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 PressureSystoleHeart Rate, FetalCeliac ArteryCoronary VesselsHomeostasisVasodilationAcetazolamideGestational AgeVasodilator AgentsDogsDoppler EffectPregnancyTime FactorsOxygenMesenteric Artery, SuperiorFetal Growth RetardationBrainCircle of WillisEchocardiography, Doppler, ColorCarotid Artery, InternalPlacental CirculationProspective StudiesCollateral CirculationReproducibility of ResultsHemorheologyTilt-Table TestFetal HeartPosterior Cerebral ArteryVasomotor SystemHypotension, OrthostaticEchocardiographySplanchnic CirculationHypercapniaFlow CytometryBlood ViscosityPulmonary CirculationBlood Circulation TimePostureBlood VolumePartial PressurePapaverineRetinal VesselsUterusEchocardiography, TransesophagealVasoconstrictionEchocardiography, Doppler, PulsedCardiac OutputAdenosineRenal CirculationCarotid ArteriesOxygen ConsumptionMitral ValveVentricular Function, LeftOrthostatic IntoleranceInfant, NewbornMicrospheresFetusInfant, PrematureHematocritAnalysis of VarianceBlood CirculationSensitivity and SpecificityDuctus ArteriosusXenon Radioisotopes