What Space Does to the Human Body: Bones, Blood, Radiation, and Brain

Living in space changes nearly every system in the human body. Microgravity alters how fluids, bones, and muscles behave, while radiation and confinement add further stress. Understanding these effects is one of the main reasons the station exists, because the answers determine whether people can safely travel to the Moon or Mars.

Fluids move upward

On Earth, gravity pulls blood and other fluids toward the legs. In microgravity that pull disappears, so fluid shifts toward the head and chest. Astronauts often develop a puffy face and thin legs in the first days of a mission. The body adjusts by reducing blood volume over time, which makes the heart work less hard in orbit.

The adjustment causes a problem on return. When astronauts stand up on Earth, their blood pressure can drop quickly because the body has less fluid to resist gravity. This condition, called orthostatic intolerance, can cause dizziness or fainting in the first hours after landing. Crews usually rest, drink fluids, and use compression garments to ease the transition.

Eyes and vision

Some long-duration crew members report changes in vision. Spaceflight-associated neuro-ocular syndrome describes a set of changes that include swelling of the optic nerve, flattening of the back of the eyeball, and shifts in the eye's structure. Scientists think raised pressure inside the skull, possibly caused by the fluid shift, plays a major role. The condition can persist after return, which makes it a serious concern for missions of several years.

Bone and muscle loss

Bones are living tissue that responds to load. In microgravity, the skeleton carries very little weight, so bone-forming cells slow down while bone-removing cells stay active or speed up. Weight-bearing bones such as the hips and legs lose mineral density at roughly 1 to 1.5 percent per month. Most of that loss stops or slows after return, but not all of it recovers quickly.

Muscle follows the same pattern. Postural muscles in the back and legs lose strength and mass because they are not working against gravity. Astronauts also lose some muscle in the arms and lower body when they are not using them intensively. The losses can be large, which is why exercise is central to life aboard.

Spine and height

Without gravity compressing the spine, the discs between the vertebrae expand slightly. Astronauts can grow a few centimetres taller during a mission, often measured at around 3 to 5 centimetres. This change reverses within days or weeks after landing, but it can cause back discomfort during the adjustment.

Radiation exposure

Radiation is one of the most difficult problems for human spaceflight. In low Earth orbit, the station receives galactic cosmic rays and secondary particles produced when those rays strike the station structure. The Earth's magnetic field and the atmosphere still block much of the most energetic material, but crews still receive a dose many times higher than people on the ground.

The table below gives rounded figures for context. Exact values vary with solar activity, orbit, and shielding, so the numbers are approximate.

Item	Approximate figure
Radiation dose on the station	About 0.3 to 0.8 millisieverts per day
Natural background dose on Earth	About 2.4 millisieverts per year
Bone loss in weight-bearing bones	About 1 to 1.5 percent per month
Exercise time on the station	About 2 hours per day
Spine lengthening in orbit	About 3 to 5 centimetres

Beyond low Earth orbit, the picture becomes harder. Deep-space travelers would face galactic cosmic rays with no magnetic shelter, and solar particle events could deliver large doses in a short time. Shielding helps, but it adds mass, and mass is expensive to launch.

Circadian rhythm and sleep

The body keeps time with a daily internal clock tied to light and darkness. On the station, the crew sees about 16 sunrises a day, so the clock is constantly pushed out of step. Many astronauts report poor sleep, fatigue, and difficulty falling asleep on schedule.

Engineers and mission planners respond with lighting systems that mimic daylight, fixed sleep periods, and schedules that avoid heavy work close to bedtime. Sleep medication is sometimes used, but planners prefer non-drug approaches when they can.

Immune and cognitive effects

Studies have found changes in the immune system during spaceflight. Some immune cells behave differently, and latent viruses can reactivate in certain crew members. These effects are measured carefully because they could matter on long missions with limited medical resources.

Cognitive performance can also shift. Astronauts face the demands of complex tasks while living in a confined environment with limited privacy. Crews train extensively for teamwork and communication, and ground teams monitor mood and workload to catch problems early.

Countermeasures

The main countermeasure is exercise. Crews use treadmills with harnesses to create ground-reaction forces, cycle ergometers for the cardiovascular system, and resistive devices that load the muscles through vacuum-based or spring-based systems. The routine is designed to protect bone density, muscle mass, and aerobic fitness, and it takes about two hours each day.

Diet is another defense. Astronauts eat a balanced menu with enough calcium, vitamin D, and protein to support bone and muscle. Medical teams track blood and urine samples to watch for early signs of kidney stones, bone resorption, and other changes. Hydration matters for the same reason: dehydration raises the risk of kidney stones, and calcium leaving the bones can end up in the urine. Crews therefore drink plenty of water and watch their intake of salt and other minerals.

Monitoring also depends on the time a crew spends in orbit. Short visits of a few days show mostly fluid and balance changes, while stays of six months or more reveal slower effects on bone, eyes, and the immune system. Researchers combine these results with data from the station's long-duration crews, which gives them a clearer picture of how the body changes over time.

Future missions to Mars would last roughly two to three years round trip, which is far longer than any station stay. Researchers are studying pharmaceutical countermeasures, artificial gravity concepts, improved shielding, and better ways to monitor crew health without frequent evacuation.

Key takeaways

Microgravity changes the way fluids, bones, muscles, and eyes behave. These changes are measurable, and the body adapts in ways that are useful in orbit but can cause problems on return to Earth.

Radiation is the hardest long-term hazard because it cannot be fully escaped on deep-space missions. Shielding, dose monitoring, and mission planning are all part of the answer.

The main practical defense is exercise, backed by diet, medical monitoring, and schedules that protect sleep. The station guide in this section shows where those countermeasures are applied in daily life, and the spacesuit article explains the gear that protects astronauts when they work outside.