Microgravity and Human Health: Lessons from the ISS and Scientific Experiments

When we gaze up at the stars, we often marvel at the vastness of space but rarely pause to consider the profound physical and biological challenges that humans face beyond our planet. Among these challenges, one of the most significant is microgravity. At first glance, weightlessness might seem fun and exciting—astronauts floating effortlessly, objects drifting around the cabin, and movements appearing graceful and free. However, behind this spectacular display lie deep and often uncomfortable changes occurring within the human body. The International Space Station (ISS) has served as a unique laboratory for decades, helping scientists study how microgravity affects human health.

What Is Microgravity?

Microgravity is a condition where the force of gravity is nearly imperceptible. On the ISS, gravity isn’t zero but so minimal that objects and astronauts appear to be in constant free fall around the Earth. This dramatically alters the environment for all living organisms, including humans. The absence of normal gravitational forces triggers a cascade of physiological adaptations, some of which can be detrimental over time.

Bones, Muscles, and Spine: The First Line of Impact

One of the most noticeable effects of microgravity is bone loss. Here on Earth, gravity constantly loads our skeleton, stimulating it to stay strong and dense. In space, this mechanical stress disappears, and the body “decides” it doesn’t need to maintain such dense bones. As a result, astronauts can lose 1–2% of their bone mass every month, a rate comparable to severe osteoporosis on Earth.

Muscles suffer similarly. Without the constant resistance provided by walking or standing, muscles—especially those in the legs and back—atrophy quickly. To combat this, astronauts spend about two hours a day exercising aboard the ISS. Their routine includes running on a treadmill secured with harnesses, strength training with resistance bands, and cycling on a stationary bike adapted for zero gravity.

Another fascinating effect is on the spine. In microgravity, the spine “unloads” and stretches, which can increase an astronaut’s height by 2 to 5 centimeters. While this might sound like a neat party trick, it often causes back pain and increases the risk of intervertebral disc injuries.

Cardiovascular System: Blood Loses Its Direction

On Earth, the heart works hard to pump blood upward against gravity to the brain and downward to the legs. In microgravity, blood redistributes evenly, pooling in the upper body. This results in “puffy face syndrome,” where astronauts’ faces appear swollen, and they often experience nasal congestion and headaches.

Over time, the heart muscle weakens since it no longer needs to work as hard. This can lead to orthostatic intolerance—a drop in blood pressure and dizziness when returning to Earth’s gravity. To prepare for this, astronauts undergo a process called “reconditioning,” which includes wearing specially designed suits that apply pressure to the lower body to simulate gravity’s effects and retrain the cardiovascular system.

Effects on Vision and the Brain

One of the most concerning discoveries of recent years is the Spaceflight-Associated Neuro-ocular Syndrome (SANS). Many astronauts experience vision deterioration during or after their missions. This is believed to be caused by increased intracranial pressure due to fluid shifts in microgravity. The optic nerve can swell, and the back of the eye may flatten, leading to blurred vision and, in some cases, long-lasting impairment.

Immune System and Genetics

Microgravity also affects the immune system in complex ways. Some immune functions become hyperactive, while others are suppressed, making astronauts more vulnerable to infections. Compounding the risk, certain bacteria appear to become more virulent in space conditions.

One of the most famous experiments illustrating space’s impact on genetics involved astronaut Scott Kelly, who spent nearly a year aboard the ISS while his identical twin brother Mark remained on Earth. Researchers compared their gene expression before, during, and after the mission. They found that hundreds of genes—especially those related to immunity, metabolism, and DNA repair—were altered in Scott. While many changes reversed after his return, some persisted for months, highlighting the profound influence of space travel on the human body at a molecular level.

Psychological Challenges and Isolation

Physical effects are only part of the story. The psychological impact of living in confined spaces, isolated from nature and separated from loved ones, is significant. Crew members face stress from limited privacy, monotonous surroundings, and the pressure of teamwork in a high-risk environment. To support mental health, the ISS provides regular psychological counseling sessions, access to entertainment like movies and music, and virtual reality experiences simulating walks on Earth or in nature.

Why Does This Matter for the Future?

Understanding microgravity’s effects on human health is essential for the future of space exploration. Missions to the Moon and Mars will take months or years, and without effective ways to mitigate health risks, long-term space travel could be impossible. Scientists are already developing technologies like artificial gravity habitats, new pharmaceuticals, and gene therapies to counteract the damaging effects of weightlessness.

Microgravity is both a challenge and an opportunity. Research aboard the ISS not only prepares humanity for interplanetary travel but also yields insights that benefit healthcare on Earth—improving treatments for osteoporosis, muscle wasting, and immune disorders. Space changes us, and through science, we learn how to change along with it.

If you’re curious about what it’s like inside the ISS, imagine astronauts floating weightlessly among high-tech equipment, exercising on specially designed machines, and adapting every day to a world where up and down lose meaning—but where human resilience continues to shine.