Tonicity

Imagine you have a big, clear glass filled with water. In that water, you put some salt. As you stir the water, some of the salt dissolves and you eventually end up with a salty water mixture. This salty water is what we call a solution.

Now let's say you have two smaller glasses. You take some of your salty water mixture and pour it into one glass, while pouring plain water into the other. If you try to taste both of these glasses, you would notice that one tastes much saltier than the other.

The difference in saltiness between the two glasses has to do with something called tonicity. Tonicity is a fancy word that describes the relative concentration of dissolved substances in two different solutions.

In the example we just gave, the glass with the salty water had a higher concentration of dissolved salt, which made it more concentrated, or hypertonic. The glass with the plain water had a lower concentration of dissolved salt, making it less concentrated, or hypotonic.

When we talk about tonicity in the body, we're usually referring to the concentration of salt (sodium chloride) in the fluids outside of our cells. Our cells need a certain balance of salt to water, and so when there's too much or too little salt in the areas surrounding our cells, it can cause problems.

For example, when we drink too much water, our cells can become swollen with excess water (a condition called hypotonicity). When we drink too little water or lose lots of salt through sweating or urinating, our cells can become dehydrated and shrink (hypertonicity).

So, to sum it up, tonicity is a fancy word to describe how much salt is in a solution. When there's too much or too little salt in the fluids outside of our cells, it can cause issues with our cells and how they function.