With hypernatremia, there’s a higher than normal concentration of sodium in the blood - above 145 milliequivalents per liter.
However, since the concentration of sodium depends on both sodium and water levels in the body, hypernatremia actually translates as too little water in the extracellular compartment.
Ok, now remember that total body water is distributed either in the intracellular compartment, meaning inside the cells, or the extracellular compartment, meaning outside the cells.
Now, both the intracellular and extracellular compartments have the same amount of solutes dissolved in that water - so the same osmolality, normally between 275 and 290 milliosmoles per kilogram.
Serum osmolality can be calculated using the formula: twice the concentration of sodium measured in milliequivalents per liter, plus the serum concentration of glucose divided by 18, measured in milligrams per deciliter, plus BUN, which stands for blood urea nitrogen, divided by 2.8, also in milligrams per deciliter - so if you remember one thing from this formula, it should be that sodium is the major determinant of serum osmolality.
Ok, now just to make matters more interesting, some of these solutes, like sodium and glucose, can’t freely cross cell membranes, so they generate an osmotic pressure inside the compartment, which is a measurement of how likely it is that this compartment will draw water out of the other one.
Normally, the intracellular and extracellular compartments are isotonic to each other - meaning, they have the same concentration of osmotically active solutes.
However, small variations in solute concentrations, like having more sodium in the extracellular compartment, can alter that equilibrium, so water moves across cell membranes in order to restore the balance.
Water moves according to the rules of osmosis - or from the hypotonic compartment, where there’s more water, to the hypertonic compartment, where there’s less water. And water does that until the two compartments once again become isotonic.
Alright then! So with hypernatremia, there is too much sodium in the extracellular fluid. This can happen because a person has gained more sodium than water, or has lost more water than sodium.
Sodium gain usually happens because of an iatrogenic overload, like in individuals who received hypertonic saline following a traumatic head injury, or sodium bicarbonate solutions used for the treatment of metabolic acidosis.