Introduction: Short-duration space flight induces circulatory blood volume alterations known as “space flight anemia” and characterized by decreases in red blood cell volume (RCV) and plasma volume (PV). Such hematological alterations may persist during long-duration space missions, potentially impacting the astronauts’ health, however this aspect remains unexplored. On the other hand, during long-duration space missions the use of hypoxia is envisaged for technical reasons, but the safety of hypoxia must be first verified since this environmental condition provokes numerous physiological alterations in humans, in particular blood volume changes potentially interacting with the hematological effects induced by space flight. Objective: Using Antarctic confinement as a high fidelity Earth-based analogue for long duration deep space missions, we hypothesize that 1) confinement at sea level reduces blood volume by concomitantly decreasing RCV and PV, and 2) chronic hypoxia counterbalances the decrease in RCV but exacerbates the decrease in PV induced by confinement. Methods: Using an innovative, automated carbon-monoxide rebreathing technique usable by the wintering staff, blood volumes will be repeatedly measured in two groups of subjects overwintering at Dumont d’Urville (DDU) (sea level group) or Concordia (3200m, altitude group).