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Biomedical subjects

H D Papenfuss

Publications and source records attributed to H D Papenfuss.

15 recordsLinked to original sources
PubMed1995

Central retinal venous outflow pressure.

BACKGROUND: Does the venous collapse phenomenon provide the possibility of venous dynamometry? METHOD: A technical model is presented which allows analysis of the conditions of the collapse of the central retinal vein in vitro. The conditions of the venous collapse were analysed with regard to intraocular pressure, intravasal pressure in the outflow of the central retinal vein and the overall perfusion. For clinical measurements dynamometry of the venous collapse is performed parallel to the experimental setting. RESULTS: The experiment reveals identical results for venous outflow pressure measured by venous dynamometry and by intravasal pressure detector. Venous dynamometry in vivo means that we use the onset of the venous collapse phenomenon to register the pressure in the central retinal vein at the point where it leaves the eye. Using this technique, retroocular obstruction of the venous outflow may be assessed. The venous outflow pressure itself depends on the venous flow resistance, intracranial pressure and arterial perfusion pressure. Any disorder of these three parameters may be assessed when the absolute venous outflow pressure is registered. CONCLUSION: The venous collapse phenomenon enables us to determine the venous outflow pressure. Clinical applications have proven promising.

Aged↗
PubMed1987

Transvascular exchange of fluid and plasma proteins.

A theoretical model of transvascular exchange of fluid and plasma proteins in the microcirculation is developed based on fundamental laws of the fluid mechanics and on phenomenological transport equations of the irreversible thermodynamics. Intravascular axial changes of the pressure, flow and plasma protein concentration are taken into account as well as axial gradients of vascular permeability. Proper nondimensionalization of the resulting equations leads to the identification of dimensionless parameters which combine the transport characteristics of the endothelial wall and the intravascular flow resistance. In the theory, the dependence of the reflection coefficient on the transport coefficients of the vascular wall and on the plasma protein concentration is established. The model is applied to the cat mesentery and the rat intestinal muscle. The numerical simulations indicate that taking into account vascular protein permeability yields considerable differences in the axial distribution of the plasma protein concentration and transvascular fluxes in comparison with the case of protein impermeability of the endothelial wall. The results show that the maximum of the transvascular fluid and plasma protein movement resides at the site of the small venules while a minimum of the exchange occurs at the site of the midcapillaries.

Animals↗
PubMed1987

The effect of the gradient of vascular permeability on fluid and plasma protein exchange in the mesenteric microcirculation.

The present work is a theoretical analysis of the intramural and transmural flows of fluid and plasma proteins. The transvascular fluxes are treated on the basis of a phenomenological approach resulting from the irreversible thermodynamics. The concept leads to a generalization of the well known Starling equation for the fluid exchange and to an additional equation which accounts for the exchange of plasma proteins. The key element in the theory is the incorporation of axial variations of the vascular permeability in the model. Numerical results are obtained for the cat mesentery taking into account the particular angioarchitecture of that tissue. The strength of the present model lies in the precise localization of the transvascular fluxes which results from the differential treatment of the fluid dynamic and mass transfer problem. The results confirm vital microscopic findings of various authors that fluid filtration and the plasma exchange occur predominantly on the arteriolar and venular side of the microcirculatory bed. In contrast, the exchange in the capillaries is relatively small regardless of the actual value of the gradient of permeability. In the organ specific situation studied, the transvascular fluxes are directed outward everywhere to the tissue. The general effect of the vascular gradient of permeability is to enhance the local transvascular fluxes on the venous side.

Blood Proteins↗
PubMed1985

Vasomotion and transvascular exchange of fluid and plasma proteins.

The effect of spontaneous arteriolar vasomotion on the transvascular exchange of fluid and plasma proteins has been studied theoretically. The model combines fluid dynamic principles with a phenomenological approach to transvascular exchange on the basis of irreversible thermodynamics. The analytical treatment of the intravascular flow together with consideration of local changes of the morphology and vascular membrane characteristics makes it possible to determine the spatial and temporal variation of transvascular fluid and plasma protein fluxes. Since the undulations of the arteriolar diameter have low frequencies (3 to 10 per cycle), the transvascular fluxes are in phase with the vasomotion. The model combines hemodynamics, microhemorheology and mass transfer at the vessel wall and provides pressures, velocities, plasma protein concentration, hematocrit and apparent blood viscosity at any position in the microvascular bed at any time. In addition, global parameters as the total filtration rate and the total transvascular mass flow rate of plasma proteins are determined as functions of time. Numerical results are obtained for the cat mesentery with the terminal arteriole exhibiting vasomotion. The vascular arrangement studied comprised a terminal arteriole, capillaries and a venule together with appropriate side branches. The results show dramatic temporal changes in the hemodynamic parameters. Fluid filtration occurs along the entire vascular length at all times except during vasoconstriction when small absorption rates occur on the venous side. The transvascular fluxes achieve maximum values either on the arteriolar or on the venular side depending on the moment of the vasomotion cycle. The transvascular exchange rates in the mid-capillaries are generally low.

Animals↗
PubMed1967

Aspects of clock resetting in flowering of xanthium.

Flowering is induced in Xanthium strumarium by a single dark period exceeding about 8.3 hours in length (the critical night). To study the mechanism which measures this dark period, plants were placed in growth chambers for about 2 days under constant light and temperature, given a phasing dark period terminated by an intervening light period (1 min to several hrs in duration), and finally a test dark period long enough normally to induce flowering. In some experiments, light interruptions during the test dark period were given to establish the time of maximum sensitivity.If the phasing dark period was less than 5 hours long, its termination by a light flash only broadened the subsequent time of maximum sensitivity to a light flash, but the critical night was delayed. In causing the delay, the end of the intervening light period was acting like the dusk signal which initiated time measurement at the beginning of the phasing dark period.If the phasing dark period was 6 hours or longer, time of maximum sensitivity during the subsequent test dark period was shifted by as much as 10 to 14 hours. In this case the light terminating the phasing dark period acted as a rephaser or a dawn signal.Following a 7.5-hour phasing dark period, intervening light periods of 1 minute to 5 hours did not shift the subsequent time of maximum sensitivity, but with intervening light periods longer than 5 hours, termination of the light acts clearly like a dusk signal. The clock appears to be suspended during intervening light periods longer than 5 to 15 hours. It is restarted by a dusk signal. There is an anomaly with intervening light periods of 10 to 13 hours, following which time of maximum sensitivity is actually less than the usual 8 hours after dusk.Ability of the clock in Xanthium to be rephased, suspended, restarted, or delayed, depending always upon conditions of the experiment, is characteristic of an oscillating timer and may confer upon this plant its ability to respond to a single inductive cycle. It is suggested that phytochrome may influence only the phase of the clock and not other aspects of flowering such as synthesis of flowering hormone.

Journal Article↗
PubMedDate not supplied

Mathematical model of the filtration in the vascular network of the ophidian glomerulus.

The present work is a mathematical model of the fluid filtration in the glomerular network occurring in snakes. The model is based on the differential form of Starling's hypothesis and takes into account the angioarchitecture of the network and the behaviour on the microrheology of blood with nucleated red cells. The model predicts the hemodynamics and the transvascular fluxes in each vascular segment within the network. The model is applied to a vascular network of the glomerulus of the garter snake. A value of 0.593 microns/(s.mmHg) was determined for the hydraulic conductivity of the glomerular capillaries using the geometrical data of the network together with experimental data for the pressures and the blood flow rate reported in the literature. The analysis shows that the local filtration rates cover a wide range. In some of the vascular segments, the filtration leads to such a high increase in colloid-osmotic pressure that the level of the transvascular hydrostatic pressure difference is reached. Mathematical simulations of the variation of the glomerular blood flow rate due to vasoactivity of preglomerular arterioles show the effect on the filtration rate and the hemorheologic parameters.

Animals↗