01.01 Fluid Compartments

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  1. Main Compartments
    1. Intracellular
    2. Extracellular
      1. Intravascular
      2. Interstitial

Nursing Points


  1. Intracellular Fluid (ICF)
    1. Inside the cells
    2. 70% of body water
    3. Primary cation = Potassium (K+)
  2. Extracellular Fluid (ECF)
    1. Outside the cells
    2. 30% of body water
    3. Primary cation = Sodium (Na+)
    4. Intravascular
      1. Inside the blood vessels
      2. 20% of ECF
      3. Primarily blood plasma
    5. Interstitial
      1. In the tissues, outside blood vessels
      2. 80% of ECF
  3. Fluid Definitions
    1. Solution
      1. A substance containing both a liquid (solvent) and particles (solutes)
    2. Solvent
      1. Liquid in which particles are dissolved or carried
    3. Solutes
      1. Particles which are dissolved in a solution
    4. Example:
      1. Solution = Salt Water
      2. Solvent = Water
      3. Solute = Sodium Chloride
  4. Semipermeable Membrane
    1. All fluid spaces are separated by some sort of  semipermeable membrane
      1. Cell membrane
      2. Vessel wall
      3. Tissue membranes
      4. Organ walls
    2. Only certain small particles and water can pass through
    3. Other particles or substances require facilitated diffusion
  5. Modes of Transport
    1. Diffusion
      1. Movement of particles through a semipermeable membrane from an area of high concentration to an area of low concentration
    2. Facilitated Diffusion
      1. Movement of particles using a transport channel/protein or carrier molecule
    3. Osmosis
      1. Diffusion of water through a semipermeable membrane from an area of low concentration of solutes to high concentration of solutes


Hey guys, my name is Brad and welcome to nursing.com. And in today’s video, we’re going to discuss our fluid compartments. I really want to discuss the various types of compartments where fluid can reside within our body, as well as how fluid moves interchangeably between these various compartments. Let’s dive in. 

So whenever we’re talking about fluid compartments, we are pretty much looking at three primary fluid compartments, and I’m gonna break these down a little bit. The first one that we’re looking at is our intracellular compartment. The second is going to be our intravascular compartment (excuse that spelling). And the third is our interstitial compartment. Let’s actually bring a little bit of clarity and make this make sense. 

So what we have here is a hand and we’ve got an arm and inside of this arm, just like yours and my arm, we have blood vessels. And within that blood vessel, we have all of these little red blood cells. This is the way that we’re going to conceptualize these three different compartments, right? Our intracellular compartment is actually the fluid inside of the cell. Okay. That’s our first fluid compartment, our intracellular compartment, it’s the fluid inside of the cell. And it’s important to know that our primary cation inside of the cells is potassium with a normal reference range of 3.5 to 5.1. 

Our intravascular compartment is actually all of the other fluid within that vessel that is not inside of cells, right? All of the plasma, the plasma proteins like albumin, all of the additional fluid inside of these vessels is our intravascular compartment. And it’s important to know that our normal, our primary cation inside of the intravascular compartment is sodium. And that our normal reference range for sodium is 135 to 145. 

And finally, our interstitial compartment is our tissue compartment. The actual tissues in that arm contain fluids as well. And the way that I like to think about these three, these three,  compartments is like a scale, right? There is a perfect homeostatic balance and movement of fluid interchangeably in between the intracellular intravascular and interstitial compartment that maintains a nice balanced fluid volume across these various compartments. And the way in which this fluid moves is something that you probably learned a long time ago called osmosis. 

So regarding osmosis and the movement of fluid you’ll recall that we have a semipermeable membrane. It’s a semipermeable cellular membrane, right? Let’s say this is our little red blood cell. And this right here is actually a little cross section of the actual cellular membrane, the lining of that cell itself. Okay. Now, we’re going to remember regarding the semipermeable membranes that they allow the passage of specific molecules, as well as electrolytes and water to pass freely across this semipermeable membrane. 

Now regarding the movement of fluid interchangeably between these various compartments, that we’ve just mentioned. We’re going to want to remember, right, the way that osmosis works, it is the movement of water from areas of low concentration of solutes across that semipermeable membrane toward areas of higher concentration of solutes. Remember solutes are little small particles, molecules, such as that potassium, such as that sodium, right? Let’s consider it something like that solvent is the actual fluid in which that those solutes are dissolved in. We’re going to consider that to be plasma. And a solution is the actual combination of the solutes and solvents together, pretty much producing blood. That’s the way I like to break down, solutes and solvent, eventually producing a solution. 

Now the best way that I like to think about conceptualizing this is dehydrated compartments, right? If you can think about a dehydrated compartment, it’s better. It’s an easier way to understand why fluid moves osmotically from areas of low solute concentration, to areas of high solute concentration. Think about a glass of water, right? You’ve got a nice little glass of water and you have poured a lot of salt into that water. I just might be an artist. Okay? So you pour a bunch of salt down into that water. And now this water is heavily concentrated in salt. Now, what happens if you were to boil that water, that glass of water, the water’s going to eventually go away. It’s going to end up leaving a little bit of water but, incredibly high and heavily concentrated amount of salt will be left behind residing in the bottom of that water. Think about if you were out in the sun at the beach, you know, sunbathing, and you were out there for several hours, you’re going to dehydrate your body. You’re going to become incredibly thirsty because what remains is a heavy amount of concentration of solutes. You’re going to want to be consuming water, to try and relieve this thirst. The reason being is you have a much more higher concentration of solutes then you do a fluid and that’s going to cause fluid to move osmotically from areas of lower concentration, to areas of higher concentration. 

But now that we’ve discussed what things look like in a homeostatic environment, whenever we’re maintaining that fine fluid balance between these three compartments, what I’d like to discuss now are what happens whenever pathology ensues. Whenever we actually have a disruption in that fine fluid balance, we actually have something that occurs called third spacing. Now, essentially what third spacing is, right, the way that I think about third spacing is what we have is we have too high of osmotic pressure. Okay. What this basically means is we have too great of a pressure within our intravascular compartment. Now we know we’ve got our cells in here and there’s the intracellular compartment as well, but there is too much pressure within this vessel. And this pressure actually pushes on the walls of the vessel and this causes fluid to seep out of the vessel and into that interstitial space. Like we see over here, this occurs in situations such as patients experiencing heart failure, their intravascularly volume overloaded causing an increased intravascular pressure, forcing fluid against that vessel wall to cross that vessel wall and to seep into the interstitial tissues. We may see this such as edema right. We see this all the time in patients who experienced CHF. 

So what are some of the therapeutic managements that we may see given surrounding this concept of fluid compartments? Right? We could see fluid based therapeutic management, not just hypotonic fluids or hypertonic fluid recalling that hypotonic means less concentrated, right? It is a less concentrated IV fluid. We could see this given to patients in situations where they are cellularly dehydrated. If we give a less concentrated fluid intravascularly, this is going to cause the fluid to want to move from the area of low solute concentration inside the vessel to areas of high solute concentration inside the cell. Hypertonic solutions could also be given, for instances, where third spacing has occurred. So let’s remember hypertonic are more concentrated fluids, right? So if we give a heavily concentrated fluid, a fluid heavily concentrated in solute such as the 3% sodium. If we give that intravascularly, it’s going to produce a much higher concentration of solutes in that vessel, which is going to promote the movement of fluid from that interstitial space back inside the vessel. And albumin is a plasma protein. This is something that we actually give all the time in the CV ICU, where I work. Same kind of concept, it is a large plasma protein that is administered intravascularly that increases the solute concentration inside that vessel causing fluid to move from the interstitial space back inside the vessel. And then we can also see electrolyte based therapies as well. We’ve spoken about how osmosis moves from areas of low solute concentration to areas of high solute concentration. It’s also important to know that electrolytes also can move across that semipermeable membrane and that electrolytes move from areas of high concentration, to areas of low concentration, a little bit different than osmosis, quite the opposite, actually. So you can think of a patient who has low blood potassium levels or low sodium levels. Well, we can correct this by administering the proper electrolytes. If a patient has low potassium, we can give them potassium containing fluids. And as this potassium containing fluid goes into the vessel, right, we’re going to remember potassium is primarily an intracellular cation. So as we administer this potassium containing IV fluid into the vessel, it’s going to cause this electrolyte diffusion from this area of high concentration inside the vessel back inside the cell. This is a way in which we’re able to therapeutically treat patients from a fluid perspective, as well as from an electrolyte perspective. 

So summarizing some of the key points surrounding fluid compartments let’s remember that fluid compartments are responsible for maintaining that fine homeostatic fluid balance interchangeably between the various compartments. Make sure you familiarize yourself with the three compartments, right: intracellular, intravascular, and interstitial.  Understanding that the fine fluid balance is maintained through the osmotic movement of fluid across semipermeable membranes from areas of low solute concentration, to areas of high solute concentration. And also understanding, what can occur whenever pathophysiologies ensue and that fine fluid balance is disturbed. Basically a high intravascular pressure, osmotic pressure pushes against those vessels, forcing fluid to seep out of those vessels and into those interstitial tissues, as well as those body cavities. And also make sure that you familiarize yourself again with the therapeutic managements that we just discussed. 

Guys, I hope that this video really helped bring further clarity to the concept of fluid compartments and how not only fluid moves via osmosis, but also how electrolytes move via diffusion. I hope that you guys have a great day, make sure that you go out there and be your best selves today. And as always, happy nursing.