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As we begin to look at fluids and electrolytes within the body, it is super important that we understand where the fluid is and how it moves throughout the body. So, we are going to start by looking at the different compartments and modes of Transport for fluid and electrolytes.
Let’s start by just understanding the different fluid compartments. Now, when I talked about compartments, I don’t want you to think about there being a little tupperware box of fluid in the body. These are more general terms and exist continuously throughout the body. So, if this is our blood vessel, we have our blood cells inside the vessel, right? We also have some cells outside of the blood vessel making up the various organs and tissues, right? So. The first general compartment you need to know is the Intracellular compartment. The fluid inside the cells is also known as intracellular fluid or ICF. Now, logically speaking, we know that fluid is either inside the cells or it’s not, right? So the second general space is any fluid found outside the cells and we refer to that as our Extracellular space or Extracellular Fluid or ECF. About 70% of our body fluid is located inside cells, and 30% is located outside the cells in the ECF. Now, for the sake of classification, we can break down our extracellular fluid as well, because we also have this space inside the vessels that we can give a specific name to. So we call this the intravascular space or intravascular fluid. It’s outside the cells, but also inside the blood vessels. The rest of the extracellular fluid is known as interstitial fluid, which is all the rest of the fluid within the tissues. So we have intracellular and extracellular. Then the Extracellular fluid is broken down further into intravascular in the vessels or interstitial in the tissues.
One big thing to note is that our body likes these fluids to stay where the belong AND to keep the same concentration of ‘stuff’ in them. One of those things is our cations. Remember a cation is a positively charged ion. The most abundant cation inside the cell is potassium. You can remember this by remembering “Circle K”. The K is inside the cell. You’ll see when we talk about potassium that the levels in the blood are actually relatively low – that’s because we keep it all in the intracellular space, not in the intravascular space. As far as outside the cells, the most abundant cation is sodium. You’ll see its levels are much higher in the blood stream because that’s where it belongs. And you’ll also notice that changes to how much sodium is in our cells can actually cause a lot of problems, because that’s not where we want it. So – potassium in, sodium out.
Now, each of these fluid spaces is separated by some sort of semipermeable membrane. A semipermeable membrane is one that is only somewhat permeable. That means that only certain particles and liquids can pass through it. It’s selective. Typically we see any kind of diffusion or osmosis happening easily, or we could see some facilitated or active transport as well – we’ll talk about what each of those mean in just a minute. Some examples of semipermeable membranes are the cell wall or cell membrane, the vessel walls, tissue membranes, and organ walls. So if we have fluid moving out of the blood vessels, it goes through the vessel wall, which is a semipermeable membrane and into the tissues. We can even see fluid shifting into organ walls or into tissues like the brain. We’ll look at that more in the Fluid Shifts lesson, so make sure you check it out. Just understand that the movement of fluids happens across semipermeable membranes and can happen really anywhere in the body. It’s not a move through a hole or down a vessel, it’s actually straight across a membrane.
Now, I want to quickly review some A&P and chemistry so that you understand some of the terms we’re going to be using. First, let’s remember the definition of a solution. A solution is a substance containing particles dissolved or distributed evenly throughout a liquid. The particles are known as solutes and the liquid is known as a solvent. In MOST cases in the body, the solutes are things like electrolytes, sugars, nutrients, etc. and the solvent is usually water. What we might see is that on one side of a semipermeable membrane you have a lot of solutes and not as much solvent – in other words it’s super concentrated. And on the other side of the membrane you have way more solvent than solutes – so it’s more diluted. When this happens, the body desperately wants to create two evenly concentrated solutions – so things are going to start moving across this membrane to make that happen.
There are 4 common ways that things move across these membranes in our body. The first is diffusion – it’s the movement of the particles, specifically – ions, sugars, salts, etc. Facilitated diffusion is the movement of particles using a carrier protein of some sort. And osmosis is the movement of water only – no particles. All 3 of these are “across a semipermeable membrane” and all 3 of them follow a concentration gradient – meaning the substances move from an area of high concentration to low concentration of that substance. The last one, active transport is moving some sort of particle using energy or ATP – that usually means it’s going against a concentration gradient. As we look at fluid & electrolytes, we’re not really going to be looking at Active Transport. We’re also not really even looking at Facilitated Diffusion. Most of what we’re going to see is regular Diffusion and Osmosis. So let’s see what this would look like with those solutions we talked about. On one side there are a lot of solutes, on the other side, not so many. In order to balance this out by diffusion, the goal would be to move the particles. Again – from an area of high concentration to low concentration. That means some of these particles would move over here. Now, you’ll end up with two solutions with even amounts of particles. That’s diffusion. Osmosis, however, is the movement of water ONLY. So if we have two solutions, just like the first time. One is more highly concentrated with solutes than the other. BUT – we’re only using osmosis to fix the problem because that semipermeable membrane is only going to allow the movement of water. Instead of these particles moving, the WATER is going to move to try to even out the concentration. The water moves from an area of high concentration of water to low concentration of water (in other words more water to less water). This is confusing, because this part is considered more concentrated. But just remember, whatever is moving (whether particles or water) it goes from where there is more of it to where there is less of it. So – the water will shift this way. Then, you’ll be left with the same number of particles as before, but more water on one side now. So the overall concentrations are now the same. The goal is always to balance out the concentrations.
So let’s do a quick recap. Make sure you know the compartments. Intracellular – inside the cell. Extracellular outside the cell. That is then split into intravascular in the vessels and interstitial in the tissues. Make sure you know the primary cation in the cell is potassium – remember Circle K. And the primary cation outside the cell is sodium. All of the compartments are separated by semipermeable membranes that allow certain particles and fluids to pass through. Remember that the liquid is called a solvent and the particles are called solutes, and the body is always trying to create balance. To do that, it will use things like diffusion – which is the movement of particles across a semipermeable membrane following that concentration gradient, and osmosis – which is the movement of water across a semipermeable membrane following a concentration gradient. There are also facilitated diffusion which requires a carrier protein and active transport, which requires ATP or energy, but those don’t really play a big role in fluid & electrolyte balance.
So I hope that introduction to the fluid compartments and the movement of fluids helped. Make sure you check out the lessons on fluid pressures and fluid shifts to learn more about how fluids move throughout our bodies. And don’t forget to check out all the resources attached to this lesson. Now, go out and be your best selves today. And, as always, happy nursing!!