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In this lesson we’re going to talk about fluid pressures. In the last lesson we talked about where the fluid is and how it moves in the body. Now, we’re going to talk about why it moves around. There are three main pressures within the bloodstream and body fluids that force the movement of fluid and electrolytes throughout the body, so let’s look at each of those now.
The three pressures are Osmotic Pressure, Hydrostatic Pressure, and Oncotic Pressure – also known as “Colloid Osmotic Pressure”. When you think of Osmotic Pressure, I want you to think concentration. This pressure refers to how water moves through the body because of concentration gradients – remember we talked about Osmosis? So if you have one solution that’s super concentrated, and another that’s more dilute – and a semipermeable membrane between them…the water is going to want to move into the more concentrated one. That force that moves the water in that direction is called Osmotic Pressure. Next is hydrostatic pressure. When you hear this I want you to think about a pushing pressure. This is the pressure exerted by the water inside blood vessels that is physically pushing outward. Think about if you filled up a cheesecloth bag with water – it would probably just drip out, right? But if you squeeze the bag, more and more water will come out. That’s hydrostatic pressure. It’s the physical force exerted by water that forces water and some particles OUT of the blood stream. And finally we have oncotic pressure. When you hear this, I want you to think “Protein Pulls”. This is the pressure exerted by proteins and they tend to pull water and fluid toward them. The most common protein in the bloodstream that does this is albumin.
So, let’s just look at what this would look like in the blood stream. Let’s say we have a super high blood sugar or some super high electrolytes in the blood. Based on Osmotic pressure, which way is the fluid going to want to shift? It’s going to shift INTO the bloodstream, right? It’s trying to balance out those concentrations. The blood develops this high osmolarity and the water shifts this way. If the blood was super dilute compared to the interstitial space, then water shifts the other way – it’s entirely based on concentrations. Now, let’s talk hydrostatic pressure. We see this mostly in the capillaries – the super tiny blood vessels. The fluid in those vessels ends up being under tremendous pressure because there’s more fluid in a smaller space, so it forces this fluid out of the vessels. A great example of this is the filtration that happens in the glomerulus in the kidneys. It’s a tuft of capillaries with a super high hydrostatic pressure and it forces the fluid and solutes out of the bloodstream. And finally we have Oncotic pressure. Remember this is about protein pulling water towards it. Most of the time we have a bunch of albumin in the bloodstream and it helps pull water in and hold it in. If we start losing that albumin, we losing our pulling power. OR if we start getting protein leaking out of the vessels, it will pull the water with it. So that’s oncotic pressure. In the next lesson we’ll look more at what conditions make fluids shift around to places we don’t really want them in our bodies.
Before we wrap up, I just want to mention one thing quickly. I’ve talked about osmolarity when I talked about osmotic pressure, but when we start talking about IV fluids, you’re going to start hearing about tonicity – so I want to explain the difference. Osmolarity is the concentration of a given solution. So we’re just looking at one solution – like the blood for example. The more solute there is dissolved in it, the higher the osmolarity and the more concentrated it is. The less solute dissolved in it, the lower the osmolarity and the more dilute it is. So, the normal blood osmolarity measurement is 275 – 295 mOsm/L. Now, when we talk about Tonicity – we’re actually comparing the osmolarity of 2 different solutions. So we may compare something to the blood for example. If the solution is more concentrated than the blood, we’d say it’s hypertonic. If it’s less concentrated than the blood we’d say it’s hypotonic. And if it has about the same concentration, we’d say it’s isotonic. So keep these things in mind and keep these pressures in mind as we start to look at fluid shifts and the different types of IV fluid solutions.
Just a quick recap. Osmotic pressure is related to the concentration and refers to the process of osmosis – the movement of water based on a concentration gradient. Hydrostatic pressure is the pushing pressure of water in a vessel that forces fluid and solutes outward, out of the vessel – like in the glomerulus. And Oncotic Pressure is the pulling pressure of proteins like albumin that help pull water into the vessels and hold it there. And also remember the difference between osmolarity and tonicity. Osmolarity looks at the concentration of one solution, whereas tonicity compares the concentration of two solutions – again usually we’re comparing something to the osmolarity of the blood.
Keep watching all the lessons in the Fluid & Electrolyte course to really see the big picture of how fluid moves throughout our bodies. Make sure you check out all the resources attached to this lesson as well. Now, go out and be your best selves today. And, as always, happy nursing!!