Only the upper body senses blood volume. In microgravity, it thinks there's too much and a human loses ten to fifteen percent of their water weight.
Go back into a gravity area and the blood goes down to the legs. The brain gets less and it's very easy to pass out. It's what led to one of the most iconic moments in history, the first human landing on Mars.
That's one small step for a man, one giant faceplant for mankind.