Bed RestHead-Down TiltWeightlessness SimulationWeightlessness CountermeasuresRestCardiovascular DeconditioningSpace FlightBedsWeightlessnessBlood PressureOrthostatic IntoleranceLower Body Negative PressureHypertensionTime FactorsPhysiciansMedical Staff, HospitalExerciseBed OccupancyPhysician-Patient RelationsSupine PositionMuscular AtrophyPlasma VolumeEarly AmbulationMuscle, SkeletalHeart RateReference ValuesHypotension, OrthostaticRegional Blood FlowAttitude of Health PersonnelBlood VolumeHypovolemiaBlood Flow VelocityFamily PracticePrehypertensionVascular CapacitanceAmino Acids, EssentialBlood GlucoseImmobilization