Body fluid compartments

Definitions & Key takeaways

Water makes up an essential part of our body weight. In our body, water is distributed in two major fluid compartments: the intracellular and extracellular compartments. The intracellular compartment consists of all the fluids inside cells; whereas the extracellular compartment consists of all the fluids outside of cells. The extracellular compartment is further divided into three subcompartments, which are the plasma, the interstitial, and the transcellular subcompartments. Plasma is the liquid part of blood. The interstitial fluid consists of fluids which surrounds the cells, whereas the transcellular compartment includes fluids such as cerebrospinal fluid, which surrounds the brain and spinal cord; lymph, which is a clear fluid that helps circulate immune cells; and peritoneal fluid, which surrounds the organs in the abdominal cavity. About 60% of our body weight is water, 40% of it is intracellular, and 20% is extracellular. This is known as the 60-40-20 rule �.

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Introduction0:00–0:35

Water is the key to life, and it takes up a big proportion of our body weight, typically around 60 percent! The precise amount of water depends on a person’s body composition.
Since fat doesn’t store any water, a person’s water content is inversely proportional to a person’s fat content. So a really muscular and lean person would have a relatively high proportion of their body weight made up of water.
Additionally, females tend to have more fat than males and so on average tend to have lower proportion of their body weight made up of water.

Body Fluid Compartments0:35–1:07

Total body water can be subdivided into two major compartments, intracellular fluid which is fluid inside cells, and extracellular fluid which is fluid outside of cell like in the blood and in the interstitial tissue between cells.
Assuming that the total body water is about 60% of their body weight, roughly 2/3 of that, or 40% is intracellular fluid, and the other 1/3 or 20% is extracellular fluid.
This is also known as the 60-40-20 rule. Intracellular fluid is important for dissolving cations which are molecules with a positive charge, and anions which are molecules with a negative charge.

Intracellular Fluid1:07–1:49

The major intracellular cations are potassium (K+) and magnesium (Mg2+), whereas the major anions are proteins and organic phosphates like ATP.
Fluid compartments always maintain the same concentration of positive charges as negative ones in order to stay electrically neutral - that’s called the principle of macroscopic electroneutrality.
So for example, the K+ in the intracellular fluid is balanced out by negatively charged proteins and organic phosphates.

Extracellular Fluid1:49–2:30

The extracellular fluid can be subdivided further into interstitial fluid, which is the fluid that can be found surrounding the cell, and plasma, which is the aqueous portion of blood.
The major cation in extracellular fluid, both in the interstitial fluid and in the plasma, is sodium (Na+) and the major anions are chloride (Cl-) and bicarbonate (HCO3-).
Now, the plasma makes up about 55% of the blood, while the remaining 45% is mostly made of red blood cells, white blood cells, and platelets.
Plasma is made up of around 90% water and 10% proteins like albumin which help transport hormones and minerals. Interstitial fluid and plasma are really similar, and that’s not surprising since plasma leaks out of the blood and goes into the interstitium through tiny pores between endothelial cells in the capillaries.

Interstitial Fluid and Plasma2:30–2:50

These tiny pores between capillaries lets small solutes and water to pass through but block large proteins and cells. In fact, because some proteins are too large to pass into the interstitium, a phenomenon called the Gibbs-Donnan effect helps the plasma remain electrically neutral.

Gibbs-Donnan Effect2:50–3:58

This Gibbs-Donnan effect is when the abundance or negatively charged large proteins repels smaller anions into the interstitium while attracting small cations into the plasma.
So compared to the plasma, interstitial fluid tends to have higher concentrations of small anions like Cl- and lower concentrations of small cations like Na+.
Ultimately, the difference between plasma and interstitial fluid is small, whereas the difference between intracellular fluid and extracellular fluid is quite large and physiologically important.
For example, Na+K+ ATPases help to establish a high concentration of K+ inside the cell and a high concentration of Na+ outside the cell.
Establishing these differences between intracellular and extracellular environments is incredibly important for allowing cells like neurons to fire action potentials, among many functions.

Dilution Method3:58–5:21

Measuring the volume of each of the body fluid compartments can be done using what’s called the dilution method. That’s where various substances which settle in a specific fluid compartment are given to an individual and their concentration is measured.
For plasma, it’s done with radiolabeled albumin, because albumin is a large protein that cannot move into the interstitial space.
For the interstitial compartment, it’s done with slightly smaller molecules like mannitol and inulin which can pass through the capillaries into the interstitium but cannot cross the cell membrane.
Finally, for the intracellular compartment, it’s done with molecules that can freely diffuse across the cell membrane like heavy water isotopes, or D2O, which can easily be differentiated from normal water, or H2O, that’s in the body.
Once adsorbed, D2O will act just like water and distribute in the 60-40-20 fashion, so 2/3s of it is intracellular and ⅓ is extracellular.
So, by injecting a known amount of the substance and allowing the substance to diffuse through the fluid compartments, it’s possible to measure the concentration of that substance in the blood, as well as if any was lost in the urine.
Ultimately, it’s possible to calculate the volume of distribution via the equation: volume equals amount divided by concentration.
Here’s a sample problem. A 70-kg man is injected with 150 mCi, or millicurie, of D2O and 650 mg of mannitol.

Case Study5:21–7:10

During a 2-hour equilibration period, he excretes 10% of the D2O and 10% of the mannitol in his urine. After that, the concentration of D2O in plasma is 0.32 mCi/100 mL and the concentration of mannitol is 4.6 mg/100 mL.
Now that we’ve got our amounts and concentrations, we can figure out some volumes. To figure out the volume of total body water, we look at D2O which will settle in the extracellular and intracellular fluid compartments.
First, we subtract out the amount of D20 excreted from the total amount, so that’s 150 mCi minus ten percent of that or 15 mCi or 135 mCi.
Then we divide that amount by the concentration of 0.32 mCi/100 mL, which gives us the volume of 42.2 Liters. Now, to figure out the extracellular fluid, we look at the volume of distribution of mannitol.
The extracellular fluid volume equals the amount of mannitol injected, minus the amount of mannitol excreted, divided by the concentration of mannitol, which gives us 12.7 liters.
Finally, if we want to calculate the intracellular fluid volume which cannot be measured directly, we need to take the difference between the total body water and the extracellular fluid volume, which comes out to 42.2 liters - 12.7 liters or 29.5 liters.
All right, as a quick recap, the 60-40-20 rule states that total body water takes up about 60% of our body weight, and more specifically, 40% is intracellular fluid, and 20% is extracellular fluid.

Review7:10–7:40

Measurements of these fluid compartments uses the dilution method where various substances are injected and allowed to distribute differently among these fluid compartments.
Final concentrations from plasma are taken to calculate the volume of fluid compartments.