Microgravity is brutal on human design, and the heart is the clearest proof. In orbit, body fluids drift upward, the torso elongates, and the spine uncompresses, so astronauts can measure several centimeters taller within days of arrival.
The heart, by contrast, starts to shed mass. With less hydrostatic pressure to fight, the left ventricle does not need to generate the same stroke work it does on Earth, and cardiac muscle follows a simple rule of biology: unused fibers atrophy. Echocardiography and cardiac MRI have shown real reductions in ventricular wall thickness and overall cardiac mass, a change similar in principle to what happens to a leg kept in a cast, only now the disuse is hemodynamic rather than mechanical.
Height gain in orbit is almost the mirror image of that loss. Intervertebral discs absorb more fluid, axial loading on the spine falls, and vertebral bodies separate slightly; radiographic measurements document spinal elongation and relaxed curvature, which together add those extra centimeters. What looks like growth is essentially decompression physics applied to anatomy, while the shrinking heart is a textbook case of cardiovascular deconditioning under sustained microgravity.
