David Elad

The air-conditioning capacity of the human nose.

The nose is the front line defender of the respiratory system. Unsteady simulations in three-dimensional models have been developed to study transport patterns in the human nose and its overall air-conditioning capacity. The results suggested that the healthy nose can efficiently provide about 90% of the heat and the water fluxes required to condition the ambient inspired air to near alveolar conditions in a variety of environmental conditions and independent of variations in internal structural components. The anatomical replica of the human nose showed the best performance and was able to provide 92% of the heating and 96% of the moisture needed to condition the inspired air to alveolar conditions. A detailed analysis explored the relative contribution of endonasal structural components to the air-conditioning process. During a moderate breathing effort, about 11% reduction in the efficacy of nasal air-conditioning capacity was observed.

Analysis of air flow patterns in the human nose

The nasal cavity is the main passage for air flow between the ambient atmosphere and the lungs. A preliminary requisite for any investigation of the mechanisms of each of its main physiological functions, such as filtration, air-conditioning and olfaction, is a basic knowledge of the air-flow pattern in this cavity. However, its complex three-dimensional structure and inaccessibility has traditionally prevented a detailed examination of internalin vivo orin vitro airflow patterns. To gain more insight into the flow pattern in inaccessible regions of the nasal cavity we have conducted a mathematical simulation of asymmetric airflow patterns through the nose. Development of a nose-like model, which resembles the complex structure of the nasal cavity, has allowed for a detailed analysis of various boundary conditions and structural parameters. The coronal and sagittal cross-sections of the cavity were modeled as trapezoids. The inferior and middle turbinates were represented by curved plates that emerge from the lateral walls. The airflow was considered to be incompressible, steady and laminar. Numerical computations show that the main air flux is along the cavity floor, while the turbinate structures direct the flow in an anterior-posterior direction. The presence of the turbinates and the trapezoidal shape of the cavity force more air flux towards the olfactory organs at the top of the cavity.

Biomechanical analysis of the keratoconic cornea.

Keratoconus is a non-inflammatory disease characterized by irregular thinning and gradual bulging of the cornea, which results in distortion of the corneal surface that causes blurred vision. We conducted three-dimensional finite element (FE) simulations to analyze the biomechanical factors contributing to the distorted shape of a keratoconic cornea. We assumed orthotropic linear elastic tissue mechanical properties, and simulated localized tissue thinning (reduction from 0.5 mm to 0.35 or 0.2 mm). We analyzed tissue deformations, stresses and theoretical dioptric power maps predicted by the models, for intraocular pressure (IOP) of 10, 15 20 and 25 mmHg. The analyses revealed that three factors affect the shape distortion of keratoconic corneas: (i) localized thinning, and (ii) reduction in the tissues meridian elastic modulus or (iii) reduction in the shear modulus perpendicular to the corneal surface, whereas thinning showed the most predominant effect. Maximal stress levels occurred at the centers of the bulged regions, at the thinnest points. The IOP levels had little influence on dioptric power in the healthy cornea, but a substantial influence in keratoconic conditions. The present FE studies allowed characterization of the biomechanical interactions in keratoconus, toward understanding the aetiology of this poorly studied malady.

Air-conditioning in the human nasal cavity

Healthy humans normally breathe through their nose even though its complex geometry imposes a significantly higher resistance in comparison with mouth breathing. The major functional roles of nasal breathing are defense against infiltrating particles and conditioning of the inspired air to nearly alveolar conditions in order to maintain the internal milieu of the lung. The state-of-the-art of the existing knowledge on nasal air-conditioning will be discussed in this review, including in vivo measurements in humans and computational studies on nasal air-conditioning capacity. Areas where further studies will improve our understanding and may help medical diagnosis and intervention in pathological states will be introduced.

Peristaltic flow in a tapered channel: application to embryo transport within the uterine cavity

Cyclic uterine peristalsis plays a central role in assisting the transport of sperm to the fallopian tube and later in the conception process in transporting the embryo to a fundal site for implantation. Fulfillment of these essential events within the time limits of fertilization and implantation depends on concomitant intrauterine fluid motion induced by uterine wall motility. A model of wall-induced fluid flow within a finite tapered two-dimensional channel was developed to simulate intrauterine fluid flow pattern and transport phenomena due to symmetric and asymmetric wall displacements. The analysis showed that the transport phenomena are strongly dependent on the phase shift of wall displacement and the angle between the walls. The velocities, flow rates, pressure and the axial transport of massless particles are reduced to zero when contractions are completely out of phase. Cases of reflux and trapping in a tapered channel are discussed for the first time. The reflux phenomenon is most likely to occur when wall motility is asymmetric, especially when the angle between the walls increases, while trapping is enhanced as the asymmetric motility and the angle between the channel walls decrease. The relevance of the results to intrauterine fluid transport phenomena, embryo transfer and hydrosalpinx was explored.

Transport Phenomena in the Human Nasal Cavity: A Computational Model

AbstractNasal inspiration is important for maintaining the internal milieu of the lung, since ambient air is conditioned to nearly alveolar conditions (body temperature and fully saturated with water vapor) on reaching the nasopharynx. We conducted a two-dimensional computational study of transport phenomena in model transverse cross sections of the nasal cavity of normal and diseased human noses for inspiration under various ambient conditions. The results suggest that during breathing via the normal human nose there is ample time for heat and water exchange to enable equilibration to near intraalveolar conditions. A normal nose can maintain this equilibrium under extreme environments (e.g., hot/humid, cold/dry, cold/humid). The turbinates increase the rate of local heat and moisture transport by narrowing the passageways for air and by induction of laminar swirls downstream of the turbinate wall. However, abnormal blood supply or mucous generation may reduce the rate of heat or moisture flux into the inspired air, and thereby affect the efficacy of the process.

Mechanics of respiratory muscles

Lung ventilation is a mechanical process in which the respiratory muscles are acting in concert to remove air in and out of the lungs. Any alteration in the performance of the respiratory muscle may reduce the effectiveness of ventilation. Thus, early diagnosis of their weakness is vital for treatment and rehabilitation. Different techniques, which are based on different measurement protocols, can be utilized for evaluation of respiratory muscle strength. Respiratory muscle strength can be assessed using pressure measurement either from the mouth or from the nostril during quasi-static breathing. However, it estimates only global performance of respiratory muscles. Techniques that are based on electromyography measurements during muscle contraction (EMG) enable the differentiation between the different respiratory muscles. Along with the above clinical and physiological techniques for assessment of respiratory muscle strength and endurance, mechanical and mathematical models of the chest wall were developed in the last few decades for analysis of chest wall movements and the contribution of its components to respiration. In this review, the different methods and the models utilized for evaluation of respiratory muscles function will be discussed.

ANALYSIS OF STRESS DISTRIBUTION IN THE ALVEOLAR SEPTA OF NORMAL AND SIMULATED EMPHYSEMATIC LUNGS

The alveolar septum consists of a skeleton of fine collagen and elastin fibers, which are interlaced with a capillary network. Its mechanical characteristics play an important role in the overall performance of the lung. An alveolar sac model was developed for numerical analysis of the internal stress distribution and septal displacements within the alveoli of both normal and emphysematic saline-filled lungs. A scanning electron micrograph of the parenchyma was digitized to yield a geometric replica of a typical two-dimensional alveolar sac. The stress-strain relationship of the alveolar tissue was adopted from experimental data. The model was solved by using commercial finite-element software for quasi-static loading of alveolar pressure. Investigation of the state of stresses and displacements in a healthy lung simulation yielded values that compared well with experimentally reported data. Alteration of the mechanical characteristics of the alveolar septa to simulate elastin destruction in the emphysematic model induced significant stress concentrations (e.g., at a lung volume of 60% total capacity, tensions at certain parts in an emphysematic lung were up to 6 times higher than those in a normal lung). The combination of highly elevated stress sites together with the cyclic loading of breathing may explain the observed progressive damage to elastin fibers in emphysematic patients.

Air-conditioning characteristics of the human nose

Nasal inspiration is important for maintaining the internal milieu of the lung, since ambient air is conditioned to nearly alveolar conditions (body temperature and fully saturated with water vapour) upon reaching the nasopharynx. This literature review of the existing in vivo, in vitro and computational studies on transport phenomena that take place within the human nasal cavity summarizes the current knowledge on air-conditioning characteristics of the human nose.

Dynamics of the intrauterine fluid-wall interface

AbstractIntrauterine fluid movements, which are responsible for embryo transport to a successful implantation site at the fundus, may be induced by myometrial contractions. Myometrial contractions in nonpregnant uteri were studied from in vivo measurements of intrauterine pressures with fluid-filled catheters and by visual observations of high-speed replaying of ultrasound images of the uterus. Transvaginal ultrasound (TVUS) images of sagittal cross sections of the nonpregnant uterus were scanned with an intravaginal ultrasound probe. Images at consecutive times (2 s apart) were digitized and processed by employing modern techniques of image processing. The sets of images were compared to evaluate time variation of the fluid–wall interface with respect to amplitude, frequencies, and wavelength of myometrial contractions. Analysis of TVUS images from 11 volunteers during the proliferative phase revealed that myometrial contractions are fairly symmetric and are propagated from the cervix towards the fundus at a frequency of about 0.01-0.09 Hz. The wavelength, amplitude, and velocity of the fluid–wall interface during a typical contractile wave were found to be 10-30 mm, 0.05-0.2 mm, and 0.5-1.9 mm/s, respectively. Additional data acquisition from a large number of normal subjects is needed to build a data base to predict normal characteristics of myometrial contractions in a nonpregnant uterus, in order to better understand their role in the preimplantation process.