Hypernatremia: Definition, Causes, Symptoms & Treatment

Instructor: Catherine Konopka

Catherine has taught various college biology courses for 5 years at both 2-year and 4-year institutions. She has a Ph.D. in cell and molecular biology.

Have you ever wondered why you're told to drink plenty of water and and not to eat too much salt? In this lesson, you will learn the causes and consequences of having too much salt in your body.


Have you ever had a doctor say you need to watch your sodium intake and drink plenty of water? The ratio of sodium to water in your body is critical for many of life's processes. If the amount of sodium gets too high, a condition called hypernatremia can result and the consequences can be deadly. Before we can discuss what's so bad about having too much sodium, let's first explore why sodium is so important.

Sodium and Water

Like all the electrolytes in the body, sodium exists in its charged form, Na+. Sodium is the most abundant positively-charged electrolyte in the extracellular fluid (ECF), which surrounds our cells and tissues. This is because cells actively export Na+ into the ECF using a sodium-potassium pump (Na/K pump). The pump forces Na+ out of the cell and imports a K+, creating sodium and potassium gradients. These gradients are essential for nerve cells to transmit electrical signals. When the level of sodium changes in the ECF, nerve cells can't receive or relay their messages. The Na/K pump is so important to life that approximately 10% of all the calories you eat go to maintaining these pumps in your cells!

Another important characteristic of sodium, is that it controls the movement of water throughout your body. Water follows the movement of ions through the various compartments of the body (ECF, blood, inside cells, urine, etc.). Since there is so much sodium in our body, it controls the movement of water more than any other electrolyte. If there is too much sodium in the ECF, water will leave the cells, causing the cells to shrink. If sodium was Mary, then water would be her little lamb!

Hypernatremia = Too Much Sodium and Not Enough Water

Ions and water move relatively freely from the ECF to the serum, the liquid part of the blood. So, the concentration of Na+ in the ECF and blood is approximately the same. The concentration of Na+ our blood ranges from 135-145 mEq/L.

Hypernatremia is an abnormally high sodium concentration in the ECF and serum. This is in contrast to hyponatremia , which is an abnormally low concentration of sodium. The Latin term for sodium is natrium, which is why the symbol for sodium on the periodic table is Na. So remember - the 'na' in hypernatremia stands for sodium. Hypernatremia occurs when serum values are greater than 157 mEq/L. To remember the difference between the prefixes hyper- and hypo-, visualize a hyper child that has had too much sugar. Another way is to remember the difference is that hypo- ends in an 'O' which looks a lot like a zero (0).

Under normal conditions (left) the concentration of sodium ions (green circles) is higher in the ECF than in the cytoplasm (CP), but the movement of water into the cell balances the movement out of the cell. In hypernatremia (right), the Na+ concentration in the ECF is even higher. This leads to an increased sodium gradient, which pulls water out of the cell causing it to shrink.
Hypernatremia, which is high extracellular Na+, causes water to leave cells

Causes of Hypernatremia

The main source of sodium is the food we eat and beverages we drink. The main way we lose sodium is through urine made by the kidneys.

A typical western diet contains 25 times more sodium than we actual need. Thank goodness it's really hard to develop hypernatremia just by eating too much salt! There have been cases where someone has died from drinking salt water or eating excessive amounts of salt, but it's very rare. This is because our kidneys are really good at removing extra sodium from the blood.

The body uses several hormones to regulate how the kidneys retain or excrete sodium and water. Although the details are beyond the scope of the lesson, it is important to know that if there is too much or too little of these hormones, it can lead to hyper- or hyponatremia.

Surprisingly, hypernatremia is not really a result of too much sodium. It's typically caused by not having enough water. Remember that the sodium concentration is a ratio of sodium to water. So if the amount of Na+ goes up or the amount of water goes down, hypernatremia can occur. The most common causes are:

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