Dear Victor,

Is there anything better than lapping up cool water on a hot day?

I talked about why we need to hydrate with my friend Ed Johnson. He teaches classes about the human body in the School of Biological Sciences at Washington State University.

It turns out that up to 60% of an adult human’s body is water. A kid’s body contains even more water than a grown up. When you were a baby, you were about 78% water.

Humans use that water for all kinds of things. It keeps you cool when you sweat. It removes waste when you pee. It moves important stuff around inside your body. It even cushions your brain and spinal cord.

But staying hydrated is really about your cells.

A cell is the smallest bit of something alive—like the smallest bit of a cat or a worm or a human. Some organisms are made of just one cell like bacteria. But humans are made of trillions of cells.

Every one of those cells is filled with watery goo called cytoplasm. It’s mostly water and electrolytes. Electrolytes are minerals with an electric charge. Your body uses them to do important things like make your nerves and muscles work.

These are cells from a waterweed plant. The movement of the stuff inside the cells comes from the flow of the clear, watery goo in the cells—also known as cytoplasm. That flow is called cytoplasm streaming. Credit: MarceloTeles/Wikimedia

There’s water outside your cells, too.

“The fluid that surrounds all the cells is their environment,” Johnson said. “It must be kept just right to enable the cells to survive.”

It’s just right when the amount of electrolytes in the water inside your cells matches the amount in the water outside your cells.

Have you ever made a glass of salt water? The salt you stir into the water is made of two electrolytes—sodium and chlorine.

If you leave the salt water sitting out, the water will start to evaporate. It will turn into water vapor and float off into the air. That means there’s less water in the glass. But the amount of salt hasn’t changed. So, the water that’s left will be more salty.

That’s how it works inside your body, too. If you don’t drink enough water, there won’t be as much water surrounding your cells. It will become “saltier” or more concentrated with electrolytes and other stuff dissolved in the water.

Remember that your cells want things to be equal inside and outside the cell. So, water from inside the cells will rush out of the cell until things even out. When water leaves the cells like that, the cells shrink up.

The opposite can happen, too. If you drank a bonkers amount of water, the fluid outside your cells would be way less “salty” or less concentrated. This time, water would rush into the cells and make them swell up.

Your cells don’t work properly when they’re too shriveled or too swollen. But good news! There are receptors in your brain that keep track of the fluid outside your cells. If it gets too concentrated—because you’re dehydrated—your brain gives you the sensation of thirst.

It’s how your body says, “Water you doing? Time to get a refreshing drink, friend.”


Dr. Universe