What does zero gravity do to the human body?

The answer

In microgravity, the human body undergoes dramatic changes. Without gravity pulling blood downward, fluids shift to the upper body and head, causing a puffy face and stuffy nose. The spine decompresses and astronauts grow up to 5 cm taller. Without gravitational load, bones lose about 1–2% density per month and muscles atrophy rapidly. The heart, no longer needing to pump against gravity, weakens and becomes more spherical. Eyesight can deteriorate as intracranial pressure increases. The immune system weakens, and some dormant viruses (like herpes) reactivate. These challenges are major obstacles for future Mars missions.