Extended periods of microgravity in space cause the human heart to lose muscle mass and transform from an oval shape into a more spherical configuration.

Why the heart changes shape in orbit

On Earth, gravity continually pulls blood toward the legs. The heart must maintain circulation against that pull, and its workload reflects the body’s upright posture. In microgravity, that force largely disappears. Fluid shifts toward the chest and head, so the heart initially receives more blood from above than it would while standing on Earth.

Over time, the body adjusts to this new arrangement. It sheds some fluid, and the heart no longer has to work in quite the same way to circulate blood. With less sustained demand, the heart muscle can lose some mass—a form of use-it-or-lose-it adaptation, though it is not identical to ordinary muscle weakening from inactivity.

What ultrasound reveals

NASA cardiologists use ultrasound to follow the heart in space, much as clinicians use it on Earth. The images can show changes in the heart’s size and shape as its loading conditions change. A heart that looks more oval under Earth’s usual conditions may appear rounder in microgravity; the shift reflects remodeling, not a heart turning into a different kind of organ.

The shape is one visible sign of a broader adjustment in circulation. It helps researchers understand how the cardiovascular system responds when gravity no longer guides blood downward and the heart’s usual workload changes.

The journey home changes the workload again

Returning to Earth reverses the challenge: gravity once again pulls blood toward the legs, and the heart must adapt to pumping in an upright body. That is why monitoring astronauts’ hearts matters beyond documenting an unusual image. It helps track how the cardiovascular system adjusts to long periods of weightlessness—and to the abrupt return of ordinary gravity.