Blood Flow VelocityRegional Blood FlowUltrasonography, Doppler, TranscranialCerebrovascular CirculationMiddle Cerebral ArteryUltrasonography, DopplerOphthalmic ArteryCerebral ArteriesPulsatile FlowUltrasonography, Doppler, ColorUltrasonography, Doppler, PulsedCoronary CirculationBlood PressureVascular ResistanceLaser-Doppler FlowmetryHemodynamicsHypocapniaCiliary ArteriesRetinal ArteryEchocardiography, DopplerEchoencephalographyRheologyUmbilical ArteriesHyperventilationMicrocirculationCarbon DioxideDiastoleUltrasonography, PrenatalAnterior Cerebral ArteryHyperemiaHeart RateUltrasonographyModels, CardiovascularArteriesReference ValuesIntracranial PressureCoronary VesselsSystoleHeart Rate, FetalCeliac ArteryHomeostasisTime FactorsVasodilationGestational AgeAcetazolamideDogsVasodilator AgentsPregnancyBrainOxygenDoppler EffectMesenteric Artery, SuperiorReproducibility of ResultsProspective StudiesFetal Growth RetardationCircle of WillisCarotid Artery, InternalEchocardiography, Doppler, ColorPlacental CirculationBone DensityCollateral CirculationHemorheologyTilt-Table TestFetal HeartPosterior Cerebral ArteryFlow CytometryVasomotor SystemEchocardiographySplanchnic CirculationHypotension, OrthostaticHypercapniaBlood ViscosityPulmonary CirculationBlood Circulation TimePostureBlood VolumePartial PressureRetinal VesselsPapaverineUterusVasoconstrictionAnalysis of VarianceEchocardiography, TransesophagealEchocardiography, Doppler, PulsedAdenosineCardiac OutputOxygen ConsumptionRenal CirculationCarotid ArteriesMicrospheresInfant, NewbornVentricular Function, LeftFetusSensitivity and SpecificityMitral ValveOrthostatic IntoleranceHematocritInfant, PrematureBlood CirculationObserver Variation