Losing large amounts of blood can reduce your oxygen levels in your blood cells, resulting in your body not properly carrying oxygen to all needed parts of the body.

Blood transfusions were used to cure injured soldiers during WWII that lost large amounts of blood.

WWII soldiers commonly lost large doses of blood from bullet wounds, diseases that destroy red blood cells, and major surgeries.

Why was blood transfusion used in WWII?

Why does your body need blood transfusions?

http://upload.wikimedia.org/wikipedia/commons/5/5d/Ww2-53.jpg

If your body has a low blood count, you cannot maintain a proper blood pressure and will go into shock.