Ventilation-Perfusion RatioPulmonary VentilationPulmonary EmbolismPulmonary Gas ExchangeRespirationKrypton RadioisotopesLaser-Doppler FlowmetryLungHindlimbTechnetium Tc 99m Aggregated AlbuminPulmonary AlveoliCarbon DioxidePerfusion ImagingIschemiaNoble GasesOxygenPulmonary CirculationPerfusionHyperventilationXenon RadioisotopesRespiratory MechanicsXenonChemoreceptor CellsBlood Gas AnalysisVentilationHypercapniaRespiration, ArtificialPhysical ExertionOxygen ConsumptionPartial PressureReflexPulmonary ArteryHeart RateAdministration, InhalationRespiratory Function TestsAnoxiaHigh-Frequency VentilationTidal Volume