Structure of the Circulatory System
Hi everybody and welcome back. Today we're going to be looking at the circulatory system. I'm going
to be doing the system in two parts. This video is going to focus on the structure of the circulatory
system while the second video is going to focus on the processes that take place in the circulatory
system. Now it's important to know the basics of how our circulatory system is structured so that
we know how best it can carry out its function because that's one of the golden rules of biology
is everything that has a structure has a specific function associated with it. Now something that
maybe you're not aware of is that circulatory systems can come in more than one kind. In
particular open and closed circulatory systems. Now the name gives it away. An open circulatory
system as we can see here on the left hand side of this diagram is in reference to our blood vessels
being open. So an example that we see in nature is with insects for example like this little
grasshopper here. Essentially what they have is a structure that is similar to a heart.
Blood moves through that pump and as it moves through the blood vessels and approaches the
tissues you will notice that the blood vessels are open at the end and that means that the blood
actually leaks out of the blood vessel and it goes in and amongst the tissues. Then on the other
side you'll notice that these blood vessels are also open on the other side and the blood somehow
enters them and goes back through into the heart. Now this is an opus open circulatory system and
essentially we find this in very small organisms. We're not going to find this in a very big animal
and essentially what happens is you are pumping a fluid through the tissues of the organism
but the actual blood component, the liquid, actually leaves the blood vessel. It means it
actually leaks out. Now in animals like ourselves that doesn't happen. Our blood is in the closed
circulatory system and an example of an animal that also has a closed system is an earthworm.
Closed is very important because what it means is that the blood never leaves any of our blood
vessels. So that means our blood moves through the heart, it goes through our blood vessels
and when it approaches the tissues that it wants to move through you will notice that it branches
into smaller pieces. We're going to learn what those are called soon and you'll notice that
they're attached to one another. Once the blood has moved through these tubes it then leaves that
area through a bigger blood vessel back to the heart and it's important to know that a closed
system allows for the organism to be bigger and it also means that the blood itself is a little
more specialized. Now that means everybody that technically all of your blood must stay inside a
blood vessel at all times. It cannot leak out anywhere. The only reason why you actually bleed
is because you've cut open one of these blood vessels, one of these tubes and the blood is
leaking out of it. The next important component about knowing our structure of the heart is to
focus on one specific function. Now I'm going to go into a lot more detail on this topic in the
next video when I do the processes of the circulatory system but it's important to know
that we have a closed circulatory system but we also have what we call a double circulatory system.
In other words what happens is the heart pumps blood twice around the heart but the destinations
are slightly different and so we refer to this as a double circulatory system. So we have a closed
and double circulatory system present in humans. Now let's start off with the systemic circulation.
In other words this is the circulation of the blood to the body and so when we look at the
diagram alongside on the left hand side you can actually divide this diagram in halves. On the
right hand side we have the systemic side and so this is how the body uses its blood that it's
receiving from the heart. Fresh oxygenated blood is going to come in from the lungs going to enter
the heart and it's going to go off into the body delivering high amounts of oxygen and it's going
to be low in carbon dioxide. As it moves through the tissues it is going to collect more and more
carbon dioxide and that then means that the blood is deoxygenated. So now what we need to do is we
need to take that blood and we need to re-oxygenate it and so that's when we enter the pulmonary
circulation. Pulmonary is another word for lungs and so what happens is the blood is now going to
return up to the heart, it's going to enter our pulmonary system and it's going to go back to the
lungs. This is the pulmonary system on the left and this pulmonary system has low oxygen levels
and high carbon dioxide levels. It's important for the body to be able to do this because essentially
what it's doing is it's pumping the blood twice through your sets of organs. The first time the
blood moves through the body it's going to have lots of oxygen in it and it's going to deliver that
oxygen but as it does so it collects carbon dioxide. That carbon dioxide we have to get rid of.
It's a waste and so that is when the blood returns back to the heart. It is pumped to the lungs and
then from the lungs it returns back to the heart where we can start the whole process again.
Sometimes this process is quite confusing because we forget that a portion of this takes place in
the lungs and a portion of it takes place in the body but if you envision this diagram here it might
help you a little bit better. The easiest way to remember the difference between the two is systemic
actually means body and pulmonary means lungs. In other words the circulation of blood to the body
and the circulation of the blood to the heart. Next on our agenda is familiarizing ourselves with
the structure of the components of our circulatory system and we are going to need to know the
external structures and we're going to need to know the internal structures of the heart and so what
I'm going to do is I'm going to highlight the most important structures because often we get diagrams
and textbooks with notes and we never really know should I know this diagram shouldn't I and it does
depend on what grade you are in and what level of high school you're in but generally at the level
that we are going to need to know a number of these labels for tests and exams. Now I want to
point out that the structure of the heart on the outside is actually quite different to what it
looks like when you cut it open and you see it on the internal structure. Many of us don't actually
realize that some structures are sort of drawn more clearly in textbooks than they are actually
in real life and some of us actually struggle to identify these things because textbooks don't
really look like the real thing and so let's go through the left hand side labels. The first label
that we see here is the superior vena cava this is a blood vessel it's the largest blood vessel
in terms of veins and it is going to enter the heart. Sitting just below that is the right atrium.
Now I want to pause here for a moment because it's important to know that the left and the right
of this diagram is opposite to your hands and that's because whenever we do anatomy and biology
the position of everything is within the body that the organ is coming from. In other words we are
looking at another person's heart which means it's their right hand side and their left hand side it
is not where your right and left hand is. Now the right atrium is the top chamber of the heart it is
where blood is received from the vena cava. You then have these coronary arteries. Now coronary
arteries are arteries that are specifically there for the heart. Now even though blood goes into the
heart itself you'd be surprised to find out that the heart actually needs its own blood vessel
separate from the bodies and that's because it needs its own oxygen and energy and so these
coronary arteries are specifically there to keep the heart alive and coronary
is associated with heart so when you see coronary think heart. The next important component of our
heart is the right ventricle. This is the lower component of the heart and it's very very muscular.
I want you to take note of the differences in the consistency and the appearance of the atriums.
You will notice that this right atrium it sits out on the outside of your heart actually very
interesting it's like a little bag it actually sits on top of the heart itself and it's very soft
it's not as muscular as the right ventricle and the right ventricle takes up the majority of this
side of the heart. This ventricle is quite thick and muscular and it needs to be as it pushes blood
out of the heart and into the lungs. That then brings me to the other side of our diagram and
we're going to look at this very large blood vessel called the aorta. It is the largest blood
vessel in the body and it carries oxygenated blood to the rest of the body and we're going
to look at its functions later on. Then we have the pulmonary artery. Now this is just the left
one we can't see the right one it's hidden but essentially this is where our blood leaves the
heart and goes to the lungs. Now often arteries and veins are confused with one another and we're
going to learn about how we're going to tell the difference very soon but essentially this is the
artery that is going to take blood to the lungs. We then have the pericardium. Now they've said
here that they've cut it away but if you look very carefully the pericardium is this sort of
almost very thin membranous layer that's sitting on the outside of the heart and it's there to
protect the heart against friction because remember the heart moves every single day
and if it moves that much it's going to create heat and friction and we want to keep our heart
safe so we put it in a membranous bag or sack. Next we need to look at the left atrium and so the
atrium is a little bit hidden here it's very hidden underneath here but there it is if I make
it circle around it. The atriums remember sit at the top of the heart in this diagram it looks as
though they're on the outside and technically they are sitting atop the heart. In other diagrams it
looks like the atria are on the inside of the heart but specifically these are the little
inflatory chambers that are going to fill with blood before it enters into the bigger chambers
below the ventricles. Then you will see there are coronary arteries. Now coronary means essentially
the heart it's referring to the blood system that supplies the heart with its own nutrients because
the heart is responsible for giving blood nutrients but it also needs to have its own
blood vessels and these are the blood vessels we can see on the surface here.
Then let's go down to the left ventricle. The left ventricle is the thickest ventricle out of the two
and the reason for the left ventricle being so thick and you'll see that now when we do the
internal structure of the heart this is due to the fact that the majority of blood that enters
the heart it needs to be pushed up against gravity. That then means that the heart needs to
really hard to push the blood up through your neck to your head to your hands to your feet
and to do that you need a really muscular structure and that's what the left ventricle is.
Finally we have the apex of the heart. The apex is essentially the pointy tip of our heart and
you'll notice that the heart is not perfectly symmetrical. It seems to lean to one side and
that has to do with making space for other organs in the body that surround the heart like the lungs
and the liver. Right moving on to the internal structure of the heart and I know that this
diagram is very intimidating. There are a lot of labels here but I promise you that some of them
you already know. We are going to add a few more for the internal structures. So we already know
the superior vena cava. Remember this is the largest vein in the body and superior means
from above. So this is all the blood coming from the top of your body. We then have the right
pulmonary artery. This is the lung artery. This is where blood is going to be leaving the heart
to go to the lungs to get oxygenated. Then below that we have a pulmonary valve. Remember valves
are there to prevent backflow. So this makes sure that the blood only flows to the lungs and not
back into the heart. We have pulmonary veins lower and these are our blood vessels that are bringing
blood from the lungs back to the heart. It's really difficult to see it in this diagram
but basically the blood is going into these tubes through the back of the heart and they're going to
pop over here on the other side but we'll get to that side soon enough. Then below that we have the
right atrium. The right atrium is the area in which blood first flows into. We can see that
by our arrows. This is where the blood comes from the top of the body. Here is where the blood comes
from the lower half of the body. Now once we move past the right atrium blood is going to flow into
the lower chamber of the heart. Now to do that it needs to go past a valve and we call this the
tricuspid valve. Tri meaning three, cuspid because of the shape of the particular valve pieces and
essentially this valve opens and closes every time the heart beats. So what happens is blood is
going to flow in from the atrium past the valve and now into the right ventricle. The right ventricle
if we have a little closer look at its structure you will notice that it is quite large. If you
look at the surface area and you compare it to its next door neighbour ventricle it's very large
because you've got to collect all the blood from the entire body and you need to push that blood
into the lungs as quickly as possible with the largest volume. So that's why it's quite large on
the inside but that doesn't mean that the left ventricle doesn't do a good job because it's a
bit smaller. I've spoken about why it's smaller but I'm going to recap that soon. Then we're going to
look at the inferior vena cava. Remember inferior means from below and this is the largest vein in
the body and then there is something called papillary muscles. Papillary muscles essentially
work and you may have heard this before if you know anything about the eye. Papillary muscles
are muscles that are attached to our little valve cusps and essentially what they do is they contract
and relax and help open the space sitting in between our two chambers and so I'm talking about
this space over here. I'm just putting a little black line there. We need to open and close that space
and so these papillary muscles are attached with these filaments, these fibres, these connective
tissue pieces up onto the actual valve itself and they open and close it. Now if we move along the
other side of our diagram, what do we need to know? Well, depending on how specific your textbook
needs to be, this is the carotid artery and the subclavian artery. The carotid artery is the one
in your neck. The subclavian is the one that runs underneath your clavicle bone, your collarbone.
It's a very important artery and then you have what we call the aortic arch and it literally gets
that name because it is this big arching blood vessel. This entire thing we can refer to as the
aorta as well. This is just the left pulmonary artery because you need one for each lung, a left
one and a right one and yet again we see some more pulmonary veins. This time, this blood is coming
from the left lung. Then sitting on top, we have the left atrium. There it is, okay. All of the
blood that's sitting in the left atrium, this area here that I'm coming in, is coming from the left
lung. It's full of oxygen now. We've gotten rid of the carbon dioxide and we're returning back to
the heart. Now, when we go past any of these boundaries, remember on the other side, we need
to move past a valve and sitting between the two chambers, the left atrium and the left ventricle,
are our mitral valves. Now, that is slightly different in appearance but it does exactly
the same job as the tricuspid valve and it opens and closes and prevents backflow.
We look a little bit lower down and we look at the left ventricle. The left ventricle, as I said
in the previous section of this video, is very, very thick and muscular. As we can see here,
if I colour it in, if you look at how thick this muscular area is, it has to be this thick in order
to force blood against the force of gravity. It has to really push your blood really hard and the
only way to do that is to be very, very muscular and this little slight angle is definitely going
to help out moving the blood. Yet again, we've got some papillary muscles running down the side
and then depending on what your textbook says, you will see something called a septum.
Now, the septum, I'm going to pick a different colour now so that we can see the difference here.
The septum is this piece of muscle that runs down the middle. Essentially, the septum divides the
left and right-hand side of the heart and it might not be very obvious by looking at the picture but
the reason for that separation is the septum needs to keep oxygenated blood separate from
deoxygenated blood. You have septums in other places in your body. Think about the septum in
your nose. It separates your left and right nasal cavity from each other and so the septum in the
heart prevents mixing of different levels of oxygen and carbon dioxide blood. Now, last but not least,
we need to take a cross-section through the heart. We need to look at the layers of the heart
starting from the outer layer and slowly working our way in and so that's what we see in this
little box at the bottom here. We've taken a sliver through the wall of the heart and sitting
right on the outside is our pericardium. Remember, the pericardium is a protective layer on the
outside of the heart. It's like a bag that contains your heart, prevents friction. Sitting just inside
of that, we are going to be able to see the epicardium. This is the just inside layer.
Then we are followed by the myocardium and whenever you see the word myo, I want you to
think of muscle because that's what it's referring to. This is the most muscular portion of our heart
and then the most inner inner layer is called the endocardium and so we have the peri on the outside,
the epi slightly inside of that, the myocardium and then the endocardium. Right, let's move into
blood vessels. We have three kinds of blood vessels and in an exam or test, you will most
likely be asked to tell the difference between the three. You will have to give visual differences
and structural differences but you'll also be asked to give functions. I'm going to start off
with looking at the structural differences and I'm going to start off with arteries. Now, arteries
are some of your thickest, largest blood vessels in the body and they have some very defining
qualities. One of them being that they have a really really thick layer of muscle. So let's run
through the labels and compare them to the others. First of all, we have a thick outer layer of
collagen fibers. Now, collagen is extremely strong, it is very flexible and what it allows it to do
is the artery can flex and move without bursting. Why does it need to do that you ask? Well, it's
because our arteries are moving blood at a really high blood pressure. The higher the blood pressure,
the more likely it is for the blood vessel to rupture or burst. So we need to keep it intact.
Sitting just below that is a thick muscle layer with a little bit of elastin in it. Elastin makes
it stretchy and basically what happens is those muscles, and this is all the muscles sitting
around here, sits around the edge of your artery and they actually move. Now, they're smooth muscles
so you don't have to think about moving these muscles, they do it on their own but it's really
important that they do because as they move, it facilitates the movement of your blood around
your body. We then have your endothelium. Endo, remember, is a biological word referring to
inside layer and so this is the innermost thin layer of cells that is just providing
essentially protection and it just makes everything nice and smooth. You don't want any
blood cells getting caught and snagged along the way. Then, if we look at the central lumen,
you'll notice that it says it's narrow and if I were to compare it to the vein, if we look over
to the vein, you'll see that the vein is a very wide lumen. So in other words, what they're saying
is if you compare the circumference of the two, so if I draw a circle around the artery and then I
compare it to the vein, we can see that the hole is much bigger, the lumen, the space is much bigger
in the vein and that has to do with the function. I'll get to the function soon but let's go over
to the vein. Now, if we look at the vein on the outside, it's got a very thin outer layer of
collagen. It doesn't have to be as flexible because blood is not moving through it at such
a high pressure. It has a thinner layer of muscle. It's still there but it's pretty thin. I mean,
if we compare it to the artery and then it also has endothelium. Any tube is going to be lined
with some kind of endothelium and we just spoke about the size of the lumen. Now, how does this
structure link to their function? Well, if arteries are there to move blood at high pressure
with not always but most of the time oxygen, then we're going to need a mechanism that's going to
make sure that blood is being pushed to those locations and to do that, arteries have narrow
lumens. So they've got this narrow opening. It keeps the blood at a very high pressure.
It allows blood to be transported more efficiently but for veins on the other hand, veins are
bringing blood back to the heart. Not always just deoxygenated blood but most of the time it is
and essentially, veins are bringing blood back to the heart. Now, that then means that the blood is
not really as pressurized as it used to be. It's gone through the whole body and it's lost pressure
as it's gone along and so the hole is a little bit bigger that's going to allow more blood to
flow through it and what's not in this picture is that veins have valves in them. So if we did a
cross-section through a vein but this time upright, there are these little flaps and these flaps are
called our valves. Now, this valve is open at the moment and it allows blood to flow past it.
The moment however, blood starts to fall back down or the heart is in between beats, what will happen
is those little flaps will meet together and close and it will prevent any blood from flowing in the
opposite direction. So it prevents what we call backflow. We can't see it in this image here but
they are definitely there. Now, last but not least in our blood vessels are the capillaries. These are
the thinnest, smallest blood vessels. Some of them are microscopic and they are one cell layer thick.
Now, they are literally only made out of endothelium. They don't have muscle layers,
they don't have collagen layers, they have no other layers but an endothelium layer which is
why they are microscopic, why they're so small and the purpose of capillaries is to join arteries to
veins. It allows for substances to diffuse into their surrounding tissues and it makes it really
easy for those substances to move because all they have to do is, for example, water or nutrients,
they have to move through one cell layer and that's it. They move through that cell, the
endothelium cell and then they move straight into your tissues. I quite like this picture and so I
included it here about how arteries and vein structures are very different and this one shows
it a bit better side by side and you can see the layers a lot clearer. Let's start off with the top.
So first of all, the endothelial cells. Now, depending on what kind of level of biology you're
doing, you might have to call it the tunica intima which is essentially the innermost layer
of cells. Now, they both have it. They also both have elastin although the amount of elastin in
arteries is greater. Then we have the smooth muscle. Now, this is a defining difference.
The thickness of the wall of the artery, if you look at it over here, is far greater in thickness
than that we see of our vein and I'll remind you it's to keep blood at a high blood pressure
because we want to move the blood around as quickly and smoothly as possible. Then below
that we have the loose fibrous connective tissues. So this is the most outer layer. This is where we
find like that collagen that we spoke about earlier and there might even be epithelial cells
on the outside. Please take note of the scientific names, the tunica intima, tunica media, tunica
externa, and the serosa. Depending on what level of biology you're doing, you might need to know
these names. If we look at a little paragraph at the bottom, I thought it summarized it very well
for us. Arteries are thick and muscular and they have an elastic wall and they gradually reduce the
harsh surge of blood to a steadier flow. What they mean is that as the blood leaves the heart,
it's quite intense, it's quite strong. So we're going to make sure that we keep that strength
in blood pressure. We also don't want to burst any blood vessels. In veins, their most defining
thing is that they have such a large lumen and they have less resistance to the blood flow.
And we can see how big the lumen is. I mean, if we have a look here, if I take my pen and I
circle around it, there's the width of the artery versus the width of the vein.
And that width tells us the pressure at which blood is moving. This is a high pressure blood,
this is a low. And as we know from physics, we are always going to move from a high pressure to
a low pressure. Lastly, in terms of structure, I included this picture just to show you what it
looks like in a micrograph. In a lot of tests and exams, we get really flustered and confused because
we see diagrams of veins and arteries in the heart, but then we see a real image, a real
micrograph, and we are a little bit confused as to what we're looking at because it doesn't look
exactly like the diagram. So let me point it out. We have an example of two micrographs here and we
have a vein in either of them. Now you'll notice the most defining thing about the vein is its
irregular shape. It doesn't seem to be circular or oval or anything like that, particularly this
one over here. You'll also notice that you can see this layer of connective tissue on the outside
and then not much else. And that's because veins are far more flexible. They are pretty
able to collapse on themselves. It means that they are going to be able to fall on themselves.
And of course, the most defining thing is look at all of this space. This is all the lumen in both
of them. If we look at our arteries, on the other hand, you can see a very, very characteristic
thick layer of muscle that sits all the way around. And then we also see the smaller lumen sitting on
the inside in both of these. So let's round up this lesson with a terminology recap. We learned
that in mammals, we used a closed system. That means that our blood is contained within blood
vessels at all times. It doesn't leak out. It must stay inside of the heart or inside a blood vessel.
We also learned that there is double circulation. That means the blood returns to the heart twice,
once to the lungs and then once to the body itself. And we call this either the pulmonary
circulation, which is blood going from the heart to the lungs, or systemic, which is the blood
coming from the heart to the body. We have two ventricles, which are the lower chambers of the
heart, and two atria, which are the upper chambers of the heart. We have the pericardium, which is
the outer layer of the heart. It's the bag that keeps the heart safe. We have the epicardium,
which is the layer of tissue just below it. And finally, we have the endocardium. There is the
tricuspid valve that keeps our right-hand side of our chambers separate. And the mitral valve
keeps the left-hand chambers of the heart separate. We then have arteries, which are the thickest
blood vessels with the smallest lumens. We have veins, which are the smaller blood vessels with
large lumens. And last but not least, we have the capillaries. These are our joining blood vessels
between an artery and a vein, and they transport nutrients and wastes between the blood and the
tissues. As always, if you like this video, I hope to see you soon. Bye!