barrier between airway tissues and inhaled toxicants by absorbing chemicals, filtering out particles, and removing them by mucociliary clearance. Studies have shown that inhaled noxious chemicals will activate the motile cilia in the mucus and increase the beating pace to speed up the mucociliary clearance (Navarrette et al., 2012). This process may be further facilitated by nasal flows, which elicit appreciable wall shear stress in the convoluted narrow nasal airway. Higher inhalation rates draw in more noxious chemicals/particles and decrease the mucus thickness. Such effects reduce the diffusion distance of motile activation as well as decrease the inertia of the mucus vibration.


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Limitations in this study include the assumptions of rigid airway wall, invariant airway geometry under different flow rates, and one rabbit sample only. Dynamic motion of the rabbit nose (sniffing, nose wiggling, etc.) is a natural pheromone and is believed to have important implications in regulating inhaled flows. In addition, the nose model hereof has been based on images of a single rabbit, which does not necessarily represent the population mean.

Conclusions In Summary, a detailed morphometric analysis of the nasal airway was presented for a New Zealand While rabbit. Functional implications associated with respiration, air-conditioning, and olfaction were evaluated using both non-dimensional analysis and computational simulations. Specific findings are:


1.

Detailed morphometric dimensions of the nasal airway, as well as the lateral recess, were measured in a New Zealand white rabbit in terms of axial distribution of the cross-sectional area, hydraulic diameter, shape complexity level, cumulative airway surface area, and volume.

2.

It is the first time to report a spiral nasal vestibule that ramifies into three apparently distinct passageways leading to three different regions: dorsal meatus, maxilloturbinate, and ventral meatus.

3.

Air flows in the rabbit nose are laminar flows; the unsteady effect is important only when sniffing.

4.

Due to a large area-volume ratio, the maxillotrubinate is highly effective in warming and moistening the inhaled air.

5.

Wall shear stress is maximal within the nasal vestibule and ventral meatus, and is much smaller in the maxilloturbinate and ethmoturbinate regions, which protects the delicate concha and the lining mucosa.

Acknowledgments The authors gratefully acknowledge the financial support from Central Michigan University Early Career Award P622911 (JX), the National Heart, Lung, and Blood Institute of the National Institutes of Health (NHLBI R01 HL073598) (RAC) and the U.S. Environmental Protection Agency (EP-C-09-006) (RAC). We thank Jun Lu for his assistance in processing the image data and Danielle Nevorski for reviewing the manuscript.

Literature Cited Author Manuscript

Bird, RB.; Steward, WE.; Lightfoot, EN. Transport Phenomena. New York: John Wiley & Sons; 1960. Bojsen-Moller F, Fahrenkrug J. Nasal swell-bodies and cyclic changes in the air passage of the rat and rabbit nose. J Anat. 1971; 110:25–37. [PubMed: 4110864] Cave AJE. Observations on the platyrrhine nasal fossa. Am J Phys Anthropol. 1967; 26:277–288. [PubMed: 6035853] Cheng RK, Yegian DT, Miyasato MM, Samuelsen GS, Benson CE, Pellizzari R, Loftus P. Scaling and development of low-swirl burners for low-emission furnaces and boilers. P Combust Inst. 2000; 28:1305–1313. Churchill SE, Shackelford LL, Georgi JN, Black MT. Morphological variation and airflow dynamics in the human nose. Am J Hum Biol. 2004; 16:625–638. [PubMed: 15495233]


Xi et al.

Page 16

Author Manuscript Author Manuscript Author Manuscript Author Manuscript

Constantinidis J, Knobber D, Steinhart H, Kuhn J, Iro H. Morphological and functional alterations in the nasal mucosa following nCPAP therapy. Hno. 2000; 48:747–752. [PubMed: 11103346] Corley RA, Kabilan S, Kuprat AP, Carson JP, Minard KR, Jacob RE, Timchalk C, Glenny R, Pipavath S, Cox T, Wallis CD, Larson RF, Fanucchi MV, Postlethwait EM, Einstein DR. Comparative computational modeling of airflows and vapor dosimetry in the respiratory tracts of rat, monkey, and human. Toxicol Sci. 2012; 128:500–516. [PubMed: 22584687] Corley RA, Minard KR, Kabilan S, Einstein DR, Kuprat AP, Harkema JR, Kimbell JS, Gargas ML, Kinzell JH. Magnetic resonance imaging and computational fluid dynamics (CFD) simulations of rabbit nasal airflows for the development of hybrid CFD/PBPK models. Inhal Toxicol. 2009; 21:512–518. [PubMed: 19519151] Craven BA, Neuberger T, Paterson EG, Webb AG, Josephson EM, Morrison EE, Settles GS. Reconstruction and morphometric analysis of the nasal airway of the dog (Canis familiaris) and implications regarding olfactory airflow. Anat Rec. 2007; 290:1325–1340. Craven BA, Paterson EG, Settles GS. The fluid dynamics of canine olfaction: unique nasal airflow patterns as an explanation of macrosmia. J R Soc Interface. 2010; 7:933–943. [PubMed: 20007171] Craven BA, Paterson EG, Settles GS, Lawson MJ. Development and verification of a high-fidelity computational fluid dynamics model of canine nasal airflow. J Biomech Eng-T ASME. 2009; 131 De Rycke LM, Saunders JH, Gielen IM, van Bree HJ, Simoens PJ. Magnetic resonance imaging, computed tomography, and cross-sectional views of the anatomy of normal nasal cavities and paranasal sinuses in mesaticephalic dogs. Am J Vet Res. 2003; 64:1093–1098. [PubMed: 13677385] Deleon VB, Smith TD. Mapping the nasal airways: using histology to enhance CT-based threedimensional reconstruction in nycticebus. Anat Rec. 2014; 297:2113–2120. El Taoum KK, Xi J, Kim JW, Berlinski A. In vitro evaluation of aerosols delivered via the nasal route. Resp care. 2015; 60:1015–1025. Green, DW., editor. Perry's Chemical Engineers' Handbook. 7th. New York: McGraw-Hill; 1997. Gross EA, Swenberg JA, Fields S, Popp JA. Comparative morphometry of the nasal cavity in rats and mice. J Anat. 1982; 135:83–88. [PubMed: 7130058] Harris AJ, Squires SM, Hockings PD, Campbell SP, Greenhill RW, Mould A, Reid DG. Determination of surface areas, volumes, and lengths of cynomolgus monkey nasal cavities by ex vivo magnetic resonance imaging. J Aerosol Med Pulm Drug Deliv. 2003; 16:99–105. Hilding A. Experimental surgery of the nose and sinuses. I. Changes in the morphology of the epithelium following variations in ventilation. Arch Otolaryngol. 1932; 16:9–18. Hornberg M, Gussing F, Berghard A, Bohm S. Retinoic acid selectively inhibits death of basal vomeronasal neurons during late stage of neural circuit formation. J Neurochem. 2009; 110:1263– 1275. [PubMed: 19519663] Kang N, Baum MJ, Cherry JA. A direct main olfactory bulb projection to the ‘vomeronasal’ amygdala in female mice selectively responds to volatile pheromones from males. Eur J Neurosci. 2009; 29:624–634. [PubMed: 19187265] Kasa W, Thwaites CJ. The effects of elevated temperature and humidity on rectal temperature and respiration rate in the New Zealand white rabbit. Int J of Biometeorol. 1990; 34:157–160. [PubMed: 2083981] Kim JW, Xi J, Si XA. Dynamic growth and deposition of hygroscopic aerosols in the nasal airway of a 5-year-old child. Int J Numer Meth Biomed Eng. 2013; 29:17–39. Kimbell JS, Godo MN, Gross EA, Joyner DR, Richardson RB, Morgan KT. Computer simulation of inspiratory airflow in all regions of the F344 rat nasal passages. Toxicol Appl Pharm. 1997; 145:388–398. Kimbell JS, Subramaniam RP. Use of computational fluid dynamics models for dosimetry of inhaled gases in the nasal passages. Inhal Toxicol. 2001; 13:325–334. [PubMed: 11295865] Kromin AA, Ignatova YP. Objective Method for Registration of the Sniffing Component of the Search Behavior in Rabbits Subjected to Food Deprivation. Bull Exp Biol Med. 2014; 156:522–525. [PubMed: 24771442] Lee RC. Size and basal metabolism of the adult rabbit. J Nutr. 1939; 18:489–500. Anat Rec (Hoboken). Author manuscript; available in PMC 2017 July 01.

Xi et al.

Page 17

Author Manuscript Author Manuscript Author Manuscript Author Manuscript

Longest PW, Xi J. Condensational growth may contribute to the enhanced deposition of cigarette smoke particles in the upper respiratory tract. Aerosol Sci Tech. 2008; 42:579–602. Lovejoy S. Area-perimeter relation for rain and cloud areas. Science. 1982; 216:185–187. [PubMed: 17736252] Maier, W. Cranial morphology of the therian common ancestor, as suggested by the adaptations of neonate marsupials. New York: Springer-Verlag; 1993. Mlynski G, Grutzenmacher S, Plontke S, Mlynski B, Lang C. Correlation of nasal morphology and respiratory function. Rhinology. 2001; 39:197–201. [PubMed: 11826688] Navarrette CR, Sisson JH, Nance E, Allen-Gipson D, Hanes J, Wyatt TA. Particulate matter in cigarette smoke increases ciliary axoneme beating through mechanical stimulation. J Aerosol Med Pulm D. 2012; 25:159–168. Negus VE. Studies on the anatomy of the nose. J Anat. 1954; 88:558–558. Orhan I, Ormeci T, Aydin S, Altin G, Urger E, Soylu E, Yilmaz F. Morphometric analysis of the maxillary sinus in patients with nasal septum deviation. Eur Arch Oto-Rhino-L. 2014; 271:727– 732. Patra AL, Gooya A, Menache MG. A morphometric comparison of the nasopharyngeal airway of laboratory animals and humans. Anat Rec. 1986; 215:42–50. [PubMed: 3706791] Ranslow AN, Richter JP, Neuberger T, Van Valkenburgh B, Rumple CR, Quigley AP, Pang B, Krane MH, Craven BA. Reconstruction and morphometric analysis of the nasal airway of the white-tailed deer (Odocoileus virginianus) and implications regarding respiratory and olfactory airflow. Anat Rec. 2014; 297:2138–2147. Richter JP, Rumple CR, Quigley AP, Ranslow AN, Neuberger T, Ryan TM, Stecko TD, Pang B, Van Valkenburgh B, Craven BA. Comparative anatomy and functional morphology of the mammalian nasal cavity. Integr Comp Biol. 2013; 53:E180–E180. Rossie JB. Ontogeny and homology of the paranasal sinuses in Platyrrhini (Mammalia : Primates). J Morphol. 2006; 267:1–40. [PubMed: 15549680] Ruf I. Comparative Anatomy and Systematic Implications of the Turbinal Skeleton in Lagomorpha (Mammalia). Anat Rec. 2014; 297:2031–2046. Salazar I, Quinteiro PS, Cifuentes JM. Comparative anatomy of the vomeronasal cartilage in mammals: mink, cat, dog, pig, cow and horse. Ann Anat. 1995; 177:475–481. [PubMed: 7645743] Schaal B, Coureaud G, Langlois D, Ginies C, Semon E, Perrier G. Chemical and behavioural characterization of the rabbit mammary pheromone. Nature. 2003; 424:68–72. [PubMed: 12840760] Schalken APM. Three types of pheromones in the domestic rabbit Oryctolagus cuniculus (L.). Chem Sense. 1976; 2:139–155. Schefer RW, Wicksall DM, Agrawal AK. Combustion of hydrogen-enriched methane in a lean premixed swirl-stabilized burner. P Combust Inst. 2002; 29:843–851. Schreider JP, Raabe OG. Anatomy of the nasal-pharyngeal airway of experimental animals. Anat Rec. 1981; 200:195–205. [PubMed: 7270920] Settles GS. Sniffers: Fluid-dynamic sampling for olfactory trace detection in nature and homeland security - The 2004 Freeman Scholar Lecture. J Fluid Eng-T ASME. 2005; 127:189–218. Shi H, Kleinstreuer C, Zhang Z. Laminar airflow and nanoparticle or vapor deposition in a human nasal cavity model. J Biomech Eng-T ASME. 2006; 128:697–706. Smith TD, Eiting TP, Bonar CJ, Craven BA. Nasal morphometry in marmosets: loss and redistribution of olfactory surface area. Anat Rec. 2014; 297:2093–2104. Smith TD, Rossie JB. Nasal fossa of mouse and dwarf lemurs (primates, Cheirogaleidae). Anat Rec. 2008; 291:895–915. Sobel N, Prabhakaran V, Desmond JE, Glover GH, Goode RL, Sullivan EV, Gabrieli JDE. Sniffing and smelling: separate subsystems in the human olfactory cortex. Nature. 1998; 392:282–286. [PubMed: 9521322] Sorensen HB, Larsen PL, Tos M. The influence of air current on goblet cell density in the mucosa of the normal uncinate process in the nasal cavity. Rhinology. 2006; 44:188–192. [PubMed: 17020065]


Xi et al.

Page 18


Subramaniam RP, Richardson RB, Morgan KT, Kimbell JS, Guilmette RA. Computational fluid dynamics simulations of inspiratory airflow in the human nose and nasopharynx. Inhal Toxicol. 1998; 10:473–502. Thrall KD, Woodstock AD, Soelberg JJ, Gargas ML, Kinzell JH, Corley RA. A real-time methodology to evaluate the nasal absorption of volatile compounds in anesthetized animals. Inhal Toxicol. 2009; 21:531–536. [PubMed: 19519153] Witmer, LM. The phylogenetic history of paranasal air sinuses. Berlin: Quintessence; 1999. Xi J, Berlinski A, Zhou Y, Greenberg B, Ou X. Breathing resistance and ultrafine particle deposition in nasal–laryngeal airways of a newborn, an infant, a child, and an adult. Ann of Biomed Eng. 2012; 40:2579–2595. [PubMed: 22660850] Xi J, Kim J, Si XA. Effects of nostril orientation on airflow dynamics, heat exchange, and particle depositions in human noses. Eur J Mech B/Fluid in press. 2015a Xi J, Kim J, Si XA, Corley RA, Kabilan S, Wang S. CFD modeling and image analysis of exhaled aerosols due to a growing bronchial tumor: towards non-invasive diagnosis and treatment of respiratory obstructive diseases. Theranostics. 2015b; 5:443–455. [PubMed: 25767612] Xi J, Longest PW. Transport and deposition of micro-aerosols in realistic and simplified models of the oral airway. Ann of Biomed Eng. 2007; 35:560–581. [PubMed: 17237991] Xi J, Longest PW, Martonen TB. Effects of the laryngeal jet on nano-and microparticle transport and deposition in an approximate model of the upper tracheobronchial airways. J Appl Physiol. 2008; 104:1761–1777. [PubMed: 18388247] Xi J, Si X, Kim JW, Berlinski A. Simulation of airflow and aerosol deposition in the nasal cavity of a 5-year-old child. J Aerosol Sci. 2011; 42:156–173. Xi J, Si X, Zhou Y, Kim J, Berlinski A. Growth of nasal and laryngeal airways in children: implications in breathing and inhaled aerosol dynamics. Resp care. 2014; 59:263–273. Xi J, Yuan JE, Si XA, Hasbany J. Numerical optimization of targeted delivery of charged nanoparticles to the ostiomeatal complex for treatment of rhinosinusitis. Int J Nanomedicine. 2015c; 10:4847. [PubMed: 26257521] Yeh HC, Schum GM. Models of human lung airways and their application to inhaled particle deposition. Bull Math Biology. 1980; 42:461–480. Zhou Y, Guo M, Xi J, Irshad H, Cheng Y-S. Nasal Deposition in Infants and Children. J Aerosol Med. 2014; 26:110–116. Zhou Y, Xi J, Simpson J, Irshad H, Cheng Y-S. Aerosol Deposition in a Nasopharyngolaryngeal Replica of a 5-Year-Old Child. Aerosol Sci Tech. 2013; 47:275–282.

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Fig. 1.

Nose of a New Zealand white (NZW) rabbit: (a) lateral and cross-sectional views of the rabbit nose, (b) MRI scans of the maxilloturbinate region and ethmoidal region, and (c) solid airway model printed with 3D prototyping technique.


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Representation of the NZW rabbit nasal airway: (a) images of sagittal airway as a function of distance from the naris, (b) 3D surface model of the rabbit nasal airway. NV: nasal vestibule, MT: maxilloturbinate region, NM: nasomaxiilary region, ET: ethmoturbinate region, NP: nasopharynx. The pink color denotes the lateral recess.


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Assembly diagram of the NZW rabbit nasal airway: (a) the whole nasal airway with the lateral recess, (b) nasal airway surface after removing the cover of the VR I (MT Cover) and the lateral recess, and (c) nasal airway surface after removing the ventral concha (VR I and II) and the ET. Anatomical details of the nasal vestibule, vemeronasal organ, ethmoidal concha, and ventral conchae are also shown.


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Fig. 4.

Airway morphometric parameters of the NZW rabbit. (a) The distribution of the crosssectional area (Ac), perimeter (P), and its hydraulic diameter (dh) versus the axial distance from the naris. (b) The distribution of the airway surface area and airway volume versus the axial location.

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Author Manuscript Fig. 5.

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Complexity of the NZW rabbit nasal airway: (a) log(Ac)–log(P) relation of the maxilloturbinate and ethmoidal regions, and (b) the distribution of the geometrical complexity versus the axial distance from the naris. For comparison, the complexity of a circle is one.


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Morphometric parameters of the left lateral recess of the NZW rabbit. (a) The distribution of the cross-sectional area (Ac) and perimeter (P) versus the axial distance from the naris. (b) The surface area and volume versus the normalized axial location relative to the length of the left lateral recess. The area that connects the main airway and the left lateral recess is 5.79 mm2.


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Fig. 7.

Distribution of airway function parameters versus the axial distance from the naris of the NZW rabbit under quiet and sniffing breathing conditions. (a) Reynolds number, Re is less than 2,000 in all regions of the airway, indicating a dominant laminar flow. (b) Womersley number, Wo is less than one under quiet breathing condition and is around one for sniffing, indicating a quasi-steady flow for quiet breathing and a transient flow for sniffing.


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Inhalation airflow inside the nasal airway under different respiration rates, (a) Q = 0.34 L/ min, (b) Q = 0.68 L/min, (b) Q = 1.36 L/min, and (b) Q = 2.04 L/min, using streamlines, cross-sectional view, sagittal view, and selected slices.


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Fig. 9.

Snapshots of particle locations at different instants after being released into the nostrils. (a) Q = 0.68 L/min, (b) Q = 2.72 L/min. The particle size is 1 μm.


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Author Manuscript

Numerically predicted pressure variation along the distance in nasal airway. (a) The pressure-drop across the nasal airway under different breathing condition, and (b) the relationship of pressure drop between selected distances with flow rate.


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Thermohumidity distributions inside the rabbit nasal airway by inhaling ambient air at 23°C and RH = 30%. (a) Temperature and (b) relative humidity (RH).

Author Manuscript Author Manuscript Author Manuscript Anat Rec (Hoboken). Author manuscript; available in PMC 2017 July 01.

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Fig. 12.

Wall shear stress in nasal airway under varying breathing conditions. (a) Q = 0.34 L/min, (b) Q = 0.68 L/min, (c) Q = 1.36 L/min, (d) Q = 2.04 L/min, (e) Q = 2.72 L/min.


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Fig. 13.

Inlet effects on flow distribution and vortices. (a) Inlet flow distribution, and (b) Instantaneous coherent structures (vortices) identified by the iso-surface of the λ2-criterion at 0.03.


Author Manuscript Table 1

Author Manuscript

Author Manuscript 1.3 133.7 1.2

RNP: area (mm2)

RNP: percentage+ (%)

144.7

LNP: percentage+ (%)

LNP: area (mm2)

NV

Anterior

2.7

308.3

2.6

12.7

1449.1

12.1

1388.6

VR-I

6.5

743.9

6.3

717.9

VR-II

0.1

11.0

0.1

12.6

VNO

Maxilloturbinate region

297.5

DR

2.3

258.6

2.5

281.5

NM

Medial

15.8

1805.9

15.4

1762.6

OL

9.4

1074.4

9.2

1051.7

Recess*

Ethmoidal region

Percentage of the regional surface area over the total area of the nasal fossa.

+

Lateral recess.

LNP: left nasal passage; RNP: right nasal passage.

NV: nasal vestibule; DR: dorsal respiratory; VR: ventral respiratory; VNO: vemeronasal organ; NM: nasomaxillary region; OL: olfactory.

*

Author Manuscript

The surface area of individual functional regions (including the lateral recesses) in left and right nasal passages of a New Zealand white rabbit

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