Search PubMed⌕ Search

SEARCH · Search PubMed

Results for “Models, Anatomic”

Search indexed PubMed citations on genomics, clinical trials, systematic reviews and public health. Explore titles, authors and supplied subject terms, then open the PubMed record.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 1,459 records · Page 81Linked to original sources
PubMed1992

Blood volume changes in microcirculation of rat intestine caused by carotid sinus baroreceptor reflex.

This series of experiments quantified the role of the rat small intestinal arterioles in capacity changes during bilateral carotid occlusion (BCO) and compared them with previous venular studies. We also determined the role of autoregulation in arteriolar constriction during BCO. First-order, second-order, and third-order arteriolar diameter changes were measured during changes in arteriolar pressure and/or sympathetic activity (via BCO). The results indicated that an arteriolar pressure drop caused an immediate significant transient diameter decrease of 14% (P < 0.001), followed by an average steady-state diameter increase of 11% (P < 0.001) over control because of autoregulation. During BCO, the arterioles demonstrated an initial 8-22% decrease in diameter (P < 0.05). The largest first-order vessels were the least responsive to changes in pressure and BCO, while the smallest third-order vessels were the most responsive. Apparently autoregulation, and not sympathetic activity, was responsible for most of the arteriolar constriction during BCO. We also developed an anatomic model of the rat intestinal vasculature which revealed that venules hold 70% of the microcirculatory volume and are responsible for 80% of the total blood shift during BCO. Venular constriction and, to a minor degree, arteriolar constriction result in a 14% decrease in microcirculatory intestinal blood volume during BCO.

Animals↗
PubMed1993

Microvascular volume contribution to hemorrhage compensation.

Regulation of systemic blood pressure during small changes in blood volume is partially achieved by blood volume shifts from microcirculation to macrocirculation that cause a decrease in systemic hematocrit [LaForte, A., L. Lee, G. Rich, T. Skalak, and J. Lee. Am. J. Physiol. 262 (Heart Circ. Physiol. 31): H190-H199, 1992]. Diameters of 4-100 microns arterioles and 5-80 microns venules in spinotrapezius muscles of anesthetized rats were measured during control conditions and immediately after a 13% (of total blood volume) hemorrhage to determine the role of the microvasculature in hemorrhage compensation. The Strahler orders, mean control diameters (microns), and posthemorrhage diameters (% of control) were as follows: TA2, 5.7 (91.2%); TA3, 8.7 (90.4%); TA4, 12.5 (85.4%); TA5, 31.4 (95.8%); AA, 39.6 (94.6%); CV2, 8.0 (94.7%); CV3, 14.8 (93.7%); CV4, 24.6 (95.2%); CV5, 41.5 (98.8%); and AV, 47.8 (97.8%), where TA, AA, CV, and AV designate transverse arteriole, arcade arteriole, collecting venule, and arcade venule, respectively. The diameter reductions were heterogeneous for arterioles and venules, with significant variation in small arterioles (< 25 microns), which exhibited dimensions ranging from 61 to 100% of control, whereas larger arterioles ranged from 81 to 100% of control. The data were used with an anatomic model to estimate the systemic hematocrit decrease (0.73%) and the blood volume shifted from the microcirculation to the macrocirculation (3.74% of total control blood volume). These results agree well with direct measurements of the systemic hematocrit decrease and thus provide the microvascular basis for the rapid blood volume shift during moderate hemorrhage.

Adaptation, Physiological↗
PubMed1994

Mechanical equilibrium determines the fractal fiber architecture of aortic heart valve leaflets.

In this work, the structure of the aortic valve is derived from its function, which (in the closed-valve configuration) is to support a uniform pressure load. It is assumed that this load is transferred to the aortic wall by a one-parameter family of fibers under tension. The equation of equilibrium for this fiber structure turns out to be equivalent to the equation of motion of vortex lines in the self-induction approximation. The method of Buttke (J. Comput. Phys. 76:301-326, 1988) is used to solve these equations and, hence, to determine the fiber architecture of the aortic leaflets. Because of a singularity at the center of the aortic valve, the computed fiber architecture has a fractal character with increasing complexity at progressively smaller scales. The computed fiber architecture resembles the branching braided structure of the collagen fibers that support the real aortic valve.

Aortic Valve↗
PubMed1989

Alveolar dimensions and number: developmental and hormonal regulation.

We used three-dimensional reconstruction 1) to determine the effect of treating rats with dexamethasone, from age 4 to 13 days, on alveolar volume (v) and number (Na) and 2) to determine if v, Na, or both change between age 14 and 60 days. At age 14 days, v, Na, and gas exchange surface area (Sa) were (2.7 +/- 0.3) x 10(4) microns3, (20.2 +/- 2.1) x 10(6), and 832 +/- 29 cm2, respectively, in diluent-treated rats; in dexamethasone-treated 14-day-old rats the same parameters were (7.5 +/- 1.1) x 10(4) microns3, (9.9 +/- 1.6) x 10(6), and 733 +/- 16 cm2, respectively. At age 60 days, v, Na, and Sa were (7.1 +/- 0.4) x 10(4) microns3, (60.8 +/- 4.1) x 10(6), and 4,495 +/- 187 cm2 in diluent-treated rats and in rats treated with dexamethasone from age 4 to 13 days, v, Na, and Sa at age 60 days were (15.9 +/- 2.4) x 10(4) microns3, (25.8 +/- 3.8) x 10(6), and 3,424 +/- 203 cm2. We conclude that treatment with dexamethasone from age 4 to 13 days (the period of normal septation) resulted in larger alveoli at age 14 and 60 days, and in diluent- and dexamethasone-treated rats the increase in Sa between age 14 and 60 days is due, at least in part, to the formation of new alveoli.

Aging↗
PubMed1985

Vascular basis for regulation of nasal heat exchange in reindeer.

A hypothesis for the operation and control of nasal heat exchange in reindeer is presented that originated from studies of the nasal vascular anatomy and has been supported by physiological measurements as well as test experiments on a physical prototype model of the reindeer nose. A central theme of our hypothesis is that the nasal mucosa possesses arterial and venous retia that communicate by way of capillaries and arteriovenous anastomoses. During heat conservation the blood runs countercurrent in these retia, whereby a temperature gradient along the nasal mucosa can be maintained. During heat dissipation, however, the retia are perfused unidirectionally in the anterior direction, whereby the temperature gradient along the nasal mucosa is reduced and heat loss facilitated. In this situation cooled venous blood, routed by way of the dorsal nasal vein, may be distributed either to the caval veins directly, for general body cooling, or, by way of the cavernous sinus that encases the carotid rete, for selective cooling of the brain.

Animals↗
PubMed1991

Three-dimensional anatomy and renal concentrating mechanism. I. Modeling results.

Simulations were performed to test the hypothesis that the three-dimensional organization of the renal medulla is essential for formation of hypertonic urine. As in previous models, representations of loops of Henle, distal tubules, collecting ducts, and vasa recta and recent estimates of tubule characteristics were included in a simulation of NaCl, urea, and fluid transport. In addition, this model specifies the relative positions of the medullary structures. By assuming that the structure of the minimum functional unit is a vascular bundle surrounded by tubules and ascending vessels, we have represented the three-dimensional organization of the medulla by a cylindrically symmetric two-dimensional model. The resulting set of equations gives rise to a nonlinear boundary value problem with linear boundary conditions, which was solved numerically via quasi linearization. Compared with previous simulations, the concentrations predicted by this model more accurately match measured quantities in two regards. First, papillary tip concentrations of NaCl and urea are significantly higher, and, second, a monotonic increase in osmolarity is observed in the inner medulla. The three-dimensional organization permitted development of local concentration gradients, which are essential to the final result.

Absorption↗
PubMed1991

Three-dimensional anatomy and renal concentrating mechanism. II. Sensitivity results.

A mathematical model has been developed to simulate hypertonic urine formation in the renal medulla. The model uses published values of membrane transport parameters, as have other models, but is unique in its representation of the three-dimensional anatomy of the medulla. The model successfully predicts measured fluid flows, osmolarities, and NaCl and urea concentrations. The model results are presented in the companion to this paper [A. S. Wexler, R. E. Kalaba, D. J. Marsh. Am. J. Physiol. 260 (Renal Fluid Electrolyte Physiol. 29): F368-F383, 1991.]. In this paper we provide tests of the sensitivity of model performance to variations in the description of the anatomy and in membrane transport parameters. From these studies we conclude that 1) strict counterflow arrangements are required in the outer stripe to prevent loss of NaCl to the systemic circulation, 2) the radial organization in the inner stripe materially improves performance of the inner medulla, 3) radial organization of the inner medulla is essential to hypertonic urine formation there, 4) the model is most sensitive to variation in collecting duct parameters, and 5) reabsorption of urea in the distal tubule improves system performance. The results support the claim that the three-dimensional structure, as captured in the model, provides a crucial framework for the production of hypertonic urine.

Absorption↗
PubMed2000

A hydrodynamic mechanosensory hypothesis for brush border microvilli.

In the proximal tubule of the kidney, Na(+) and HCO(3)(-) reabsorption vary proportionally with changes in axial flow rate. This feature is a critical component of glomerulotubular balance, but the basic mechanism by which the tubule epithelial cells sense axial flow remains unexplained. We propose that the microvilli, which constitute the brush border, are physically suitable to act as a mechanosensor of fluid flow. To examine this hypothesis quantitatively, we have developed an elastohydrodynamic model to predict the forces and torques along each microvillus and its resulting elastic bending deformation. This model indicates that: 1) the spacing of the microvilli is so dense that there is virtually no axial velocity within the brush border and that drag forces on the microvilli are at least 200 times greater than the shear force on the cell's apical membrane at the base of the microvilli; 2) of the total drag on a 2.5-microm microvillus, 74% appears within 0.2 microm from the tip; and 3) assuming that the structural strength of the microvillus derives from its axial actin filaments, then a luminal fluid flow of 30 nl/min produces a deflection of the microvillus tip which varies from about 1 to 5% of its 90-nm diameter, depending on the microvilli length. The microvilli thus appear as a set of stiff bristles, in a configuration in which changes in drag will produce maximal torque.

Animals↗
PubMed1979

Deposition of particles in model airways.

The deposition of monodisperse aerosols in models of single lung airway bifurcations was studied. The particles, generated by spinning disk, were in the size range of 4.2--8.8 microns (density 1.3 X 10(3) kg . m-3). The model bifurcations were symmetrical, of physiological size, and based on established morphological data. The effects on percentage deposition (P) of flow rate (expressed as Reynolds' number. Re), particle diameter (d), tube diameter (D), and bifurcation angle (2 theta) were studied and a generalized expression for deposition based on these variables was obtained, viz., P = 11.04 X 10(5) Re (d/D)2.5sin3 theta. The applicability of this equation is discussed.

Aerosols↗
PubMed1980

Steady flow in a model of human central airways.

We studied the pressure-flow relationships and flow distribution under steady conditions in a model of human central airways, over a range of tracheal Reynolds' numbers (350-30,000) by using air or helium. We found that the Moody diagram [log coefficient of friction CF = delta P/[1/2 rho (V2/A2)] vs. log Reynolds' number (Re)] had a slope of -1 for Re less than 500, a slope 0 for Re greater than 10,000, and slopes between -1 and 0 for 500 less than or equal to Re less than or equal to 10,000. The distribution of flow among branches was dependent on tracheal Reynolds' number so that, as tracheal Reynolds' number increased, the upper lobes received proportionally less of the total flow than the lower lobes. Because the airways in the upper lobes generally had greater branching angles than those in the lower lobes, this result was consistent with the hypothesis that the effective resistances introduced by branching angles was flow dependent, increasing proportionally more the greater the angle.

Airway Resistance↗
PubMed1981

Oscillatory flow and quasi-steady behavior in a model of human central airways.

We studied biased oscillatory flow in a model of human central airways to examine under what conditions oscillatory flow deviated from steady flow. Although the steady flow resistance of the model was 25% less than the oscillatory flow resistance of the model at 15Hz, the overall inertance of the model did not change over the range of frequencies from 5 to 15 Hz, suggesting that frequency-dependent inertial distortion of velocity profiles did not alter central airway pressure-flow relationships over this frequency range. In a given terminal branch of the model, instantaneous oscillatory flow at 2 and 5 Hz could be predicted well from the steady flow distribution, but with increasing frequency the oscillatory flow from the branch deviated more from the steady flow predictions. A significant component of this deviation was due to a phase shift between predicted and measured oscillatory flow. We conclude that the major frequency-dependent behavior flow in the human central airways is a phasic redistribution of flow above 7 Hz, resulting from the asymmetric distribution of inertances in this structure.

Airway Resistance↗
PubMed1983

Lobar flow patterns in a hollow cast of canine central airways.

To demonstrate that the regional partitioning of flow in central airways is direction dependent, we compared inspiratory and expiratory lobar flows in a hollow rigid cast of canine lungs. Lobar flows were measured by a hot-wire anemometer, simultaneous tracheal flows by a pneumotachograph, and data displayed as (lobar) flow-(tracheal) flow curves. For tracheal airflows between 50 and 370 ml/s, we found that inspiratory apical flows were smaller, and basal flows larger, than corresponding expiratory distributions. This effect was largely extinguished by substituting a gas mixture of 85% He-15% N2; resulting regional flows were comparable to expiratory distributions obtained on room air. Thus canine central airways structure induces a kinematically irreversible flow pattern, consistent with a model of "cascade flow" in which branching structure modifies the distribution only of inspiratory flow. These results support Fowler's concept that lung regions fill and empty sequentially and may be pertinent to analyses of regional washout.

Animals↗
PubMed1983

Technique for making hollow central airway casts.

We have developed a method for producing rigid hollow casts of the central airways. Woods metal (melting point 70 degrees C) was poured into the airways at the trachea of an excised canine lung air-dried at a transpulmonary pressure of 30 cmH2O. The tissue was dissolved and removed, and the resulting negative cast was carefully trimmed leaving only airways with a diameter of 2 mm or greater. The cast was then electroplated with copper that provided a rigid jacket encasing the Woods metal. Heating this structure to 100 degrees C liquefied the Woods metal allowing the majority of it to flow out. Residual amounts were readily removed by a high-pressure steam hose attached to the trachea. Comparisons were made between airway diameters from in vivo radiographic bronchograms (DB) and direct measurements of the resulting trimmed Woods metal cast (DC). Linear regression analysis of these data indicated a high degree of correlation (DC = 0.98 DB + 0.03, r = 0.99). We concluded that this technique produces positive casts which provide a good representative model of the central airways.

Animals↗
PubMed1986

Beyond the principle of similitude: renormalization in the bronchial tree.

The average dimensions (diameter, length, and volume) of the airways in the mammalian bronchial tree, long thought to be exponential functions of the generation number, are shown to be power laws in generation number modulated by a harmonic variation. These data are satisfactorily described by means of a functional scaling relation--renormalization group property--between successive generations for the average variable of interest. This type of scaling may provide a mechanism for the morphogenesis of complex but highly stable structures.

Animals↗
PubMed1988

Convective exchange between the nose and the atmosphere.

It is generally accepted that there is little rebreathing of gas exhaled through the nose. A detailed physical model system has been used to quantify and identify the mechanisms responsible for this phenomenon. By the use of a cast of the upper respiratory tract and oscillating flows with a Reynolds number of 500 and nondimensional frequency of 1.6, corresponding to quiet tidal breathing through the nose, dye dilution measurements indicated an efficiency of tidal exchange of 0.95. Flow visualization studies performed to trace the expiratory flow, as well as the streamlines during steady inspiratory flow, support the hypothesis that the high efficiency of exchange is due to radical differences in the velocity fields between inspiratory and expiratory phases of this oscillatory flow. These findings confirm that convective gas exchange between the nose and the atmosphere is highly efficient; however, the underlying mechanism responsible for this exchange also maximizes the exposure of the respiratory system to aerosols contained in the ambient atmosphere.

Atmosphere↗
PubMed1990

Inability to separate airway from tissue properties by use of human respiratory input impedance.

Respiratory input impedance (Zrs) from 2.5 to 320 Hz displays a high-frequency resonance, the location of which depends on the density of the resident gas in the lungs (J. Appl. Physiol. 67: 2323-2330, 1989). A previously used six-element model has suggested that the resonance is due to alveolar gas compression (Cg) resonating with tissue inertance (Iti). However, the density dependence of the resonance indicates that is associated with the first airway acoustic resonance. The goal of this study was to determine whether unique properties for tissues and airways can be extracted from Zrs data by use of models that incorporate airway acoustic phenomena. We applied several models incorporating airway acoustics to the 2.5- to 320-Hz data from nine healthy adult humans during room air (RA) and 20% He-80% O2 (HeO2) breathing. A model consisting of a single open-ended rigid tube produced a resonance far sharper than that seen in the data. To dampen the resonance features, we used a model of multiple open-ended rigid tubes in parallel. This model fit the data very well for both RA and HeO2 but required fewer and longer tubes with HeO2. Another way to dampen the resonance was to use a single rigid tube terminated with an alveolar-tissue unit. This model also fit the data well, but the alveolar Cg estimates were far smaller than those expected based on the subject's thoracic gas volume. If Cg was fixed based on the thoracic gas volume, a large number of tubes were again required. These results along with additional simulations show that from input Zrs alone one cannot uniquely identify features indigenous to alveolar Cg or to the respiratory tissues.

Acoustics↗
PubMed1991

Applications of fractal analysis to physiology.

This review describes approaches to the analysis of fractal properties of physiological observations. Fractals are useful to describe the natural irregularity of physiological systems because their irregularity is not truly random and can be demonstrated to have spatial or temporal correlation. The concepts of fractal analysis are introduced from intuitive, visual, and mathematical perspectives. The regional heterogeneities of pulmonary and myocardial flows are discussed as applications of spatial fractal analysis, and methods for estimating a fractal dimension from physiological data are presented. Although the methods used for fractal analyses of physiological data are still under development and will require additional validation, they appear to have great potential for the study of physiology at scales of resolution ranging from the microcirculation to the intact organism.

Animals↗