Taken from the InBody Newsletter

You buy a brand new car.  You’re excited.  You vow to take incredibly good care of it forever.  As part of your promise to your new car, you decide that you’ll never miss an oil change every 3 months (or 3,000 miles).

Why is changing your car’s oil so important?  Oil lubricates all the moving parts and keeps the engine running smoothly.  Without oil, your car couldn’t run.  It’s no surprise that many people refer to engine oil as the “blood” of the car.

That’s a pretty strong metaphor, calling oil “blood.”  On one hand, it shows how important people feel oil is, but on the other, it shows the universal acceptance of the significance placed on the word “blood.”  There’s no fluid more important.

With your car, have to change the oil every three months, so you’re forced to think about your car’s “blood” at least 4 times a year.  But what about your own blood?  Do you think about that 4 times a year?  You know you need to make sure your car’s insides are running smoothly, but what about yours?

As it turns out, your blood makes up part of something called your total body water.  It’s no small thing;  your overall weight can be 45-65% due to total body water, depending on age, gender, and fitness level.  In other words, you are made up of mostly of water.  But how much do you really know about it?

Body Water, Defined

Your body water can be found inside not only in your blood, but in your muscles, your fat, your organs, and inside every cell in your body.  To account for all this, your body water can be divided into two basic groups.


Extracellular water is the water located outside your cells.  The water in your blood falls into this category.  Roughly 1/3 of your body water is attributed to ECW, and this water is found in your interstitial fluid, transcellular fluid, and blood plasma.

Extracellular water is important because it helps control the movement of electrolytes, allows oxygen delivery to the cells, and clears waste from metabolic processes.


Intracellular water is the water located inside your cells.  It composes 70% of the cytosol, which is a mix of water and other dissolved elements.  In healthy people, it makes up the other 2/3 of the water inside your body.

The intracellular water is the location of important cellular processes, and although it has many functions, a very important one is that it allows molecules to be transported to the different organelles inside the cell.  Essentially, the ICW picks up where the ECW left off by continuing the pathway for fuel to be transported to the cells.