4 “Boring” (But Important!) Secrets of Fluid and Electrolytes (Part 1)

Today, we’re talking about some seemingly boring, “unsexy” topics in nursing school – but they’re foundational if you hope to understand the more difficult topics you’ll need to know later in nursing school! Just like building a house, you HAVE TO build a strong foundation of knowledge related to “normal” concepts so that you can build on it. Without that understanding, your house (and potentially your nursing school experience!) will collapse.

So what are the 4 boring, everyday topics you need to fully understand in Fluids & Electrolytes? They are: osmosis, diffusion, oncotic pressure, and hydrostatic pressure. But you can’t just memorize academic definitions; you need to take the time to develop a deeper understanding so that you can use this knowledge to think like a nurse when it comes to disease symptoms and nursing care.

Water Balance across the Lifespan

One of the first things you study in Nursing Fundamentals is developmental issues across the lifespan, and one of the most important is how the percentage of water in our body changes as we age. Remember that chart in your textbook (or on your instructor’s powerpoint slides…) that showed the little baby with around 70% of body water? Remember how that percentage decreased into adulthood, then increased even further in the elderly?

Changes like that do need to be memorized, but they are best memorized as a pattern. Our brains are wired to remember patterns and stories easier than random facts, so it’s a great memory trick. In this case, think about the juice, plump, squishy, oh-so-watery, little baby.

Then as that baby grows up, it gradually dries up like a shriveled old prune, lol! Now it’s going to be easier for you to remember that younger people have higher levels of water content in their body, whereas the elderly have lower percentages of water content in their body.

This is important, because now you have a basis for understanding why the very young and the very old have a higher risk for fluid imbalances…it’s because their body water is on either end of the spectrum, so there’s less “wiggle room” for error (refer back to my previous blog post on homeostasis…because that’s what I mean by “wiggle room”!).

And this is also why Osmosis, Diffusion, Oncotic Pressure, and Hydrostatic Pressure are all such important, foundational concepts when you want to understand Fluids & Electrolytes (or really anything in nursing school…). It’s because they are play a HUGE role in how water moves around our bodies…and our bodies are mostly made up of water!

Osmosis for Nursing Students

So what is Osmosis? Let me give you the definition for 4th graders (as a reminder, this is not because you’re dumb, it’s simply because it’s easier to understand that way for everyone! Including me, lol!).

Osmosis requires 3 things:

  • Water (duh)
  • A semi-permeable membrane: Since water is a liquid, it’s going to automatically fill whatever container it is in. That’s just what liquid does. In order to move from one area to another, there needs to be a semi-permeable membrane that allows the water molecules to pass through, but doesn’t allow the other “stuff” to pass through.
  • Stuff: “Stuff” is my highly technical term for anything that isn’t water, but is hanging out in the water, such as the electrolytes and, well…stuff.

So Osmosis is where water wants to move around and dilute stuff as evenly as possible.

(Of course, water doesn’t want anything at all, because it’s an inanimate object. And osmosis is a passive process that doesn’t require any energy…it simply happens. But personally I think it’s easier to talk about and understand it this way, don’t you agree?)

So if you have more “stuff” on one side of a semi-permeable membrane than on the other, then more water is going to want to move to the side with more stuff so that it can dilute it. Think about ratios or fractions. The goal of osmosis to have have an even distribution of stuff to water (stuff/water) on both sides of the semi-permeable membrane.

Osmosis to Predict Disease Symptoms (instead of trying to memorize them)

In the human body, there are tons of semi-permeable membranes. But when it comes to nursing school, the 2 that are most important to fluids and electrolytes are 1) the blood vessel walls and 2) red blood cell membranes.

That’s because blood is mostly made up of water (seriously…look up what’s in the blood. It’s mostly plasma, and plasma is mostly water. No surprise, given that your body is mostly water!)

Since the blood is mostly water (and so are IV fluids, fyi), then you can use your understanding of osmosis to predict where that water will move in the body based on where the stuff is going. Because water follows the stuff, remember?

So if a patient has a problem where their kidneys are filtering out too much sodium (Na+), then in most cases the water is going to follow the Na+ (because that’s a lot of “stuff”!). And you should already be predicting that this patient is going to be at risk of fluid volume deficit or dehydration, because they are going to lose water. Osmosis is going to cause the water to follow the sodium right out of the body.

If there’s a condition where the sodium (Na+) moves from inside the blood vessels to the interstitial space, then what do you predict would happen then? The water would follow and also move into the interstitial space, too! And that’s one of the possible causes of edema.

Diffusion for Nursing Students

Diffusion, while similar to osmosis, focuses on the movement of the stuff instead of the water. Here’s how I like to define it: stuff likes to spread out as equally as possible.

Now, it’s important to note that when we’re talking about the human body, this stuff is almost always going to be dissolved in water. BUT diffusion does not require water in order to occur. Just like osmosis, it is a passive process that does not require energy. But it can also happen in the air.

I’m going to give you an example of diffusion you will never forget. If you are squeamish…you might want to skip the next paragraph, lol!

Think about dog poop. When there is a solid pile of dog poop, those dog poop molecules are packed closely together. They want to spread out! So some of the dog poop molecules on the surface diffuse out into the air (where there is currently little to know dog poop molecules present). When those dog poop molecules diffuse into your nose, they connect to your olfactory receptors, and that’s how your brain knows that you smell dog poop. So the next time you smell dog poop…it means you have dog poop in your nose. And you can’t unknow that fact, sorry!

A less gross analogy of diffusion is to think about a part in a large ballroom. Let’s say that 100 people attend. Are they all going to crowd into the same corner? Of course not! That would be totally weird.

Instead, the people would mill about and generally spread out throughout the entire room. Is every single person going to be exactly equidistance from each other? No, of course now. And that’s not going to be true in diffusion either, because molecules (like people!) are always moving about. But in general, the trend is going to be that people will be equally distributed around the room.

Diffusion to Predict Disease Symptoms (instead of trying to memorize them)

Diffusion is essential to understand a lot of the normal process of the body. Again, diffusion is a big part of how we urinate! Urine is simply filtered blood, and if you review the Loop of Henle from A&P, then you’ll quickly see that it involves a lot of electrolytes diffusing OUT of the descending loop, then whatever is kept diffuses back INTO the ascending tubule.

This can happen because the direction of diffusion changes. Remember, diffusion causes the “stuff” to want to move from crowded areas to less crowded areas. And in the Loop of Henle, the “stuff” starts out crowded in the descending loop so it leaves. But then the loop is kind of empty of stuff…so now the “stuff” wants to diffuse back in again!

Diffusion is also essential in the lungs. We literally wouldn’t be able to breath without diffusion. It’s the reason why the aveoli walls are so incredibly thin…because oxygen and carbon dioxide molecules both need to diffuse through.

Think about it…when you breath in, you have relatively more oxygen in your lungs than the blood cells returning to your lungs carrying all that carbon dioxide. So the carbon dioxide wants to move away from the red blood cells (because there’s a lot of carbon dioxide there) and move toward the lungs (where there’s a lot of oxygen, but not a lot of carbon dioxide…yet).

And the oxygen wants the reverse…there’s a lot of oxygen in the lungs, and not a lot in the capillaries. So the oxygen diffuses into the capillaries to spread out.

Eventually, there is enough oxygen that has moved into the blood vessels, and enough carbon dioxide that has moved into the lungs, that diffusion slows down. And that is when you exhale to get rid of all that diffused carbon dioxide waste, and breathe in more oxygen to restart the process.

Isn’t the human body amazing?!?

The Silver Bullet Study System Helps You Apply Osmosis & Diffusion to Exams

Are you ready to transform the way you study and think in nursing school? Check out my free 10-minute overview of the Silver Bullet Study System. This system isn’t just about passing exams; it’s about teaching you to study in a way that forces your brain to think like a nurse, making a real difference in your future patients’ lives.

And in my Part 2 of this article, I’ll explain Oncotic Pressure and Hydrostatic Pressure, so that you can be fully prepared to tackled Fluids and Electrolytes for yourself!

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