When we start trying to understand the difference between hydrostatic and oncotic pressure, first of all we have to understand what osmosis is. Now, we probably learned about osmosis in our Bio 101 class and in Intro to Chemistry but basically what osmosis is, is it’s really the passage of liquid through the semipermeable membrane from an area of low concentration to an area of high concentration of solute.
Basically what we have is, we have our liquid here and inside our liquid we have a bunch of solutes. Okay, here’s all of our solutes. Between that we have a semipermeable membrane, which means that fluid can pass but things of a specific size are not able to pass. Liquid can pass through but these little solutes can’t pass through that membrane.
What the liquid’s going to do is, it’s going to pass through this membrane until homeostasis is reached, until the concentration of the solute is equal on either side of that membrane. So that’s what osmosis is, and that’s an important concept to understand so basically it’s the passage of liquid from an area of low concentration of solute to an area of high concentration of solute to achieve equilibrium and equal concentration of the solute on either side of the semipermeable membrane.
What is oncotic pressure? This definition comes from Wikipedia but it says that oncotic pressure or colloid osmotic pressure is a form of osmotic pressure exerted by proteins, notably albumin, in a blood vessel’s plasma that usually tends to pull water into the circulatory system.
What does that mean? First of all, what is a colloid? Okay, if we go back to that previous slide there, we saw that a colloid … What a colloid is essentially, it’s a substance that does not diffuse easily across a semipermeable membrane so as we saw there in that previous slide, so all these little solutes in here, those are going to be referred to as colloids.
They’re not going to profuse across this membrane and so that’s what we’re talking about when we’re talking about our colloids. Then, what again is oncotic pressure? Basically, it’s the pressure that those colloids, the proteins like albumen within the blood, exert to draw water into the capillary system.
We have our capillary system and what oncotic pressure is, and within our capillary system we have proteins like albumen and albumen is a large protein and it’s not going to be able to get out of the capillary system under normal circumstances so what’s going to happen is, we have all this albumen in here and we have all this fluid outside the capillary system and what that albumen’s going to do is, it’s going to exert a pressure or a force to draw fluid into the capillary system.
That’s why it’s called colloid osmotic pressure. It’s the osmotic pressure that these colloids are exerting within the capillaries to draw fluid inside the capillary system. Albumen is going to be the one that we really focus on within the body because it’s going to be the one that exerts the most pressure. It’s a very large protein within the capillary system.
What is hydrostatic pressure, then? This definition comes from CVPhysiology, it’s a great site to go check out but what hydrostatic pressure is, it’s the pressure that drives the fluid out of the capillary system and it’s highest on the arterial end of the capillary and lowest at the venular end so here again we have our capillary.
Remember we have all these colloids, all this albumen that’s drawing water in to the system and what our hydrostatic pressure is is, it’s that pressure so over here we have our heart and as blood leaves the aorta, it’s leaving that under pressure and this capillary system, or the arterial system is under pressure as well.
It’s under our pressure to push that blood throughout the system so it’s a highly pressurized system and as it gets to the capillaries it remains under pressure, and so what’s going to happen in these capillaries, these capillaries are very permeable so let’s show that this is a very permeable wall that the capillaries have.
What happens is, as the blood reaches the arterial end of the capillary, what’s going to happen is that force that that blood is under, that pressure that that blood is under is going to push the fluid out of the system of the capillary and that’s what they’re referring to here, that’s called filtration.
It’s going to filter the blood. It’s going to push some of the fluid out of the capillary system and then as the blood passes along the capillary system, it’s going to reach the venular end so this is … Then we’re going to go back to the veins and back to the heart. As we reach this side, we have all this albumen that’s still here and that’s going to draw some of the fluid back in so it’s going to get rid of what we don’t want, bring in what we do want, and keep that homeostasis with our blood.
What’s going to … Here, let’s go to the next slide here. Like I said, what creates, so over here we have our heart. What’s creating this hydrostatic pressure? Our aorta’s leaving our heart and it’s passing along in here to the arterial end of our capillary bed, so here’s our capillary bed and the blood in the heart is highly pressurized, right and that squeeze from the arteries is going to keep that blood pressurized so as it comes into the capillary system here, this capillary is semipermeable and that pressure within that artery is going to actually push some of that fluid out.
That’s going to, when we’re talking about capillary hydrostatic pressure, that fluid is going to go out into the third space and things like that, so that’s what we’re talking about there, then as that blood passes through … So that blood’s been filtered out there. As it passes through here we have all this albumen within the capillary and that’s going to draw some of that fluid back in.
Then the blood’s going to go back to the vein and it’s going to come back in through the superior vena cava and back into the heart of course. Back into that high pressurized system and just repeat that process over and over.
How do hydrostatic and oncotic pressure actually work within the body? So far, this is just incredibly overly simplified but here we go. If our capillary hydrostatic pressure is greater than our oncotic pressure, we’re going to have excess fluid leaving the capillary system and where our capillary hydrostatic pressure is less than our oncotic pressure, we’re going to have fluid enter the capillary system.
Let’s talk about the capillaries really quick. Capillaries are very thin walled vessels. They are actually only about one cell thick and they’re highly permeable as you can see. They’re very permeable to that fluid, so that’s going to allow for that osmotic pressure and that hydrostatic pressure to actually work okay, so that’s why this is possible.
It’s possible due to how thin these capillaries are and the pressure going into them and the pressure inside of them as they pass from the artery to the vein. We have our artery that branches off into our capillary system and that comes back together to our veins, so heart, artery, capillary, vein, back to the heart.
That’s where the capillaries come into play, very thin. This is what helps to feed the tissues. Let’s see, here we go. Here’s our heart. It’s going to leave the heart here and it’s going to pass out into the upper body here to the capillary system to feed the upper body. We’re going to feed the liver, we’re going to feed the kidneys, we’re going to feed the lower body. In a lot of ways that’s really how, that’s how the body is getting its oxygen. That’s how it’s getting its nutrients is through the capillary system feeding these tissues.
Here we go. This is where we’re going to actually show what we’ve been drawing this whole time. Here is … Let’s put our heart over here. Here’s our heart. Here’s the arterial end of the capillary so this is a capillary and here’s the venal end of the capillary so this is going back to the heart. This is deoxygenated blood heading back to the heart, oxygenated blood leaving the heart, going into the capillary system here.
As blood enters the capillary system our colloid osmotic pressure throughout this system is going to remain steady at about 25 millimeters of mercury so that’s the pressure that that albumen, for example those colloids inside the capillary are going to exert to draw fluid into it so that’s pretty constant as it passes through here. It’s going to be about 25.
What’s going to change here is that our hydrostatic pressure is going to, as the blood enters the heart here it starts at about 35 millimeters of mercury and so because that hydrostatic pressure is greater than our osmotic pressure here, what’s going to happen is, it’s going to force some of that fluid out and we’re going to get that filtration there.
Then as it passes towards the venal end, what’s going to happen is our hydrostatic pressure actually decreases as it goes down and so our hydrostatic pressure here is actually going to be less than our oncotic pressure and so that’s going to allow fluid to come back in, the resorption of that fluid so that’s really what happens.
Again, what’s really exerting a lot of this force is going to be these albumen molecules here that are very large. It’s coming in at a high pressure and that’s forcing fluid out. It’s passing along the capillary system and that hydrostatic pressure’s decreasing and as we lose some of that fluid, we’re forcing some of that fluid back in to the system.
That’s really kind of how that works. If you want to get a copy of this PowerPoint, you can go to OncoticPressure.com and you can get a free copy of this PowerPoint presentation or you can go to NRSNG/freebies to get it as well. Okay, so let’s just draw the system again real quick.
We have our heart, aorta, so what happens is this is all arteries. The aorta branches into this capillary bed or well, I mean the arteries branch into this capillary bed but then comes out into our vein and that eventually comes back to the superior vena cava, back into the heart.
As we’re leaving the heart here, we have hydrostatic pressure. That hydrostatic pressure coming in to the heart, it’s about 35 millimeters of mercury, passes through and our hydrostatic pressure decreases so what we have here on the venal end is going to be our oncotic pressure.
With our hydrostatic pressure, that’s pushing fluid out and with our oncotic pressure we’re drawing fluid in so when you think hydrostatic, think heart. When you think oncotic, think albumen so hydrostatic, heart, out. Pushing fluid out. Oncotic pressure, albumen. Think in. That’s really how those two play in, so as we have with a situation like heart failure, what’s happening is we’re getting that build up of fluid inside the system.
We’re getting that fluid that the heart is not pumping as well and it’s not pushing fluid through so we’re getting a buildup of that hydrostatic pressure so … What that’s going to do is that increase, so with CHF. I’m sorry, with CHF what’s going to happen is we’re going to get that weak heart and it’s not going to be circulating the fluid as well and so we’re going to get a buildup of that hydrostatic pressure. What that’s going to lead to is that’s going to lead to edema.
On the other end, in a situation like malnutrition, we’re going to have decreased albumen and that decrease in albumen is going to lead to a decrease in our oncotic pressure so what that’s going to do, if we have a decrease in our oncotic pressure here, we’re going to draw less fluid in so that’s going to lead to the third spacing and the edema as well.
Date Published - Mar 17, 2015
Date Modified - Apr 18, 2019