With hypernatremia, hyper- means high, and -natrium is latin for sodium, often shortened to Na+, and -emia refers to the blood, so hypernatremia means a higher than normal concentration of sodium in the blood, generally above 145 mEq/L.
The concentration of sodium depends on both sodium and water levels in the body.
About 60% of our body weight comes from just water, and it basically sits in two places or fluid compartments—it either outside the cells in the extracellular fluid or inside the cells in the intracellular fluid.
The extracellular fluid includes the fluid in blood vessels, lymphatic vessels, and the interstitial space, which is the space between cells that is filled with proteins and carbohydrates.
One third of the water in the body is in the extracellular compartment, wheres two thirds of it is in the intracellular compartment.
Normally, the two compartments have the same osmolarity -- total solute concentration -- and that allows water to move freely between the two spaces.But the exact composition of solutes differs quite a bit.
The most common cation in the extracellular compartment is sodium, whereas in the intracellular compartment it’s potassium and magnesium.
The most common anion in the extracellular compartment is chloride, whereas in the intracellular compartment it’s phosphate and negatively charged proteins.
Of all of these, sodium is the ion the flits back and forth across cell membranes, and subtle changes in sodium concentration tilts the osmolarity balance in one direction or another and that moves water. This is why we say “wherever salt goes, water flows”.
So with hypernatremia, someone can have a high concentration of sodium in the extracellular fluid and therefore the blood, by either losing more water than sodium, or gaining more sodium than water. Either way this increases the sodium concentration in the extracellular fluid, draws water out of the cells.
When hypernatremia develops over a long period of time, the cells get time to adapt and they start generating osmotically active particle, which ends up preventing water from being lost via osmosis.