‘Boring’ Fluids & Electrolytes (Part 2): Oncotic Pressure and Hydrostatic Pressure in Nursing School

In Part 1 of this series, I reviewed Osmosis & Diffusion (including examples of how understanding them can improve your exam grades!). In Part 2, I’m reviewing the 2 less common forces at play in Fluids & Electrolytes: Oncotic Pressure and Hydrostatic Pressure. But just because they are “less common”, don’t be fooled…you still need to understand these be better at thinking like a nurse!

What is Oncotic Pressure in Simple Terms for Nursing Students?

You should know now that I like to explain things like I’m talking to a 4th grade…simply because it’s easier to understand that way! So what’s the 4th grade definition of Oncotic Pressure?

Oncotic Pressure can be defined as the magnetic-like pull that proteins have on water. Take a minute to practice your active learning strategies (it’s the #1 thing that helps with retention!) and think about how that differs from Osmosis and Diffusion that we discussed in the last article. With Oncotic Pressure, the water is, in a sense, being pulled by the protein.

Role of Albumen on Oncotic Pressure

It’s important to know what protein we’re talking about. Now, there is water (and protein) all throughout the body. But in nursing school, we are most often discussing water/fluid shifts in and out of the blood vessels. So it makes sense that the protein we are most interested in related to Oncotic Pressure is also located inside the blood vessels. And that protein is albumen…ever heard of it?

Albumen is the most common protein inside the blood vessels, which makes it the biggest factor in regulating Oncotic Pressure. It’s a relatively big molecule, and therefore is not able to leave exit the semi-permeable membrane of the blood vessel walls. At least…not under normal circumstances…

Using Oncotic Pressure to Predict Disease Symptoms (so you don’t have to memorize them!)

Here’s an example of how understanding Oncotic Pressure can make studying other nursing school topics easier. It’s because in certain medical situations, like severe burns, the normal variables change. And when you understand oncotic pressure, you can make predictions about what will happen as a result (spoiler alert: that would be the symptoms!)

See, albumin is normally too large to pass through the blood vessel walls. However, when a patient suffers a severe burn, the permeability of the blood vessel walls change. Basically, the “holes” get bigger, which allows the larger albumin molecules to escape into the interstitial spaces.

Because Oncotic Pressure causes those albument molecules to have a magnetic-like pull on water…guess what also escapes out into the interstitial space! The water.

This movement of albumen–followed by water–OUT of the blood vessels can lead to circulatory collapse, one of the common, acute complications after a serious burn injury. Now YOU practice thinking like a nurse…leave a comment below to tell me: What do you think will happen to the blood pressure when albumen leaves the blood vessels?

That’s an example of how understanding the foundational basics can help you study better and more efficiently in nursing school. You don’t have to memorize the symptoms, when you are able to use your existing critical thinking skills to figure out what you need to know. (FYI…this is Step 3 of my Silver Bullet Study System!)

And then let’s talk about the Medical and Nursing Care…what do you DO in this situation? Well, if the albumen is leaving the blood vessels, then obviously we would want to replace it ASAP. Which is why one of the expected treatments in this situation would be to administer a colloidal IV solution, which often will contain (you guessed it!) albumen! (And that would be an example of Step 4 of the Silver Bullet Study System!)

Hydrostatic Pressure: The Force of Water

Now, let’s turn to hydrostatic pressure. This is probably the least used of these 4 F&E concepts in Nursing School, but it’s about the force exerted by the fluid itself.

Think of filling a water balloon – the more water you add, the more it stretches, right? That’s hydrostatic pressure in action. The water itself takes up space, and in order to take up that space it must exert a certain amount of pressure on the molecules around it. In our bodies, this pressure is what blood exerts against the walls of blood vessels.

Using Hydrostatic Pressure to Predict Disease Symptoms (so you don’t have to memorize them!)

When the Hydrostatic Pressure is high, like in cases of increased fluid volume or high blood pressure, fluid can leak out of the blood vessels into surrounding tissues – hello, edema! On the other hand, if the pressure inside the blood vessels drops too low, then the fluid in the interstial space can push its way back into the blood vessels.

Applying Oncotic Pressure & Hydrostatic Pressure on Nursing School Exams

Just like with Osmosis and Diffusion, we can’t only understanding these principles in isolation; you have to practice the “mental gymnastics” that help you understand how each concept influences each other, too. That’s where homeostasis comes in. When studying diseases using the 4-steps of the Silver Bullet Study System, for example, you can make really good predictions about what symptoms you’ll see based on which direction each of these 4 forces is trying to move the water, and which one is strongest.

For example, under normal circumstances, Oncotic Pressure pulls water IN the blood vessel. Hydrostatic Pressure pushes water OUT of the blood vessel. But the Oncotic Pressure (combined with Osmosis, which is also a big factor keep water inside the blood vessels) is a much stronger force than Hydrostatic Pressure. This keeps the appropriate amount of water safely inside our blood vessels so that homeostasis can be maintained.

Study Better in Nursing School

I hope this deep dive into oncotic and hydrostatic pressure has shed some light on the intricate workings of our body and how you, as a future nurse, can apply this knowledge. Remember, understanding normal is required to understand the more complex topics you’ll encounter later in your nursing school journey…and your future nursing career.

For more resources and guidance, grab my free Silver Bullet Study System Overview.

And if you found this article helpful, have a request for a future topic, or simply want to tell me where you’re at on your nursing school journey…leave a comment, share with your classmates, and even tell your instructor about Your Nursing Tutor.

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