Welcome back folks. We're continuing our lectures on vascular network of your body. The last mini lecture we talked about just some general principles, how the cardiovascular system itself is responsible for managing fluid volume around the body.
Managing it so that we can have a particular amount of volume of fluid so that other parts of the cardiovascular system can manage. pressure so it can help push those fluids around the body. So we had our little mantra from our last little video of managing volume so that we can manage pressure.
We're going to take that a step further. We're going to look at arteries and veins and see how they manage some of these volumes and pressures that are kind of moving around them. And so in this little mini lecture, we're going to pay real specific attention to those volumes and pressures. So let's get started. In this first image, let me get my laser pointer here.
If you notice along the bottom of the axis or the bottom axis here on this graph, it's telling you arteries are sitting in this particular region, arterioles, capillaries, venules, and then veins. And over on the left, you're looking at pressure in millimeters of mercury. And this pressure you can see is actually...
pretty separate as far as the actual amounts that we're talking about here when we're looking at systemic circulation meaning the blood vessels associated with going around to the most of the body in comparison to the pulmonary circulation the blood vessels associated with the lungs and the oxygenation of of that blood if you look at this image i want you to focus on the top Component first, you can see in this systemic circulation that when you look at pressures, when you look at pressures in the arteries, when you're looking at pressures in the arteries, they fluctuate wildly. Now, I say that term kind of cautiously. This fluctuation in pressure is due to the contraction of the heart and the pumping of blood into these large vessels, the arteries. So think of the aorta. The aorta is getting a huge bolus of fluid every time the ventricles of the heart contract, sending this large pulse of fluid into the aorta.
The aorta, with its structure, is having to maintain and deal with this large bolus of fluid, causing tremendous amounts of pressures on the wall. And so as you can see here during This one lumped amount of pressure here that's changing, the bottom part is diastolic pressure, the top part is systolic. You can see over to the left, 80 millimeters of mercury, 120 millimeters of mercury. If you remember from our discussion of the heart and we talked about the ventricles having to generate enough force to open up the semilunar valves and push fluid into the aorta.
that they were probably going to have to generate somewhere in the area of about 120 millimeters of mercury to push that fluid up and into the aorta. Here's that 120, here's that pressure being that fluid and the pressure that it is generating showing itself in the aorta. But this pressure gets pretty high.
Once the ventricle stops contracting, fluid starts to fall back towards the heart. Of course it's going to get stopped because of the semi-lunar valves, but pressure in the aorta drops off dramatically. Here's diastolic pressure sitting down somewhere approximately around 80. The big thing I want you to remember about this is this huge change, this 40, 50, sometimes 60, 70 millimeters of mercury pressure that these large vessels are having to withstand.
Remember the construction of these large vessels like the aorta? Large amounts of connective tissue, large amounts of muscle, all forming around that large vessel to help maintain it, keep it from exploding because of the large amounts of pressure that are occurring. You will also hear that arteries like the aorta are called elastic arteries.
If you look at them under microscope, you will actually see elastic fibers, like very, very strong, powerful rubber bands, literally wrapped. around the aorta so that when these pressure changes occur from the pumping of blood into these large vessels, it will expand just slightly to deal with the fluid coming in. But because of those rubber bands and their density, they will pull back and squeeze, help the muscles squeeze the vessel and help contain that pressure. I often tell students, When we are dealing with this large change in pressures from diastolic to systolic situations in the blood vessels, that it's kind of as if you're trying to stuff a watermelon into a garden hose.
The garden hose, because of its construction, may be strong enough to handle it, but putting that watermelon into that garden hose is a very difficult proposition. And that's literally what you're doing when you have this bolus of fluid coming from the heart, stuffing itself into the aorta. Now, have you ever watched old cartoons where the cartoon character is going over to a faucet outside or inside the house and there's a hose attached to it and the cartoon character turns on the faucet?
Well, it's hard to see. There's no three dimensions with this cartoon. But you know water is moving through it because you can see these large bulges in the hose after the cartoon character has turned on the the faucet.
That's what you should be thinking about when you see these bulges here. Fluid has been pushed or the faucet has been turned from the heart to go into fluid moving into the aorta and now you can see the bulges created by the fluid moving through the aorta. Pretty fantastic, huh?
Down below, you can see that the pulmonary vessels also have to deal with the same sorts of pressure kinds of changes. But the pressure changes aren't as great. If you actually look at the pulmonary artery coming from the heart, its construction isn't as powerful as what you see with the aorta coming from the heart. The next part of this image I want you to pay attention to is looking at the arterioles. And as you see fluid moving from the arteries into the arterioles, you see pressure dropping off.
This pressure dropping off is mainly due to resistance. The resistance is because the vessels, the arterioles, are getting smaller. And so that means this bolus of fluid having to move into these arterioles that are smaller is going to have to squeeze itself into it. Will that cause...
large amounts of pressure? Yes, it will. But because there's so many more arterioles than there are arteries, each one of the arterioles can take up a little bit of pressure. So that means pressure overall will drop off. You can see how the pressure, the area colored in the light bluish color, is starting to drop off both in systolic and diastolic phases.
Now, this is going to be incredibly important. These arterioles Their main job being to control this pressure. We want to be able to drop pressure down to at least 40 millimeters of mercury.
40 millimeters of mercury, or that number 40, is going to be really important to you. 40 millimeters of mercury of pressure is about the maximum amount of pressure that a capillary can withstand. If you looked at the arteries up here, we were dealing with pressures up in the 120s to 130, maybe larger, depending on what's going on in your body. That's not going to be good for capillaries.
The idea of the arterial system moving into the arteries moving into the arterial system is that the arterial system be able to reduce the overall pressure so that we can move this fluid. into the capillaries without destroying them. Remember, the capillaries do not have a very thick wall. They only have the endothelial layer and a little bit of connective tissue.
And so we want the pressure to be lower here or else it will literally cause the capillaries to explode. Not a good thing. Not a good thing. As we move through the capillaries and over to the venules, you'll see pressure. starts to level off.
It's getting very low at this point, but dropping off. This falling into the venules and into the veins, this pressure remaining low over in this area, you can see you don't see the lumps of the systolic contraction occurring any longer. This is all mainly due to the compliance or the flexibility of the venules and veins.
Remember, they do have connective tissue around them. They do have some muscle, but these vessels tend to be larger and much more flexible. Remember, their job is to hold fluid.
Their job is not necessarily to hold the pressure. Everybody understand that? Now, this whole situation here of arteries being able to manage the high pressures being caused by the contraction of the heart, the arterioles being able to reduce that pressure and having the flexibility and resistance to lower the pressure to help manage that fluid pressure before we get down to the capillaries.
Capillaries, remember. their job to distribute materials and so the walls have to be very very thin. How are we doing folks?
This is going to be a very important component for you to remember. You'll see if this changes, if the ability of the arteries or the ability of the arterioles changes to the point that they cannot manage these pressures, pressure moving into the capillaries is going to be much higher. And remember, if pressure is above 40, these capillaries are going to break. They're going to explode.
Remember that as we move into some of our next parts here. In this image, just trying to show you different arteries moving down to the arterioles and the pressure changes that occur because of the construction of these different vessels. So at the top here, the aorta. You can see very clearly all the different components of the pressure changes occurring because of systole and diastole.
You can see what's called the incensura. The incensura means incisor or the notch, a sharp notch. This notch is when, as fluid is being pushed in because of systole, contraction of the heart, when the heart starts to move into diastole, fluid starts to back up in the aorta.
And then... That'll cause the semilunar valves to close shut. This, in Sunsura, is when that vessel, or excuse me, that valve shuts. And because of that, fluid hits that valve, starts to back up a little bit, so you see pressure increase a little, and then it'll start to back off even further as pressure evens out throughout the aorta.
As we move from the aorta down towards the femoral artery, you can see still some huge changes. Huge changes in pressure from systole to diastole. Look at what happens as we look over at the radial artery. The radial artery, in comparison to the femoral artery, is smaller.
And yes, it's still considered an artery, strong walls, but because it is smaller and there are other arterioles kind of branching off the radial artery at this point, pressure has fallen off. Still pretty... Big changes from systole to diastole, but not the large changes that you see from the femoral artery. As we move from either the femoral artery or the radial artery down to the arterioles, pressure is starting to be smoothed out. We still see a little bit of fluctuation with systole and diastole, but it is smoothed out because, remember, we have to get it to a particular level so we can get that fluid moving through the capillaries.
without destroying the capillaries. These vessels utilize a couple of principles to help manage this fluid. So we're trying to dampen, that's the term that I'm putting up here for you to look at, dampening. We're trying to dampen the difference in pressure between systole and diastole. Dampening means to lower or weaken.
And these vessels are going to do it using, again, two principles. The first one, resistance. And that resistance comes from vessels getting smaller, and because the larger vessels like the aorta and the arteries, femoral and radial artery, have quite a bit of muscle, so they can constrict.
They can make themselves even smaller. This resistance helps smooth out fluid movement so that we don't have big changes in pressure and volume. So resistance is a big deal for these vessels to be able to maintain the pressure situation in these vessels. component is compliance.
Now compliance means how flexible a vessel is. And I know we've talked about resistance and compliance here and that they probably shouldn't be the same, it shouldn't be in the same sentence, but they are. That's what makes these vessels like the arteries and the aorta so amazing is that they have the ability to constrict and increase resistance, but they also have the ability to be compliant, meaning flexible, flexible with the fluid.
This is, again, very, very important. If these vessels lose one of these two abilities, either to increase resistance or to increase compliance, we have major problems in trying to manage pressures. For those of you thinking, hopefully, of high blood pressure, high blood pressure is usually set off because of the blood pressure.
Vessels are losing their compliance, their ability to stretch and manage the pressure. So if we lose compliance, and that means if plaque, fatty formations around the vessels form, the vessels lose their ability to stretch, to deal with the fluids. When you lose compliance, you lose the ability to manage these pressures.
Everybody got that? We can talk more about this a little bit later. Now, let's bring in some numbers here for everyone to kind of think about.
The first, an easy one. Normal, what we may consider to be normal blood pressure, and there's a caveat to this. So remember we talked a little bit about this when we were talking about heart contractions, the 120 over 80 millimeters of mercury. 120 being systolic pressure, 80 being diastolic pressure.
And you know what those refer to. Now, this... The term normal blood pressure, again, we have to take that with a grain of salt. Everyone has literally their own normal blood pressure. And we have to kind of measure that and understand that in relations to the individual's body and how that body is being used for us to understand what really normal is.
But we use this 120 over 80 as kind of a benchmark for folks to understand. blood pressure. Let's talk about what's called pulse pressure. Pulse pressure, well, if we take systolic pressure, 120, and we subtract diastolic pressure, 80, from it, we get what's called pulse pressure. So if we do that calculation, that's going to be 40 millimeters of mercury.
40 millimeters of mercury. That number... is back with us again.
Remember I said that capillaries can't normally withstand pressures above 40 millimeters of mercury? Well, now we understand pulse pressure a little bit. That pulse pressure usually is sitting at about 40 millimeters of mercury. So all these things are starting to tie together. Systolic pressure will increase pretty easily depending on what you're doing with the body.
Just a couple of ways that stroke that... systolic pressure will increase is if stroke volume in the heart increases. If stroke volume increases, that means we're trying to pump more fluid into the aorta or into the pulmonary artery, which means we're going to have to generate more force. So systolic pressure being that force would have to increase, would have to increase.
So when you're working out, you're probably going to have a higher systolic pressure because you're pumping more fluid. Pumping more volume by way of the heart, stroke volume is increasing, you're going to see a systolic pressure probably increase. One of the ways, again, that systolic pressure could increase because of a problem that's going on is when we lose compliance. So again, remember we talked about that in the previous slide. Compliance is referring to the flexibility of the artery or the flexibility of the vessels.
Systolic pressure. Being the pressure the heart is having to generate to push fluid into the aorta, if the aorta is losing compliance, losing its flexibility, you're going to have to generate more force to push fluid into it because it's not going to be able to give very easily. So remember that. Normally, even in the best of us taking care of our bodies, compliance decreases with age.
So as you get older, compliance has a tendency to drop off. Why? Well, just the normal way. wear and tear of our particular vessels. So think about that as you're getting older and you're eating that McDonald's hamburger next week.
All right. In hospitals, because blood pressure vary, can vary so dramatically depending on what's going on with the individual, they're starting to look at a new way of measuring pressures. One of those is something called mean blood pressure, where you take the systolic pressure, from the individual and two times the diastolic pressure.
And then you divide that by three and that gives you kind of an average blood pressure, a mean blood pressure. Instead of having to worry about two numbers, 120 over 80, you have a single number to worry about, 93 in this case. If you took 120 plus two times 80, which would be 160, or excuse me, I'll let you do the calculation on that. It'll give you 93, 93 millimeters of mercury.
All right. Now, venous pressure. Remember, the veins are not necessarily dealing with pressure. They're dealing with volume.
And one real important number to kind of remember, and we talked a little bit about this in previous lectures, is something called central venous pressure. This is the pressure. at the junction point between the superior and inferior vena cava, as that area where it's moving fluid into the right atrium.
We said fluid, the pressure in the right atrium should be somewhere around zero. We don't want any pressure holding up fluid moving into the heart. And so having that central venous pressure be zero is really important.
And again, measured at the right atrium. okay, measured at the right atrium. Pressures from thoracic veins sitting within the chest wall usually have a little bit higher pressure, somewhere in the area of seven to 10 millimeters of mercury. Now this is mainly due to the lungs expanding and putting pressure on those vessels from when you're taking air into the lungs and they're expanding, they squeeze those vessels, causes some pressure increases. So anywhere from seven to 10 millimeters of mercury.
However, with most of the other veins of the body, we have to worry about gravity and gravity's effect on water. Gravity's effect on water. One place we really see this pretty clearly is the venous pressure in legs. So venous pressure in legs can be pretty high due to what is called hydrostatic pressure. Hydrostatic pressure, meaning the water pressure.
Because of gravitational forces, us as an animal deciding to stand up, gravitational forces are pulling on water. And that water gets pulled from the upper parts of our body towards the lower parts. So now we have all this fluid accumulating down in the lower parts of our body.
Well, the veins are responsible for kind of hanging on to fluid. They start to get overloaded or distended because of all of this fluid. Building up in the legs.
Hydrostatic pressure. Pressure because of gravitational forces. Because of this, you'll hear about people having issues with venule networks down in the lower parts of their body. So you'll hear about varicose veins. But not everybody gets varicose veins.
And even with those with varicose veins, the problem could be much larger if it wasn't for something called the venous. pump, the venous pump. So let's start talking about what this venous pump is, all right?
Now, before we get to the venous pump, I just want to show you here in this image, this image is trying to show you pressures on the venous system. I want to make sure that you all pay attention that this is, we're talking about the venous system here. I know everything is in red over on the illustration, but we're talking about veins.
And so if you look at here, Pressure around where the heart is at and vessels that are associated with right at the tip of the heart. These are the veins. Pressure is sitting somewhere around zero, so fluids should be able to move into the heart very, very easily. If you look at vessels, these veins, as we move down lower in the body, you're going to see pressure rise.
This is due to hydrostatic pressure. Water. building up or accumulating because of gravitational forces.
Look over in the arm as you move out and in. To the radial artery, pressure 6 to 8 millimeters of mercury. That's a pretty good amount of pressure.
If you have your arm lowered, that pressure can jump to 35 millimeters of mercury. What do you think would happen if we raised our hand above the heart? I'm hoping most of you guessed that that pressure would drop. In fact, it would become negative, meaning fluid is literally getting sucked out of your hand, out of your arm, as your hand is above your head. That's why you can't hold your hand above your head and Definitely.
Look up here. When you look at these vessels coming up and into the head, look. The pressure is negative 10 millimeters of mercury.
Literally because we stand on our two legs here, fluid in our head is literally getting getting sucked out every moment. That's why the heart has to maintain pressure with the arteries to make sure fluid can actually get to the head or get to the other parts of the body that are fighting gravitational forces. Everybody got that? So hydrostatic pressure is causing fluid to back up in the venule system and cause pressure to increase, all right?
This is a number just for kind of your identification. One millimeter of mercury for every 13 point six millimeters of height. That's how much pressure we're actually increasing because of the gravitational forces. Now, arteries also experience some hydrostatic pressure, but because they have more muscle and their walls have a little more strength because of the connective tissue, they don't express it or don't have to worry about it as much. Alright, let's look at the the venous valves and talk more about the venous pump.
In this image here, we're focusing on the muscles down in the lower calf just as kind of an example. So we're talking about the gastroc muscle here and vessels, blood vessels, veins that are sitting inside of that gastroc muscle. When the person contracts that muscle, so when you stand on your toes and that gastroc muscle contracts, the muscle shortens and fattens in the middle and pushes on the vessel.
As you can see here, it's pushing on the vein, squeezing it, pushing fluid or increasing the pressure, pushing fluid through the valves and up into the next segment of the vein. Fluid that is sitting below, well, yes, pressure is increasing and it's going to want to backflow, but because the valve is there, it can't move. As soon as you let off of that muscle, this opens up. Fluid from below, because pressure has slightly increased, pushes through the valve, fills. this area and literally we're going to use muscle contraction to help push fluids up against gravity.
Push fluid up against gravity. This is what's called the venous pump. The venous pump. And this venous pump will actually help reduce the pressure.
Help reduce the pressure that the lower extremities will feel because of gravitational forces. So instead of being... a positive 90 millimeters of mercury, it's going to get dropped down to 65, 70 millimeters of mercury. Everybody got that? Alrighty, a last few things for us to kind of remember here about this managing of fluids around the body here.
For veins, remember, they're acting as reservoirs. They're holding blood for the body. And in fact, this term reservoir is an important one for you.
Not all the blood that is sitting within the veins is going to get pushed around the body all the time. It kind of takes turns moving. And yet when there are times that the body may need extra blood moving it can push it into service what do I mean well the body has the capability of losing approximately 20% of its blood volume now why would we want to we don't want to but remember when we're talking blood we're also talking about water so we're not talking about you having some sort of injury to your body where you're losing fluids that's part of this that could be part blood volume loss. But think about you working out or you being in a situation where you're losing fluids because of sweating.
That fluid is water that's coming from blood vessels. So this extra fluid that we have sitting in reserve in the reservoir can be pushed into service. Our body has that stored, stored away. Got that? Now, sympathetic nervous Remember I was talking about that last time?
The sympathetic nervous system has inputs to veins and can cause some constriction. Remember, veins don't have a lot of muscle. So the constriction that is taking place is not a huge amount, but it is enough to literally push some of this extra fluid that it may be storing into service in case of times where we're losing fluids, a hemorrhage.
Alright. Well there are other around the body that it will use to store blood. Some of those, like the spleen, the spleen can manage about 100 milliliters of extra blood. The liver can manage about 300 milliliters of blood. The large abdominal veins, just us talking about it, will hold 300 milliliters more than what they actually need.
The subcutaneous plexus, these are the venous plexus that sit just literally underneath the skin layers and actually innervate particular parts of the muscles. They all hold about 300 milliliters of blood. And the heart can actually hang on to an extra 100 milliliters.
milliliters. We do have one other area that kind of can hold blood and that's the lungs. The lungs. Folks, that was a longer little lecture but there was a lot to kind of go on with there. Go back over this.
Look at how the cardiovascular network is managing pressure and managing volume. All right? We'll talk to you later.