HypovolemiaFluid TherapyIsotonic SolutionsColloidsPlasma SubstitutesHyponatremiaBlood Volume DeterminationRehydration SolutionsWater-Electrolyte BalanceIntraoperative CarePerioperative CareColorectal SurgeryHydroxyethyl Starch DerivativesBlood VolumeLower Body Negative PressureGoalsShockHemodynamicsPlasma VolumePostoperative CareSodium ChlorideDehydrationHypotension, OrthostaticFluid ShiftsVascular CapacitanceDiureticsFurosemideShock, HemorrhagicPostoperative ComplicationsThirstHypernatremiaBed RestHead-Down TiltHemodilutionInappropriate ADH SyndromeIntracranial HypotensionHemorrhageProspective StudiesBlood PressureTreatment OutcomeCardiac OutputHeart RateSupine PositionHypotensionDrinkingStroke VolumeSpectroscopy, Near-Infrared