May 31, 2011

058 Net Hydrostatic Pressure and Filtration Pressure

How do the differences in hydrostatic and osmotic pressures affect the flow of blood within the circulatory system and to the different parts of the body? What is filtration pressure and how are these affected during abnormal conditions such as having a high blood pressure?

Watch and learn with Leslie as he explains further about this topic. Have fun!

Transcript of Today’s Episode

Hello and welcome to another episode of Interactive-Biology TV where we’re making Biology fun! My name is Leslie Samuel and in this episode, Episode 058, I’m going to be talking about “Net Hydrostatic Pressure and Filtration Pressure.” Let’s get right into it.

Now, we’ve been looking at the circulatory system and we’ve shown that the blood leaves the heart and then, it goes via the aorta to the arteries and then, to the capillaries, to the venules, to the veins and then, to the vena cava, and then, ultimately, back to the heart. If you need a review of that, you can always check out Episode 054 where we go into more detail about that.

What we’re going to be doing today is we’re going to be looking at what happens between the arterioles, the capillaries, and the venules. That’s what we’re showing here. We have an artery leading to the arteriole and then, that goes to the capillary bed and then, that goes via the venules to the vein. What we’re going to look at is what happens specifically right here. The goal here is we want to get blood coming to the tissues delivering nutrients and so on, oxygen to the tissues and then, taking stuff away. So, taking away waste and so on.

What I’m going to do here is I’m going to simplify this a little bit. I’m going to show this like this. I’m going to take from the arterioles to the venules. I’m going to simplify it showing one arteriole that connects to one capillary, and then, that’s going to connect to one venule. I’m simplifying this significantly. Here we have the arteriole, here we have the venule, and here we have the capillary (I’m not going to put the ‘c’ here, but, here we have the capillary).

The main things that we’re going to focus on are the different pressures that we have in this setup. Now, of course the heart is pumping and the blood is coming in this direction, and then, it’s going via the capillaries. This is where the exchange happens because this is where we have the tissue and, this is where we want to get stuff delivered and we want to pick up stuff to take away from the tissues.

The first thing we’re going to talk about is ‘net hydrostatic pressure.’ I’m just going to write NHP for net hydrostatic pressure. When we’re talking about hydrostatic pressure, we are talking about pressure due to the fluids. Of course, in the blood we have fluids. In the tissue we also have fluids. The net hydrostatic pressure, as the blood is coming in here, of course there’s going to be a blood pressure because the heart is beating, it’s pumping the blood, and we’ve looked at blood pressure in previous episodes, and as the blood goes through the capillaries, there’s going to be friction that it’s encountering. It’s going to be bumping against the walls of the capillaries, and that is going to actually reduce the pressure. What we’re going to end up with is a high amount of pressure here and that’s going to drop down as we go along the capillaries. But, not only that, we have tissue here that’s filled with fluid also and that’s also going to exert a pressure on the capillaries.

The net hydrostatic pressure, we’re talking about the total hydrostatic pressure, that is going to be equal to the blood pressure, so the blood is pumping out, and we’re going to subtract the tissue pressure. So, the blood pressure, how much it’s pumping out and how much it’s pushing in from the fluids in the tissues. That net hydrostatic pressure is going to be greatest going out closer to the arterioles. So, we have a lot of hydrostatic pressure pushing out and, as we go down, and the blood is bumping against the walls and so on, that amount of pressure is going to decrease. This is what I’m illustrating here, it’s going down, going down, and when I reached to the end, we’re going to have the least amount of hydrostatic pressure which makes sense, as I said before, the blood is coming via the arteries to the arterioles. It’s coming because the heart is pumping it, and with that pumping we’re going to get a lot of pressure.

That’s going to be highest here, but, because this tube is so small, we have a tiny tube almost to the point that the blood cells can only get through one at a time. That is going to cause a lot of friction, and that is going to decrease the amount of pressure. If I take something and I push it across a surface, because there’s going to be friction with that surface it’s going to slow down. And, that’s exactly what we’re getting here. The amount of pressure is going to decrease as we go away from the arterioles and towards the venules. So, the net hydrostatic pressure is what we’re looking at here.

We’re going to get stuff leaving, but, not everything can leave. Blood cells aren’t going to leave but, water is small enough to get through the pores in the capillaries. So, water is going to leave. It’s going to take some oxygen with it. It’s going to take nutrients with it and so on. That is going to leave the capillaries which is exactly what we want because we want to deliver that stuff to the tissues or to the muscles or to whatever it is this capillary is going through. This is a good thing but, as the water is leaving, of course, we’re going to get less and less water inside the capillaries. Because of that, we’re going to have a change in osmotic pressure. Now, if you remember osmosis is the movement of water across a selectively permeable membrane. We have a selectively permeable membrane here.

What is going to happen is water leaves here, there’s going to be a little bit of osmotic pressure for water to come back in. As more water leaves, we’re going to get an increase in osmotic pressure and then, we’re going to get an even greater increase in osmotic pressure. That’s going to continue of course, and this is exactly what we’re going to see.

As the net hydrostatic pressure goes down, the net osmotic pressure is going to increase and increase and increase. When we take this together, we’re going to get the filtration pressure, (I’m going to write FP), and that’s going to be equal to net hydrostatic pressure minus net osmotic pressure: FP = NHP – NOP. These are not the official symbols. This is just what I’m writing for simplification. But, you can see that we’re going to get a filtration pressure. If we’re over here and we’re taking this hydrostatic pressure minus this osmotic pressure, we’re going to see that we’re going to have a net filtration pressure moving stuff out. That’s going to decrease as we go here to where in the center, the filtration pressure is going to be zero because we have this amount coming out and this amount going in. And, just to make that more equal, I’m just going to draw the rest of that arrow here. Then, of course, as we go down here, we have more pressure, going in, the osmotic pressure is significantly greater, so, we’re going to get a filtration pressure that’s pointing into the capillaries, moving stuff in.

So, over here, we’re moving stuff out. Over here, we’re moving stuff in. And if I were to draw a graph, I’m just going to draw a graph over this, I’m not sure why I did that as a dotted line, this is the y-axis and we’re dealing with filtration pressure, and let’s say this is the zero line. What we’re going to have is a filtration pressure that looks something like this. Here we have it moving out, so it’s going to be somewhere around here, and of course, that’s going to go down to zero at this point, and then, continue going down, showing that we have a negative filtration pressure or in other words, a pressure moving stuff into the capillaries. Here is ‘moving out of’ and here we’re ‘moving stuff into.’

This point right here, where we have a filtration pressure of zero, that is called the ‘dynamic center.’ This is where the net hydrostatic pressure is equal to the net osmotic pressure — equal but, and opposite of course — and that is called the dynamic center. In a perfect world, this dynamic center is exactly where we want it to be so that, we have a good amount of distance for stuff to leave and a good amount of distance for stuff to come in. So, we’re delivering the nutrients and the stuff that we need to the tissue, and we’re taking away the waste and the stuff that we don’t want sending that away from the tissues.

I want to look at a different scenario, where we have the same setup. We have the arteriole, the capillary and the venule going back to the veins, and vena cava, and back to the heart. But, in this situation, we’re going to be dealing with someone that has high blood pressure. So, here, we’re going to have a significant amount of net hydrostatic pressure pushing stuff out. So, it’s much higher. What that’s going to do, as you can see here is the pressures are going to be greater all the way along. Yes, it’s decreasing but, because we’re starting with a higher amount, we’re also going to end with a higher amount.

We also have the osmotic pressure doing the same thing that it was doing before and, stuff is leaving, and let’s say, we have it here, and this is kind of extreme and let’s say that because of this high blood pressure, we still have the filtration pressure here, we have the osmotic pressure here. But, the dynamic center, instead of being over in the center, the dynamic center is somewhere around here. Now, that is a significant problem because this is what we have. In this entire section we have this fluid leaving and, it’s not until here that we have a net amount of fluid coming in. What’s that going to do is it’s going to cause more fluid to be leaving than the amount of fluid that’s coming in, and that is going to result in accumulation of fluid in the tissues or, we can also call that, edema. So, this can be a result of high blood pressure because we have more fluid leaving the capillaries than coming into the capillaries. We have more going towards the tissues and that can cause a significant amount of problems resulting in edema.

So, the take home message, net hydrostatic pressure is blood pressure minus tissue pressure. That’s what we’re showing here. And, if we want to find the filtration pressure, we take net hydrostatic pressure minus net osmotic pressure. That will give us that filtration pressure. To this side of the dynamic center, the filtration pressure is moving fluids and dissolved molecules out of the capillaries. As we come to this side, it’s moving fluids into the capillaries. If we have high blood pressure, that can shift the dynamic center significantly resulting in accumulation of the fluids in the tissues or edema.

That is pretty much it for this episode. As usual, I’d like to invite you to visit the website at for more Biology videos and other resources. That’s it for this video and I’ll see you on the next one.

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