Dehydration
Shock.  Fluid is an important part of human blood.  Water makes up most of blood plasma, which carries blood cells and other important things to the entire body.  If a person gets very dehydrated, the amount of blood in the body drops.  This causes low blood pressure.  If a person does not have enough blood and oxygen in their body, they can go into shock.  In medicine, this is called hypovolemic shock.