The heart of an astronaut actually gets smaller when in outer space.
Why Astronaut Hearts Shrink in Space (Despite Exercise)
Your heart is a muscle, and like any muscle, if you stop using it the way it was designed, it gets smaller. For astronauts, that process happens faster than you'd think. Research on astronaut Scott Kelly during his year-long mission showed his heart's left ventricle lost about 0.74 grams per week—roughly three-tenths of an ounce—despite exercising six days a week for several hours.
On Earth, gravity is constantly pulling blood downward, forcing your heart to work hard to pump it back up to your brain. Remove that resistance, and suddenly your heart doesn't need to be as strong. It's like going from lifting heavy weights to lifting feathers—your muscles adapt accordingly.
The Workout Regimen That Still Isn't Enough
It's not like astronauts are lounging around up there. On the International Space Station, crew members follow a rigorous exercise protocol: 60 minutes of aerobic training and 40-60 minutes of resistance work daily, averaging about 600 minutes per week. They use specialized equipment like the Advanced Resistive Exercise Device (ARED), a second-generation treadmill, and a cycle ergometer.
But here's the problem: even this intensive regimen can't fully replicate what gravity does naturally. Your heart doesn't get the same workout pumping blood in weightlessness, no matter how many miles you run on a space treadmill. Studies from 2024-2025 continue to show that microgravity causes cardiac atrophy, reduced blood volume, and changes in heart rhythm that exercise alone can't prevent.
What Happens to Space Hearts
The changes go beyond just size. Astronauts experience:
- Myocardial atrophy (heart muscle shrinkage)
- Reduced total blood volume
- Endothelial dysfunction (blood vessel problems)
- Arterial wall thickening
- Changes in maximum heart rate and blood pressure
- Decreased aerobic capacity
Some astronauts return to Earth with age-related cardiovascular conditions and irregular heartbeats. The good news? Most effects are reversible. Dr. Benjamin Levine, who studied Kelly's heart, noted that while it did shrink, "the function remained good."
The Mars Problem
This becomes a critical issue for future deep space missions. A trip to Mars could take 6-9 months each way, plus time on the surface at only 38% of Earth's gravity. NASA researchers warn that up to 17% of astronauts could experience significant loss of muscle performance, bone health, and cardiorespiratory fitness with current countermeasures.
Recent research is exploring artificial gravity systems, high-intensity interval training protocols, and pharmaceutical interventions. Until then, astronauts will continue their grueling workout schedules, knowing their hearts are shrinking anyway—a small price to pay for living among the stars.