An average adult has 10-12 pints of blood in their body. Should he lose 5-6 pints the man would definitely die. While he dies at once he losses 5 or more, that did not mean he would be okay to lose more than 1.
Just losing 2.5 to 3 pints of blood would already send a human to hemorrhagic shock. Hemorrhagic shock was when there was a significant loss of blood, leading to a critical reduction in the volume of circulating blood and a decrease in the delivery of oxygen and nutrients to tissues and organs.
To put it simply, much like to a gas was to a car, a body cannot run without blood. Once you no longer got blood to the specific parts of your body, they will let you know.
When your muscles no longer get the blood, you would begin to feel weak. Your brain without blood would go into confusion and become lightheaded. As for the heart, it begins to "gasp" and increases the pace of its beating in order to circulate more blood.