PubMed Health⌕ Search

SEARCH · PubMed Health

Results for “Models, Theoretical”

Explore indexed PubMed citations for clinical trials, systematic reviews and public health research. Read source abstracts and follow each citation to its original PubMed record.

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

At least 631 records · Page 35Linked to original sources
PubMed2000

A dynamic game-theoretic model of parental care.

We present a model in which members of a mated pair decide whether to care for their offspring or desert them. There is a breeding season of finite length during which it is possible to produce and raise several batches of offspring. On deserting its offspring, an individual can search for a new mate. The probability of finding a mate depends on the number of individuals of each sex that are searching, which in turn depends upon the previous care and desertion decisions of all population members. We find the evolutionarily stable pattern of care over the breeding season. The feedback between behaviour and mating opportunity can result in a pattern of stable oscillations between different forms of care over the breeding season. Oscillations can also arise because the best thing for an individual to do at a particular time in the season depends on future behaviour of all population members. In the baseline model, a pair splits up after a breeding attempt, even if they both care for the offspring. In a version of the model in which a pair stays together if they both care, the feedback between behaviour and mating opportunity can lead to more than one evolutionarily stable form of care.

Animals↗
PubMed1994

A theoretical model for gas transport and acid/base regulation by blood flowing in microvessels.

An investigation was made of the coupling between O2 and CO2 transport by blood flowing in microvessels. The blood was treated as two continuous coexisting phases: a red blood cell (RBC) phase and a plasma phase. The microvessel was divided into two regions: the central, RBC-rich and the outer, cell-free region. The radial distribution of RBCs and transport of various species due to bulk convection and radial diffusion were taken into account. Chemical and transport processes which were included in the model are (1) interactions of hemoglobin with O2 and CO2, (2) the Bohr and Haldane effects (the inter-dependence of O2/CO2 transport), (3) CO2 hydration-dehydration reactions, (4) buffering actions of hemoglobin and plasma proteins, and (5) anion exchange across the red cell membrane. The governing equations of the model subjected to the imposed inlet and boundary conditions were solved numerically to provide the concentration distributions of various species in blood that are important in the simultaneous gas exchange and pH regulation process. Predictions of the new model of simultaneous O2/CO2 transport by flowing blood were shown to be in excellent agreement with prior workers' experimental results from large artificial membrane tubes. A previous mathematical model which treats blood as a homogeneous continuum and uses a local chemical equilibrium approximation to describe the gas transport was shown to satisfactorily predict the amount of O2 transport for blood oxygenation accompanied by CO2 elimination. However, the previous model significantly underpredicts O2 transfer for blood deoxygenation accompanied by CO2 uptake. Furthermore, the previous model disagrees substantially with the CO2 transport results under both oxygenation and deoxygenation conditions.

Acid-Base Equilibrium↗
PubMed2002

Can we tell how a community was constructed? A comparison of five evenness indices for their ability to identify theoretical models of community construction.

Evenness indices provide a simple measure of community structure. It might therefore be possible to use them to identify the process by which a community has been assembled. To test whether this is practicable, we constructed simulated community samples using five stochastic models of community construction, proposed by Tokeshi. We then examined the ability of five evenness indices to identify the model under which the community samples were produced. Each model produced samples with a range of evenness values, but mean evenness differed between the models. The dominance decay (DD) model produced community samples with the greatest evenness, followed by MacArthur fraction (MF). Evenness was lowest under the dominance pre-emption (DP) model. The differences between models were quite consistent across indices and consistent between 5-species and 15-species communities. Samples produced under the DD and MF models varied least in evenness between 5-species and 15-species samples, and those produced under the random assortment (RA) model varied most, irrespective of the evenness index used. Evenness varied considerably between replicate samples, as expected with stochastic processes. In 5-species communities, the greatest robustness in evenness across replicates was seen using indices O or E'. In 15-species communities, O and E(Q) were the most robust. The index best able to identify the model which had generated the sample differed between models and with species richness. If the model of interest is not known in advance, the best index for identifying the generating model is E(var) for communities of more than 10 species for RF, RA and DP models and O for other communities. The number of samples required in a data set before it could be effectively identified was, for example, more than 30 for 5-species samples produced under the random fraction (RF) model, and 3 for 15-species RF samples. In contrast, a 5-species DD data set could be effectively identified when it contained 15 or more samples. We conclude that evenness can be used to identify the process by which the community has been constructed, out of the five models considered here. The best evenness index for doing this varies with the species richness and the evenness of the community but we can suggest the use of O without knowing the model because its the more stable one against species richness and the more robust and unbiased against simulated samples and it encompass a good discriminating power between the models in most of the cases studied.

Abstracting and Indexing↗
PubMed1976

A theoretical model of the respiratory function of blood.

The model of the respiratory function of blood as outlined here is judged to be a powerful tool for the evaluation of the potential effects of changes in blood as well as tissue parameters upon the supply of oxygen to tissue. The erythrocyte-plasma disequilibria in lung and systemic capillaries which is a consequence of the slowness of plasma CO2 hydrolysis are not significant in terms of the delivery of oxygen to tissue, but result in slightly lower unloading of CO2 in the lungs compared to that which might occur at full equilibration. However, the effects might be of greater interest when relating pH values measured by in vivo sampling to actual values prevailing within the capillaries. The physiological importances of hyperventilation and elevated DPG levels in anemia cannot as yet be clarified. If anything, it appears that increased DPG is a compensatory mechanism to restore proper O2-Hb affinity in alkalosis, while the purpose for an alkalosis is not obvious.

Anemia↗
PubMed1991

A theoretical model for testis thermoregulation.

Studies going back as far as the early 1920's show that there is a clear relationship between testis temperature and semen quality. The most intriguing question is whether there is a mechanism of thermoregulation which, in the human, maintains testis temperature within certain limits that permit euspermia. Thermoregulation is defined as maintaining some specified (optimum?) temperature plus or minus an error over internal and ambient loss factors. A computer Model has been evolved which contains no regulation or feedback. It appears to predict human testis temperature data gathered in earlier studies. The Model accounts for countercurrent heat exchange in the pampiniform plexus and predicts, with an open loop analysis, that there is no feedback or regulation. As far as thermoregulation is concerned, there appear to be no first-order effects taking place in the human testis. This suggests that ambient temperature changes cause corresponding changes in testis temperature. Also any internal changes in thermal properties such as core temperature variations or variability of the countercurrent heat exchanger will also cause temperature change. Testis temperature as predicted by this Model is the result of the heat energy entering the testis from arterial inflow minus the venous outflow and heat loss from the scrotum. The Model predicts that the heat exchanger will function to provide precooling of arterial blood as external temperatures drop but will fail to precool effectively as temperature rises. This is predicated on the fact that the countercurrent heat exchanger becomes less effective as the temperature gradient across the exchanger becomes smaller and less heat energy is able to be transferred from arterial flow to venous flow. The Model also predicts that any diminution of the heat exchanger mechanism either from reduced venous flow or restricted scrotal heat loss will result in higher testis temperature. Lastly, the Model correctly predicted that febrile patients would experience elevated testis temperature during periods of elevated core temperature. Since more heat energy is available in the arterial blood, more heat energy is delivered to the testis under these conditions. In the human it appears that any internal or external factor causing a temperature change will not trigger or activate a feedback mechanism to control the resulting testis temperature. A major factor in subfertile semen may be the inability to check excessive temperature of the testis which impairs the ability to produce and mature fertile spermatozoa.

Body Temperature↗
PubMed1998

The relationship of pulsatile cerebrospinal fluid flow to cerebral blood flow and intracranial pressure: a new theoretical model.

An electrical-equivalent circuit model of the cerebrovascular system is proposed, components of which directly relate to cerebrospinal fluid (CSF) compartment compliance and the determination of intracranial pressure (ICP). The model is based on three premises: 1) Under normal, physiologic conditions, the conversion of pulsatile arterial to nonpulsatile venous flow occurs primarily as a result of arterial compliance. Nonpulsatile venous flow is advantageous because less energy is required to maintain constant flow through the venous system, which comprises 75-80% of total blood volume. 2) Dynamic CSF movement across the foramen magnum is the primary facilitator by which intracranial arterial expansion occurs. Interference of the displacement of CSF during systole results in pulsatile venous flow and increased venous flow impedance. 3) Tissue hydrostatic pressure (here defined as ICP) is a dependent variable which is a function of capillary hydrostatic pressure and the osmotic/oncotic pressure gradient created by the blood-brain-barrier (BBB). An interference of transcranial CSF movement results in a decrease in cerebral blood flow (CBF) due to inertial effects impeding pulsatile venous flow. Feedback regulation in response to this decreased CBF leads to arteriolar vasodilatation (decreased resistance), thereby lowering the pressure difference between internal carotid and capillary pressures. Assuming no changes in the BBB potential, ICP increases linearly as capillary pressure increases.

Blood Flow Velocity↗
PubMed1980

A theoretical model for studying the rate of oxygenation of blood in pulmonary capillaries.

A mathematical analysis of the process of gas exchange in the lung is presented taking into account the transport mechanisms of molecular diffusion, convection and facilitated diffusion of the species due to haemoglobin. Since the rate at which blood gets oxygenated in the pulmonary capillaries is very fast, it is difficult to set up an experimental study to determine the effects of various parameters on equilibration rate. The proposed study is aimed at determining the effects of various physiological parameters on equilibration rate in pathological conditions. Among the significant results are that 1. dissolved oxygen takes longer to achieve equilibration across the pulmonary membrane and carbon dioxide attains equilibration faster, 2. the equilibration length increases with increase in blood velocity, haemoglobin concentration, calibre of pulmonary capillaries and fall in alveolar PO2, 3. the alveolar PCO2 and forward and backward reaction rates of haemoglobin with CO2 do not materially affect the equilibration rate or length. 4. At complete equilibration, by the end of the pulmonary capillary 92% of the total haemoglobin has combined with oxygen and 8% free pigment is left which is present as carbamino haemoglobin, met haemoglobin, carboxy haemoglobin etc. These results are of some importance for anaemic conditions, muscular exercise, meditation, altitude physiology, hypo-ventilation, hyperventilation, etc.

Capillaries↗
PubMed1976

A theoretical model for the eye of new-born infants.

A Gullstrand model for the eye of new-born infants is proposed, based on intra-ocular distances obtained by ultrasonography and lens radii measured on sections of deep-frozen preparations (v. Pflugk, 1909). Refractive indices of the media are assumed to be equal or nearly so to those of the adult eye, since no experimental data are available. The focal length of the model is 15.74 mm, the power 84.8 dptr, that of the lens alone 43.4 dptr, and the refractive error +2.8 dptr. A simplified model where the inner surface of the cornea is omitted, has a focal length of 15.63 mm, a power of 85.3 dptr and a refractive error of +2.4 dptr. When the data of v. Pflugk are analyzed statistically it is found that the spread of the lens power is considerably smaller than that which would be expected from the spread of the radii.

Anterior Chamber↗
PubMed1981

Oxygen transport and the function of myoglobin. Theoretical model and experiments in chicken gizzard smooth muscle.

We studied the steady-state oxygen transfer across thin layers of respiring chicken gizzard smooth muscle and compared three models for oxygen consumption with respect to their influence on the facilitation of oxygen diffusion by myoglobin. These models assumed zero-order, Michaelis-Menten or exponential kinetics. The transport equation was solved for these models with simultaneous oxygen facilitation assuming chemical equilibrium between oxygen and myoglobin. Experimental flux data were obtained in two situations: a) high oxygen pressure throughout the layer of tissue providing maximum oxygen consumption and oxygen permeability, and b) anoxic conditions in part of the layer and with submaximal oxygen consumption and desaturation of myoglobin. Measurements in the presence of functional myoglobin were compared with data obtained after abolishing the transport function of myoglobin by application of 1 kPa carbon monoxide. It was found that oxygen consumption interferes with the facilitation effect. The oxygen pressure at half maximum oxygen consumption in the Michaelis-Menten model was 0.3 +/- 0.1 (S.E) kPa. The facilitation of the oxygen transport by myoglobin was 50 to 100% of the maximum value to be expected on the basis of the prevailing myoglobin concentration.

Amobarbital↗