On Earth, gravity pulls roughly a litre of blood and interstitial fluid down into the legs, and bones and muscles carry the constant load of body weight against gravity. In microgravity both gradients vanish at once. Fluid that used to pool in the legs now redistributes evenly and shifts toward the head and chest — the classic "puffy face, bird legs" look — raising pressure in the skull and pressing on the eyes. Sustained for months, that pressure is believed to drive SANS (Spaceflight-Associated Neuro-ocular Syndrome): optic disc swelling and vision changes seen in long-duration astronauts. Meanwhile bone and muscle, no longer resisting body weight, are actively remodelled away — astronauts lose bone mineral density at roughly 1–2% per month, five to ten times faster than post-menopausal osteoporosis on Earth, concentrated in the weight-bearing hip and spine.
fluidShift(t) = F_max · (1 − e^(−t/τ_f))
boneLoss(t) = rate_bone · t · (1 − 0.75·countermeasures)
muscleLoss(t) = rate_muscle · t · (1 − 0.70·countermeasures)
ICP(t) ∝ fluidShift(t)
- Mission duration — drag to any point on a real mission timeline (0–365 days, ISS expeditions run ~180, the longest single stay on record is 371).
- Countermeasures — toggles daily resistance exercise (ARED-style squats/deadlifts loaded by vacuum cylinders) and lower body negative pressure, a chamber that seals around the waist and sucks blood back down into the legs to mimic gravity's pull. Both are real ISS hardware and cut bone/muscle loss by roughly 70–75% but do not eliminate it.
- Fluid shift saturates within the first days (fast fluid redistribution); bone and muscle loss instead accumulate roughly linearly for as long as the mission continues.
Real-world relevance: these two effects are the leading medical constraints on how long humans can safely stay in space, and directly shape countermeasure hardware design for Moon and Mars missions where the trip alone can exceed a year.