Thesis (Ph. D.)--Oregon State University, 2008. / Printout. Includes bibliographical references (leaves 126-138). Also available on the World Wide Web.

On sait depuis longtemps que l’exposition à l’altitude est associée à une réduction de l’aptitude aérobie. Différentes hypothèses ont été posées pour expliquer cette limitation à l’effort en hypoxie (une limitation ventilatoire ou diaphragmatique, une altération de la diffusion pulmonaire et une disconcordance entre de la perfusion et la diffusion tissulaire, etc.) mais généralement, la limitation de l’effort aérobie en hypoxie est attribuée à une diminution du transport sanguin de l’O2 (TO2) parc convection vers les muscles. Le TO2 dépend du débit cardiaque (Q) et du contenu artériel en O2 (CaO2). <p>Le CaO2 est diminué en altitude à cause d’une diminution de la pression partielle inspirée en O2. Cependant, le chémoréflexe hypoxique tente de contrebalancer cet effet en élevant la ventilation et en diminuant la pression alvéolaire en CO2 afin de maintenir la pression alvéolaire en O2 constante. De plus, avec l’acclimatation, le rein produit de l’érythropoïétine permettant au taux d’hémoglobine d’augmenter. Ces deux principales adaptations à l’altitude ramènent le CaO2 à sa valeur de base du niveau de la mer en 2 à 3 semaines passées à 5000 m d’altitude mais sans amélioration de l’aptitude à l’effort aérobie.<p>L’exposition à l’altitude est aussi associée à une diminution du Q maximal. Les mécanismes à l’origine de cette limitation du Q maximal restent, à l’heure actuelle, incompris. Les principales explications évoquées sont, une diminution de la réserve chronotrope, une diminution de la commande nerveuse centrale vers le cœur ou une diminution de la demande périphérique. Récemment, des études sur des sujets sains en hypoxie suggérèrent qu’au moins une partie de la limitation du Q maximal à l’effort est liée à une élévation de la postcharge ventriculaire droite suite à l’hypertension pulmonaire induite par l’hypoxie. C’est cette hypothèse que nous avons voulu vérifier dans une première étude.<p>Nous avons étudié l’effet d’une inhibition de l’hypertension pulmonaire d’altitude par le sildénafil, un inhibiteur de la phosphodiestrérase-5, chez des sujets sains, en normoxie, en hypoxie aiguë et en hypoxie chronique. Les résultats de cette étude ont confirmé l’effet vasodilatateur pulmonaire du sildénafil et une augmentation de la VO2max en hypoxie aiguë. Cependant, la prise de ce dernier était couplée à une amélioration de l’oxygénation, si bien que l’élévation de la performance aérobie observée en hypoxie aiguë sous sildénafil ne pouvait être entièrement attribuée à une réduction de l’hypertension pulmonaire. <p>Nous conclurent que cette amélioration de la performance était probablement d’avantage liée à une amélioration de l'oxygénation qu’à un effet vasodilatateur pulmonaire.<p>Les résultats équivoques obtenus lors de cette première étude nous ont incité à tester les effets d’une amélioration de l’oxygénation sur la performance aérobie en haute altitude. Pour ce faire, quinze sujets sains ont été testés au niveau de la mer et après acclimatation à 4700 m d’altitude soit sous placebo, soit sous acétazolamide, un inhibiteur de l’anhydrase carbonique augmentant l’oxygénation par stimulation ventilatoire en réponse à une acidose métabolique. La prise d’acétazolamide n’eut aucun effet sur l’hémodynamique pulmonaire et sur la VO2max et la charge maximale. Nous avons toutefois observé qu’une amélioration de l’oxygénation durant l’effort retarde l’apparition du seuil ventilatoire améliorant ainsi la phase aérobie de l’effort. Cette étude confirme donc qu’une élévation du CaO2 permet une amélioration de l’aptitude aérobie. <p>Finalement, la dernière étude a pour but d’étudier les effets isolés d’une vasodilatation pulmonaire sur la performance aérobie en altitude. Les résultats d’une étude préliminaire montrent que l’inhibition de la vasoconstriction hypoxique par un agent pharmacologique antagoniste des récepteurs de l’endothéline ETA et ETB, le bosentan, permet une élévation de l’aptitude aérobie en hypoxie aiguë, sans effets sur l’oxygénation, confirmant ainsi notre hypothèse initiale qu’une postcharge ventriculaire droite augmentée en hypoxie peut contribuer à une limitation de l’aptitude à l’effort aérobie en hypoxie. <p><p>Conclusions :<p>L’ensemble de nos résultats suggère que l’aptitude aérobie en altitude est déterminée par le transport d’O2 qui peut être augmenté par manipulation pharmaceutique du débit ventriculaire droit maximal après inhibition de la vasoconstriction pulmonaire hypoxique (bosentan), amélioration de l’oxémie (acétazolamide) ou des deux (sildénafil).<p> / Agrégation de l'enseignement supérieur en kinésithérapie et réadaptation / info:eu-repo/semantics/nonPublished

No description available.

The profusion of observational data made available by the Venus Express and previous space missions, increases our need to develop numerical tools to interpret the data and improve our understanding of the Venus meteorology. The main objective of this work is to develop an improved Venus general circulation model and to study the most likely mechanisms driving the atmosphere to the current observed circulation. Our new model is an extension of a simplified version and includes a new radiative transfer scheme and convection and an adapted boundary layer scheme and dynamical core that take into account the dependence of the heat capacity with temperature, at constant atmospheric pressure. The new radiative transfer formulation implemented is more suitable for Venus climate studies than previous works due to its easy adaptability to different atmospheric conditions. This flexibility of the model was very important in this work to explore the uncertainties on the lower atmospheric conditions such as the gas absorption and the possible presence of aerosols near the surface. The new general circulation model obtains, after long periods of integration, a super-rotation phenomenon in the cloud region quantitatively similar to the one observed. However, this phenomenon is sensitive to some radiative parameters such as the amount of the solar radiative energy absorbed by the surface and the amount of clouds. The super-rotation in the model is formed due to the combined influence of the zonal mean circulation, thermal tides and transient waves, and the main mechanisms involved are identified and studied. In this process the momentum transported by the semidiurnal tide excited in the upper clouds has a key contribution. These migrating waves transport prograde momentum mainly from the upper atmosphere to the cloud region. In this work we also explored the model parameters to gain a better understanding of the effect of topography, the diurnal cycle and convective momentum mixing. In general the results showed that: the topography seemed capable of sustaining stronger global super-rotation; without diurnal cycle the strong winds in the cloud region are not produced; the convective momentum mixing experiment did not lead to significant changes. A simple experiment done advecting the UV absorber in the atmosphere, qualitatively showed several atmospheric phenomena that are important for the distribution of clouds. Among them is the presence of a region of low permeability isolating the polar vortex. This last experiment also showed that when increasing the amount of UV absorption in the upper cloud region the winds get stronger. Following the interpretation of observational data using numerical models, we also used a simplified version of the general circulation model to assess the accuracy of zonal wind retrievals from measured temperatures using the cyclostrophic thermal wind equation in the Venus mesosphere. From this analysis we suggest a method which better estimates the lower boundary condition, and improves the consistency of the results at high latitudes when compared with cloud tracking measurements.

It has been deduced from indirect evidence that significant adjustments of vascular resistance take place in the venous network when blood flow changes in a organ. In the following experiments, we attempted to test the hypotheses that changes in postcapillary resistance in skeletal muscle may be due to changes in venous diameter, changes in the number of venules with blood flow, and/or changes in the apparent viscosity of blood in venules. The hypotheses were tested by observing the response of cat sartorius muscle venules (7-200 μm diameter) during arterial pressure reduction and muscle contraction. There was no observable change in venular diameter during any of the above perturbations. There was a significant decrease in the already low normalized velocity of blood in venules from a mean of 13 sec⁻¹ under control conditions to 5 sec⁻¹ during arterial pressure reduction to 20 mm Hg. At very low pressures, the number of venules with blood flow decreased. Combining our findings with Lipowsky's (1975) in vivo viscometry data, it was predicted that resistance in venules would increase 100% as a result of increases in blood viscosity when blood flow was reduced 60%. During post-contraction hyperemia the normalized velocity of blood in venules increased from 16 sec⁻¹ to 38 sec⁻¹ and the number of venules with blood flow increased a modest amount. Combining our observations with Lipowsky's data, we predict that venular resistance would fall 54% when blood flow increased 250% If shear rate changes cause substantial changes in blood viscosity in venules as suggested by the findings cited above, hydrostatic pressure in the small venules should tend to remain relatively constant as flow is altered. To determine whether this is the case, pressures of venules were measured using the servo-null technique during arterial pressure reduction. Pressures in the larger venules were a linear function of blood pressure and blood flow with an intercept not significantly different from the systemic venous pressure. Pressures in the smallest venules studied (24 μm), however, were somewhat insensitive to alterations in blood pressure (intercept of 10.4 mm Hg) and blood flow (intercept of 13.2 mm Hg). The stability of pressure in 40 venules supports the hypothesis that variable blood viscosity maintains the pressure drop in the venous network and the capillary hydrostatic pressure somewhat constant during changes in blood flow.

published_or_final_version / Business Administration / Master / Master of Business Administration

published_or_final_version / Mechanical Engineering / Doctoral / Doctor of Philosophy

Finite difference numerical models based upon the Navier-Stokes equations with the Boussinesq approximation have been utilized to study the dynamics of a rotating liquid with horizontal density gradients. There are two configurations analyzed: a cylindrical annulus of water rotating about a vertical axis (parallel to the body force), and a hemispherical shell of silicone oil with a radial body force, rotating about the polar axis. In both the cylindrical and spherical configurations, the thermal and mechanical forcings (boundary conditions) are symmetric about the axis of rotation. The physical parameters varied are the rotation rate and the amplitude of the horizontal thermal forcing. Two numerical models have been developed for each geometrical configuration: one to calculate axisymmetric flows and another to test the stability of those flows to non-axisymmetric perturbations. The primary purpose of the models is to determine whether axisymmetric or non-axisymmetric flow will be observed in a corresponding laboratory experiment. For the cylindrical annulus, the predictions of axisymmetric and non-axisymmetric flow are in good agreement with laboratory experiments previously performed. In the spherical experiment considered, which has not been performed in the laboratory, there is evidence that if the rotation rate is fixed and the latitudinal thermal forcing is reduced, there exists a transition from non-axisymmetric to axisymmetric flow, but that as the rotation rate is decreased for a fixed latitudinal thermal gradient on the boundaries, the flow does not become axisymmetric. The structures of some of the fastest growing eigenmodes are presented for both cylindrical and spherical cases. Analyses of the energetics indicate that the waves in all cases considered are essentially baroclinic in nature.

No description available.

No description available.