My blood pressure is about 78/52 and the nurse said that I cannot donate blood because they would have to take me to the hospital and put it back in. Is that correct and if it is, does it mean that if I was injured and losing blood I would die more quickly than someone with normal bp?
Blood pressure is partly determined by your blood volume. If you suddenly remove a pint of blood without replacing the volume, it is normal for BP to go down (less blood in the same space = lower pressure)
Your BP is low, but that is probably normal for you, and is just fine. Taking a pint out, though, would likely drop your BP to the point where you won’t adequately perfuse your vital organs. This can cause fainting, heart problems, kidney damage and other bad things.
If you were injured and had a major blood loss, you might see adverse effects more quickly. However, if you are young and otherwise healthy, your sympathetic nervous system would kick in big time and the adrenaline would probably keep your BP high enough to keep you going.
People who have high blood pressure are at risk if they lose a lot of blood because of something called autoregulation. Their organs are used to the higher pressures, and they are unable to tolerate pressures that would be normal for you.