Blood Flow VelocityUltrasonography, Doppler, TranscranialCerebrovascular CirculationCerebral ArteriesRegional Blood FlowPulsatile FlowBlood PressureHemodynamicsMiddle Cerebral ArteryUltrasonography, DopplerOphthalmic ArteryUltrasonography, Doppler, ColorUltrasonography, Doppler, PulsedVascular ResistanceCoronary CirculationLaser-Doppler FlowmetryHypocapniaCiliary ArteriesEchocardiography, DopplerRetinal ArteryEchoencephalographyRheologyHeart RateUmbilical ArteriesHyperventilationMicrocirculationCarbon DioxideDiastoleHyperemiaUltrasonography, PrenatalAnterior Cerebral ArteryUltrasonographyArteriesReference ValuesModels, CardiovascularSystoleVasodilationTime FactorsIntracranial PressureAcetazolamideHomeostasisHeart Rate, FetalCeliac ArteryCoronary VesselsDogsVasodilator AgentsMesenteric Artery, SuperiorOxygenGestational AgePregnancyProspective StudiesDoppler EffectBrainHemorheologyHypotension, OrthostaticReproducibility of ResultsHypercapniaSplanchnic CirculationFetal Growth RetardationCircle of WillisEchocardiography, Doppler, ColorCarotid Artery, InternalPlacental CirculationEchocardiographyCollateral CirculationBlood VolumePulseTilt-Table TestHypertensionFetal HeartPosturePosterior Cerebral ArteryCardiac OutputVasomotor SystemFlow CytometryVasoconstrictionBlood ViscosityPulmonary CirculationRenal CirculationCarotid ArteriesAnalysis of VarianceHypotensionBlood Circulation TimeHematocritOxygen ConsumptionPartial PressureInfant, NewbornVentricular Function, LeftPapaverineBlood CirculationVeinsRetinal VesselsUterusEchocardiography, TransesophagealInfant, PrematureEchocardiography, Doppler, PulsedRestAdenosineSupine PositionCarotid Artery, Common