Gravity hits the cardiovascular system first when a spacecraft’s hatch opens again. Months of microgravity have allowed blood and other fluids to drift toward the chest and head, while leg muscles and the heart have done less work against weight.
To avoid fainting, space agencies treat circulation like a deconditioned athlete that needs a training plan. On board, crews follow strict aerobic and resistance exercise to maintain cardiac output and vascular tone, slowing the drop in plasma volume that would otherwise undermine blood pressure. Closer to reentry, many drink saline and other fluids in a protocol known as fluid loading, boosting blood volume so gravity has more to work with when it suddenly pulls blood toward the legs.
The core problem is orthostatic intolerance: the tendency to feel dizzy or collapse when standing because the baroreflex, the feedback loop that stabilizes arterial pressure, has adapted to weightlessness. To bridge the gap, some astronauts wear inflatable garments that squeeze the lower body, mimicking the pressure normally supplied by gravity and helping venous return. After landing, medical teams guide gradual standing, monitored tilt tests, and targeted rehabilitation to rebuild autonomic regulation and muscle pump function, until ordinary posture no longer threatens consciousness.
