How does the heart and cardiovascular system react to space conditions?
When the human body is first exposed to microgravity, blood shifts from the lower body towards the heart and head in what is called a headward fluid shift. This initially causes the central venous pressure to drop and the heart to temporarily enlarge, but as soon as the body senses what it interprets as an increase in total blood volume, it works to eliminate the excess fluid via urination. Within 2 days and after more frequent urination, the heart shrinks in order to pump a smaller volume of blood. Once an astronaut returns to Earth, the heart has difficulty adjusting to an environment in which it has to pull blood up to the brain against the force of gravity.
