By John K. Roberts, MD MS MEd - 22nd Dec 2021 - Course previews

Identifying body fluid compartments

In this video, we discuss body water distribution and influence of muscle mass on body water amount. We also walk you through clinical calculations. Have a look to find out how much body fluid bodybuilders have!

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Video transcript

The simplest way to think about the body is to recognize that for the most part, it is simply a whole bunch of particles dissolved into a solution of water. There are a wide variety of solutes in our body, and each solute behaves differently when it is dissolved in our body fluid. Knowing the basics of water and solute balance in the various body fluid compartments is critical to managing patients with fluid balance disorders.

In order to study body fluid, we need to determine an estimate of total body water. To estimate total body water for a person with high muscle mass, calculate 0.6 times the total body weight. To estimate total body water for a person with low muscle mass, calculate 0.5 times the total body weight. Let's do some examples. These muscular individuals have a weight of 90 kilograms. Using the formula we calculate that their total body water is about 54 liters. That is approximately 60% of the total body weight.

Alternatively, these individuals who have less muscle mass weigh 70 kilograms. That would make their total body water about 35 liters or 50% of body weight. Because each person is different, you should use clinical judgment when deciding which formula to use based on muscle mass. Now that you can estimate total body water, let's talk about how it is distributed. Two thirds of total body water is intracellular and 1/3 is extracellular. Intracellular, meaning the cytoplasmic fluid inside of cells. Extracellular meaning the fluid surrounding these cells.

Extracellular fluid can be further broken down into two categories, interstitial and intravascular. Three quarters of the extracellular fluid is composed of interstitial fluid, which is the fluid taking up the space in between cells. The remaining quarter of the extracellular fluid is composed of the intravascular fluid, or IVF, which is the circulating fluid contained in arteries and veins. It is important to note the exclusion of hematocrit in this category. Hematocrit, which is made up of red blood cells is normally 45% of the total circulating fluid volume and is technically part of the intracellular fluid. Therefore, when we refer to the extracellular intravascular fluid, it is the portion that does not include circulating red blood cells.

Let's do some examples, starting with an individual that weighs 70 kilograms. Because this individual shown has normal muscle mass, we will multiply the body weight by 0.5. First we calculate the total body water to be 35 liters. Two thirds of the total body water is intracellular. Therefore, we calculate two thirds times 35 liters and find it to be 23.3 liters. 1/3 of the total body water is extracellular so, 1/3 times 35 liters is about 11.7 liters. Three fourths of the extracellular fluid is interstitial fluid. Thus, three fourths times 11.7 gives us 8.75 litres of interstitial fluid and the remaining fourth or 3 liters would be the intravascular fluid.

Now let us consider an individual who weighs 90 kilograms and has high muscle mass. In this case, we will multiply the total body weight by 0.6. This gives us a total body water of 54 liters. Two thirds times 54 liters gives us 36 liters of intracellular fluid. 1/3 times 54 liters gives us 18 liters of extracellular fluid. Of this 18 litres, three fourths is interstitial fluid, which would be 13.5 liters. The remaining quarter is intravascular fluid which is 4.5 liters. Now, your assignment is to use your own body weight to estimate your total body water. Go ahead and calculate how much of that fluid is intracellular, extracellular and then interstitial and intravascular.