Osmosis

• Drinking water causes the process of osmosis in the body. During osmosis, liquid passes through cell walls and hydrates the body. Because salt water is heavier than fresh water, the salt water will not pass through the cell wall.
  
  

Osmoregulation

• The unsalted water inside the cells passes through the cell wall to join the salt water outside the cells. The reason the fresh water leaves the cells is because water in the body is equilibrated through the body through osmosis. That means water inside the cells and outside the cells needs to be in balance. That state of balance is called an isotonic solution.
  
  

Dehydration

• The person who drinks salt water is actually thirstier than they were before they drank the salt water. Also, kidneys cannot eliminate such a high concentration of salt as is present in salt water. It would take extra water to eliminate the salt water and you can't eliminate more water than you drink.
  
  

Decline and death

• The body reacts to dehydration by using all of the water molecules it can find within the body. This causes the body's systems to shut down, eventually causing seizures, convulsions and brain damage. The kidneys eventually shut down because they cannot process the salt. Death follows.