Which of the following most likely happens when astronauts go to space?

1. their heart rate speeds up because the blood vessels have less blood to circulate.

2. their heart rate speeds up because the blood vessels have more blood to circulate.

3. the blood vessels in their neck caves in because the heart pumps less blood per beat.

4. the blood vessels in their neck bulges out because the heart pumps more blood per beat.

4. The blood vessels in their neck bulge out.  

Explanation:

This option is closest to being correct.

On Earth, the force of gravity pulls blood to the lower body. In space, the force of gravity is cancelled, so the blood spreads out evenly.

More blood in the upper body leads to increased blood pressure there, so the blood vessels in the neck bulge out.

The heart does not pump more blood per beat. Instead, it becomes lazy because it does not have to do as much work pumping blood back from the legs against the force of gravity.

A and B are wrong. You don't gain or lose blood simply by going into space.

C is wrong. The blood vessels in the neck bulge out.