You'd want one of the eggheads to explain it. Best I can remember from school? Epstein drive, which is basically a nuclear fusion rocket, creates a super high gravitational field when it accelerates.
To make up for high gs, we usually inject ourselves with blood thinners, blood vessel reinforcers, adrenalines and some other crap I can't remember. Keeps us from losing consciousness and, well, popping a vessel. Still happens, though, from time to time.