This is a very simple question, and I thought that it would be easy to answer. Not so!
The general consensus seems to be that a blood loss of 30-40% would be fatal, but even that is an over-simplification. It turns out that it depends on the SPEED of the blood loss. If the blood is lost quickly, 33% can be fatal; if it’s lost slowly then up to 66% could be lost before death! Why the difference? Well, a sudden loss can result in either of these two conditions (both of which can be fatal due to heart attack or stroke):
- too little fluid in the blood vessels for flow to happen properly (kind of like when the pipes in your house are drained of water)
- not enough blood cells left to carry enough oxygen.
Here’s the best reference: http://www.merck.com/mmhe/sec14/ch172/ch172b.html