"endlessly riding in a rollercoaster" implies that you think the astronauts' blood is forced to their head by negative G. That's not the case.
There's a simple test that can answer this. If blood flow to the head is caused by being in orbit of a planet, you'd find a measurable difference between astronauts positioned with their feet pointing towards Earth or away from it. You'd be able to solve the blood flow problem by declaring the zenith side of the ISS to be the floor, and have everybody work with their feet pointed away from Earth.
No such effect has been found.
Say an astronaut's blood is pushed somewhere by an outside force (gravity). That same force acts on the whole astronaut. Since the astronaut is not restrained (unlike when you're in a roller coaster), that force would eventually move the astronaut to one of the walls of the spacecraft. That doesn't happen.
Astronauts wind up with increased blood flow to the head because the body, including the heart, is designed to function in 1G. The heart pushes the blood "uphill" from the feet to the head against the force of gravity. Blood vessels have the right size so every body part gets enough blood in 1G even though blood flows more easily to the feet than the head.
In freefall, your blood is as weightless as the rest of the body, so the heart has much less resistance to work against. In the first few weeks after launch, the heart keeps working at the rate it's used to. Without gravity, this means increased blood flow, especially to the areas that are 'hard to reach' under normal gravity, i.e. the head.