Ventilation-Perfusion RatioPulmonary VentilationPulmonary EmbolismPulmonary Gas ExchangeRespirationKrypton RadioisotopesLaser-Doppler FlowmetryLungHindlimbTechnetium Tc 99m Aggregated AlbuminCarbon DioxidePerfusion ImagingIschemiaNoble GasesOxygenPerfusionPulmonary CirculationHyperventilationXenon RadioisotopesRespiratory MechanicsXenonChemoreceptor CellsVentilationBlood Gas AnalysisHypercapniaPhysical ExertionRespiration, ArtificialOxygen ConsumptionReflexPartial PressurePulmonary ArteryHeart RateAdministration, InhalationRespiratory Function TestsAnoxiaHigh-Frequency Ventilation