EPO and anemia (phenotype): The SVRR has been studied most closely in spaceflight anemia. Upon release from gravity, interstitial fluid from gravity-dependent areas re-enters the circulation. This increases intravascular volume and end-diastolic volume, activating stretch receptors and initiating the SVRR. Plasma volume decreases within hours, followed by decreased erythropoietin concentrations. Upon return to gravity, fluid returns to gravity-dependent spaces. Intravascular volume is restored by the action of antidiuretic hormone, resulting in dilutional anemia until red cell mass is restored [2].