Hello everybody, we're back. We are going to focus this mini lecture on a part of the vascular network that we haven't had a chance to talk a lot about yet, and that's the capillaries. In our previous mini lectures, we spent a great deal of time talking about how we get materials to the capillary beds and take materials away from the capillary beds by focusing on arteries and veins.
These capillaries that we're going to focus on in this mini lecture have some very, very special properties associated with them. And so to get this moving here, we need to look at this image that is coming up next. So let's get started. This image, I'm hoping you can see just using the colors. We have an arterial end and we have a venous end.
The arterial end, well, we're actually looking at arterioles. So arteries are bringing fluids, bringing materials over to the arterioles on the left-hand side of the image here. And on the right side of the image, you can see the venules that will feed into veins, bringing materials back towards the heart, back towards the lungs for oxygenation and for other purposes as well.
Now, in looking at... the arterial and the venule. Remember, they still have the structure that we've talked about before.
Connective tissue around the outside, fairly significant muscle layers, and the endothelial layer sitting on the inside. In this image, I want you to notice as we move down to these arterials that are the smallest of that group and getting ready to... feed into the capillary networks, you can see that along the outside of these arterioles, the muscle layer is actually, well, it's not continuous.
It's not continuous. You have these little ringlets of smooth muscle instead of a layer of smooth muscle. Everybody understand that?
This is a single ringlet or a single smooth muscle cell wrapped around the arteriole. You can see it here. You can see it all along here. Now, this is showing that it's not terribly continuous, and that's correct. It's kind of sporadic.
But working together, it still gives this arterial its properties of being able to constrict. What I want you to notice is as we move from the arterial into the capillary network, you can start to see that even the smooth muscle ringlets start to become a little more sparse. They aren't around as much. And in fact, you'll only see those little smooth muscle ringlets in very strategic spots to help manage movement of fluid. And we'll talk about that management of fluid movement here in a little bit.
So arterial end moving into the capillary networks. And these capillary networks, again, are going to be made up mainly of the endothelial cells, single layers. of epithelial cells, simple squamous epithelium, with a little bit of connective tissue around it.
Anybody got that? As we move over to the venous end of this, or the venule end, you can see, much like we've talked about before, there is not much muscle associated with the venules and veins. We still have our smooth muscle ringlets here, again, sitting in fairly strategic spots for movement of this fluid, allowing it to kind of distribute in a particular way. Everybody got that? Okay.
Now let's bring up a few other things here. When we talk about these arterioles that are getting ready to branch off into the capillary networks, the diameter of these arterioles are sitting somewhere in the area of 20 micrometers. 20 micrometers.
Got that, folks? That's a pretty small vessel. That's a pretty small vessel.
Now, as we move into the capillary network, these capillaries, their diameter sits somewhere in the area between 5 to 9 micrometers. 5 to 9 micrometers. Everybody got that?
Now, I know these diameters, you probably can understand that that's really small, but how small is that? Well, Do you know what the size of a red blood cell is? Do you guys remember? You should have been taught this in anatomy. Anybody out there?
The size of a red blood cell is always seven micrometers across. Seven microns. That is something that is very, very consistent in our bodies. Red blood cells are seven microns across. It's so consistent that physiologists call that the biological yardstick.
Why do they call it that? Well, when you look under a microscope for trained anatomists or histologists looking at tissues, one of the first things they try to do is to look for a red blood cell someplace because that red blood cell being seven microns, if you have it in the field of vision, you can literally measure everything in that field of vision with the size of the red blood cell. So, Red blood cells, seven microns across. So if we're talking about this arterial here being approximately 20 microns across, you can get an idea of how much red blood, or how many red blood cells, or how large a quantity can probably travel through that structure. Now, red blood cells, while they're seven microns across, are very pliable.
They are able to squeeze into small spaces and so they can bunch up and squeeze on top of each other. They're very flexible. And that probably could help you with understanding how these red blood cells can get through the capillaries.
Because if capillaries are sitting somewhere in the area of 5 to 9 microns, well that means there's going to be a capillary that's smaller than a red blood cell is in diameter. So that means the red blood cell is literally going to have to squeeze through that structure, squeeze through the capillary. Everybody got that? So if we have red blood cells, they're going to be traveling within these vessels as well as all the fluids, nutrients, everything else that we've been talking about with this tissue.
Now, I want to remind you about these smooth muscle ringlets. These smooth muscle ringlets, these sphincters, as you'll see on this image, all have a particular amount of control associated with them. Meaning, as you can see over on the left here, smooth muscle regulated mainly by what are called local tissue conditions. Meaning, these smooth muscle ringlets are trying to pick up, hmm, what's the content of the fluid sitting around them? What's the content of the fluid inside of the vessels?
That will signal them whether to constrict or dilate, allowing fluids to move into particular areas. In a future lecture, we're going to spend a lot of time talking about these local controls for these smooth muscle ringlets. Everybody good? Let's start talking about capillaries specifically.
In these two images here, I want you to focus on, first of all, how thin the walls are. these capillaries are. In the upper left-hand image, these capillaries are a single cell layer thick or their walls are a single cell layer thick. And these cells, these epithelial cells, are attached to each other to form this wall. Now, sitting around them, there is a thin, very light covering of connective tissue, kind of like a spiderweb-like.
bit of connective tissue sitting around it. So that means if materials can pass from inside of the capillary to outside, it can get through that connective tissue fairly easily. Now, this first image up at the top here, this what's called a continuous capillary, you can see the cells in their entirety.
You can see the nuclei for each of these epithelial cells. And if this is a cell, that means what you're looking at around the nucleus is the cell membrane. And that cell membrane is made up of phospholipids that we've talked about before from the beginning of class.
And so this is a continuous sheet of a cell, which means if something sitting inside of the capillary is actually going to try to get outside of the capillary, it has to pass through a cell. It has to be able to pass through two bilipid membranes. Remember that?
So in this image over to the right. of this continuous capillary. It's a cross-section through the membrane.
One cell, here's another cell. This is the inside of the capillary, the plasma. The space in between cells, this gap, this intercellular gap, there are probably proteins linking the two cells together to make sure that they have as tight a fit as possible. But there's also probably a narrow gap where if a structure is small enough that's sitting on the inside, it may be able to pass through.
Or something from the outside, if it's small enough, may be able to pass to get into the capillary. Make sense? Let's look at this image down below.
called a fenestrated capillary. This fenestrated capillary, again, you can see the cells, you can see the nuclei of these simple squamous epithelial cells, but embedded in the cells, in the cell walls, are channels, are channels. These channels, we will call them fenestrations, openings, pores is another name you can see. If we zoom into the wall and we look at this cell membrane, like we...
or this capillary membrane like we were looking at the one above, you can see these channels or pores literally going from one side of the cell to the other. It's as if we had a door that went, or I'll say a hallway that went from inside of the building all the way out to the outside of the building, all right, without crossing any other rooms. This image here is also trying to show you this intracellular gap. much like the one up above is a little bit bigger.
And it may be big enough for cells like white blood cells, which are incredibly flexible, for them to be able to move from the plasma, from the inside of the capillary, into the tissues, or from the tissues back into the capillary. These pores, these fenestrations, may be big enough to allow certain things through. They could act as channels, allowing molecules like sodium.
potassium, glucose to actually move from inside of the capillary out or outside of the capillary in. If you look at the image down below, it's trying to show you here are some of those pores that we were looking at a few minutes ago or a couple minutes ago when looking at the fenestrated capillary. Sodium, potassium, glucose, they could in many cases be able to move across these pores using diffusion to be able to get across. Proteins?
Well, they're most likely too big to be able to use the fenestrations. So they're still going to have to use some sort of cytosis, some sort of transport mechanism, to be able to move across the cells. That means moving across a whole cell, being transported across a whole cell, to move from inside of the capillary out or from outside in.
Everybody got that? Over here, you can actually see a water port. those fenestrations could be big enough for water to move back and forth. And you'll see how that comes into play a little bit later in another lecture when we talk about fluid distribution. Last but not least, up at the top here, you can see fats.
Now, remember, what is the membrane of a cell made of? It's made of fatty acids, fats. So fats have this special property. Since the membranes of the cells that make up.
The capillary are made up of fats. Fatty acids, steroids, which are made of fats, and other hormones that are made of fats, literally can move across the membrane of or the wall of a capillary at will, easily, because they're lipid soluble. O2 and CO2 also have that property of being able to move across the walls of these cells.
So remember. that lipid soluble substances can move across the wall fairly easily. Fairly easily. How's everybody doing?
Okay. Now, let's talk about some of these molecules or substances and their permeability across the wall of a capillary. So this table is taken from, as you can see, a paper many, many, many years ago.
But it illustrates the point that I really want to bring up here. These molecules that are listed over to the left, and this is just a few of what you would find inside of the capillary or out in the tissues, we're looking at their relative permeability, relative permeability across a capillary, across a capillary wall, utilizing the pores, utilizing those fenestrations. All right, now water.
Its molecular weight, this is its size, 18 molecular weight here, seems to be able to get across the walls of a capillary pretty easily, pretty easily. And because it can get across pretty easily and it's a substance we know fairly well, we use that as kind of our reference point. Water. In the concentration of water inside of a capillary versus outside, the amount of water that can move per a particular unit of time, we call whatever amount that moves across, we say that is equal to one. And that one is our permeability for water.
Using water as our reference, we can go to any other substance now and we can... We can compare it to water, meaning sodium chloride. With its size, we can measure, well, if sodium chloride has a high concentration on one side of that capillary wall versus the other, and we use the same unit of time that we use for water, we can measure how much of it gets across. And then whatever that number is, we compare it to the amount of water that gets across, and we say, okay, did it get across as well as water or not as well as water? In this case, it's trying to show that the permeability of sodium chloride across the wall of a capillary is sitting at 0.96.
Not as good as water, not as good as water, but pretty good. So it has free movement across the wall of a capillary using those pores. Urea, you can look at this. It's just a little bit bigger than sodium chloride, but look, it's a little tougher for it to get across the wall. Glucose.
three times the size of urea. And yet, huh, it seems to be able to get across the wall, well, almost as well as urea. Sucrose, fairly large molecule. It can still use those pores to get across the wall of the capillary. Look at some of these other larger molecules.
Inulin we'll come back to, but let's look at myoglobin and hemoglobin. Hemoglobin, remember, from a red blood cell. It's a large protein, 68,000 molecular weight. But look, it can barely, if at all, get across the membrane, which means it's too big for those pores, meaning that if hemoglobin is going to get across the wall of a capillary, it's going to have to somehow be shuttled.
And that's not something that's very easily done. The last structure here, albumin. Albumin is an incredibly large protein that our body relies on.
It's a protein that will help stabilize concentration of solutes and water movement across a membrane. And one of the reasons why it can do this is because, well, at 69,000 molecular weight, you can see here its permeability across the membrane, it cannot get across. It cannot get across.
Because of this, physiologists... have used albumin and its molecular weight is kind of the standard. If a molecule is smaller than 69,000 molecular weight, it can probably get across the membrane pretty easily.
If it's larger than 69,000 molecular weight, it is not going to get across the membrane of a capillary. Everybody got that? That's going to be very important to us later when we start talking about it. other proteins and how they get across the membrane.
All right, folks? We have a little story here with inulin that we'll talk about in lab, okay, at 5,000 molecular weight here. Well, it can get across the membrane, but it's got a history behind it, so we'll talk about it later.
Knowing what can get across a capillary wall and what can't get across a capillary wall is going to be very important to us. In our next set of lectures, we're going to be talking about how materials from inside of the capillary versus materials outside of the capillary are going to provide forces. I know you hate me using that term.
Forces for pushing other molecules back and forth across the capillary. It's going to set up concentrations and concentration gradients. And so...
Capillaries love using concentration gradients to be able to move materials out of the capillary or move materials into the capillary. So in this image here, we have a capillary here on the left. And outside of that capillary, this is the intracellular space, the space in between other cells that may be in this particular area of the body.
These cells, the cell that you see at the top and this cell over here, These are cells that are from whatever tissue we're looking at. And so what we're going to have to work on now is... How do we manage to move materials from inside of the capillary out and outside of the capillary in? So that these cells can get the nutrition they need, to get the oxygen they need, to get rid of the waste products that they need, and be able to survive.
We'll get you on your next mini lecture, folks.