Hyponatremia

With hyponatremia, hypo- means under or low, and -natrium is latin for sodium, often written as Na plus, and -emia refers to the blood, so hyponatremia means a lower than normal concentration of sodium in the blood, generally below 135 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—one third of it is in the extracellular fluid, meaning outside the cells, and two thirds of it is in the intracellular fluid, or inside cells.

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.

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 moves 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”.

That being said, hyponatremia, or low concentration of sodium in the extracellular fluid and therefore the blood, can be caused by either losing more sodium than water, or gaining more water than sodium.

Broadly speaking, hyponatremia can be divided into three categories based on water volume status.

The first is hypervolemic hyponatremia where there’s an enormous increase in total body water with a less significant increase in total body sodium.

Typically this is seen in conditions like congestive heart failure, cirrhosis, or nephrotic syndrome where a lot of fluid leaks out of the blood vessels and into the interstitial space, causing edema especially in the ankles.