Bed RestHead-Down TiltWeightlessness SimulationWeightlessness CountermeasuresRestCardiovascular DeconditioningSpace FlightBedsRegional Blood FlowWeightlessnessBlood Flow VelocityOrthostatic IntoleranceLower Body Negative PressureFlow CytometryPhysiciansMedical Staff, HospitalBed OccupancyTime FactorsSupine PositionMuscle, SkeletalExercisePhysician-Patient RelationsMuscular AtrophyPlasma VolumeEarly AmbulationHypotension, OrthostaticAttitude of Health PersonnelPulsatile FlowHypovolemiaVascular CapacitanceFamily PracticeGene FlowAmino Acids, EssentialImmobilizationBlood PressureVascular Resistance