When serum sodium levels rise in the vascular system due to a fluid loss, osmosis causes water molecules to shift from the cell into the blood vessel to maintain homeostasis. Normally, this restores the fluid volume. When less fluid is available to exchange among the compartments of the body, however, the circulating blood volume decreases, causing hypovolemia and hypotension. Here we look at fluid volume deficit, the causes, and how the manifestations can start as thirst and end in hypovolemic shock.