That's exactly what happens. The human body cannot withstand more than 2-3 G for a prolonged period. Above that, the heart just can't pump blood and you die if you stay there too long. For instance, Stapp, the world record holder only withstood 46.2 Gs for a brief instant. Normally your blood weighs about 8-10% of your total body weight, so for the instant he was at 46.2 G, he weighed about 7700 lbs and his blood weighed between 616 lbs and 770 lbs.