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Biomedical subjects

E G Cravalho

Publications and source records attributed to E G Cravalho.

At least 37 records · Page 2Linked to original sources
PubMed1989

A model of acoustic transmission in the respiratory system.

A theoretical model of sound transmission from within the respiratory tract to the chest wall due to the motion of the walls of the large airways was developed. The vocal tract, trachea, and the first five bronchial generations are represented over the frequency range from 100 to 600 Hz by an equivalent acoustic circuit. This circuit allows the estimation of the magnitude of airway wall motion in response to an acoustic perturbation at the mouth. The radiation of sound through the surrounding lung parenchyma is represented as a cylindrical wave in a homogeneous mixture of air bubbles in water. The effect of thermal losses associated with the polytropic compressions and expansions of these bubbles by the acoustic wave is included and the chest wall is represented as a massive boundary to the wave propagation. The model estimates the magnitude of acceleration over the extrathoracic trachea and at three locations on the posterior chest wall in the same vertical plane. The predicted spectral characteristics of transmission are consistent with previous experimental observations. This theoretical approach suggests that the locations of the spectral peaks are a strong function of the geometry and the wall properties of the airways, while the attenuation at higher frequencies is primarily associated with the absorption of sound in the parenchyma.

Animals↗
PubMed1986

Low temperature light microscopy and its application to study freezing in aqueous solutions and biological cell suspensions.

The freezing of biological cell suspensions can be understood in terms of ice formation in the external suspension medium and the cellular reactions to the changing environment. Cryomicroscopy allows a quantitative analysis of both categories of phenomena. Besides freezing stages of appropriate thermal design, the components used for that purpose include a microcomputer (PSI 80) based control system, an image analysis system (Intellect 100) and a spectrophotometer (MPV compact). The investigation of extracellular ice formation is focused on the following effects: The redistribution of solutes in the residual liquid and the resulting concentration profiles are determined photometrically or densitometrically. The transitions between various morphologies of the ice-liquid phase boundary (planar-cellular-dendritic) can be related to interface instability theories. With respect to solute segregation, the studies also involve the formation of bubbles from supersaturated gaseous solutes and freezing potentials resulting from the differential incorporation of cations and anions into the solid phase. The interaction between particles or cells and the advancing ice front is determined from critical interface velocities marking the transition between repulsion and entrapment. The effects of freezing on biological cells are studied mainly with blood cells, especially lymphocytes. The water efflux due to osmotical gradients across the membrane yields volume shrinkage curves which are recorded and analysed from video images for various cooling rates. Beyond a certain threshold cooling rate, intracellular ice starts to form, and different crystallization morphologies can be detected. The intracellular crystallization temperatures depend on cooling and warming rates as well as on the presence of penetrating cryoadditives. A fluorescence viability is used to determine the percentage of damaged cells immediately after thawing.

Cells↗
PubMed1986

Finite difference analysis of respiratory heat transfer.

A numerical computer model of heat and water transfer within the tracheobronchial tree of humans was developed based on an integral formulation of the first law of thermodynamics. Simulation results were compared with directly measured intraluminal airway temperature profiles previously obtained in normal human subjects, and a good correlation was demonstrated. The model was used to study aspects of regional pulmonary heat transfer and to predict the outcomes of experiments not yet performed. The results of these simulations show that a decrease in inspired air temperature and water content at fixed minute ventilation produces a proportionately larger increase in heat loss from extrathoracic airways relative to intrathoracic, whereas an increase in minute ventilation at fixed inspired air conditions produces the opposite pattern, with cold dry air penetrating further into the lung, and that changes in breathing pattern (tidal volume and frequency) at fixed minute ventilation and fixed inspiratory-to-expiratory (I/E) ratio do not affect local air temperature profiles and heat loss, whereas changes in I/E ratio at fixed minute ventilation do cause a significant change.

Asthma↗
PubMed1984

An analytical method to evaluate cooling rates during cryopreservation protocols for organs.

A simple, graphical procedure has been established to estimate the cooling rates at a specified location and temperature in a biological medium frozen by imposing a constant cooling rate at the outer surface. The graphical procedure also facilitates the determination of the outer surface temperature at the instant a given temperature has been attained in a specific location. The results obtained through a numerical study using finite elements can be applied to biological media with various chemical compositions including different concentrations of cryoprotective agents.

Freezing↗
PubMed1981

Blood freezing to nearly absolute zero temperature: -272.29 degrees C.

Human erythrocytes were collected in ACD anticoagulant, glycerolized with an equal volume of 8.6 M glycerol, 8 percent glucose, 1 percent fructose, and 0.3 percent Na2 EDTA solution, sealed in glass ampules, and frozen to -272.29 degrees C (0.86 degrees K) in liquid helium under high vacuum. Biological assays performed after thawing and three-stage dialysis to remove the glycerol showed less than 1 percent hemolysis. Intracellular potassium, 2,3 DPG, and ATP were within the normal range.

Blood↗
PubMed1979

Water permeability of yeast cells at sub-zero temperatures.

A combined cryomicroscopic-multiple nonlinear regression analysis technique has been used to determine the water permeability of the yeast cell Saccharomyces cerevisiae during freezing. The time rate of change in volume of "supercooled" yeast cells was photographically monitored using a "cryomicroscope" which is capable of controlling in a programmable manner both the temperature and the time rate of change in temperature of the cell suspension being studied. Multiple nonlinear regression analysis together with a thermodynamic model of cell water transport during freezing was then used to statistically deduce the subzero temperature dependence of the cell water permeability. The water permeability process for S. cerevisiae being cooled at subzero temperatures was found to be rate-limited by the passage of water through either the plasmalemma, the cell wall, or a combination of these two permeability barriers. The hydraulic water permeability coefficient for yeast at 20 degrees C is approximately 1--2 x 10(-13) cm3/dyne sec, if extrapolation from subzero temperatures to room temperature is permissible, while the apparent activation energy governing the permeability process at subzero temperatures is approximately 45--68 kJ/mol (11--16 kcal/mol).

Cell Compartmentation↗
PubMed1977

Effect of solution non-ideality on erythrocyte volume regulation.

A non-ideal, hydrated, non-dilute pseudo-binary salt-protein-water solution model of the erythrocyte intracellular solution is presented to describe the osmotic behavior of human erythrocytes. Existing experimental activity data for salts and proteins in aqueous solutions are used to formulate van Laar type expressions for the solvent and solute activity coefficients. Reasonable estimates can therefore be made of the non-ideality of the erythrocyte intracellular solution over a wide range of osmolalities. Solution non-ideality is shown to affect significantly the degree of solute polarization within the erythrocyte intracellular solution during freezing. However, the non-ideality has very little effect upon the amount of water retained within erythrocytes cooled at sub-zero temperatures.

Biophysical Phenomena↗
PubMed1976

A membrane model describing the effect of temperature on the water conductivity of erythrocyte membranes at subzero temperatures.

Thermodynamic models show that the loss of intracellular water from human erythrocytes during freezing depends heavily upon the water conductivity of the erythrocyte membrane. These calculations, which are based on the simple extrapolation of ambient conductivity data to subzero temperatures, show that more than 95% of cell water is transferable during freezing, whereas experiments show that at least 20% of cell water is retained. A study of the effects of different published values for the membrane water conductivity on cell water retained during freezing shows that this discrepancy may be a consequence of the simple extrapolation procedure. For a homogeneous membrane system, absolute reaction rate theory was used to develop a surface-limited permeation model that includes the resistance to the flow of water not only through the interior region of the membrane but also across possible rate-limiting barriers at the solution-membrane interfaces. The model shows that it is unlikely that a single rate-limiting process dominates water transport in the red cell as it is being cooled from ambient to subzero temperatures. The effective membrane conductivity at subzero temperatures could possible be much lower than a simple extrapolation of existing data would predict. With the aid of this model analytical predictions of intracellular water during freezing are more consistent with experimental observations.

Blood↗