Welcome back to Bogobiology!
In this video, we’ll be learning about the structure and function of the heart and circulatory
system.
We’ll cover the major loops of the circulatory system, the various types of blood vessels,
and of course the path of blood through the heart and the heart’s internal structure.
There’s also a corresponding study guide handout to go with the video that’s linked
in the video description.
Ok, here we go!
The heart is a complex muscular organ that pumps blood throughout the body.
It is a major part of the circulatory system, which is the network of vessels that transport
blood.
Red blood cells carry oxygen on their surface attached to a molecule called hemoglobin.
They deliver the oxygen all over the body, and pick up one of the cells’ waste products;
carbon dioxide.
Remember, virtually every cell in the human body needs a constant supply of oxygen.
If they don’t get it, they start to die within a few minutes.
The blood must constantly cycle through the three divisions of the circulatory system:
the pulmonary circuit, the systemic circuit, and the coronary circuit.
In the Pulmonary circuit, the blood circulates through the lungs and the blood is saturated
with oxygen.
In the Systemic circuit, the oxygenated blood is delivered to the rest of the body.
The Coronary circuit supplies the heart itself.
Remember that the heart is made of muscle, and muscle cells require a lot of oxygen to
function properly
There are three major types of blood vessels that the circulatory system uses to deliver
blood to the tissues of the body; arteries, veins and capillaries.
Arteries carry blood away from the heart.This is their defining characteristic.
USUALLY this means they carry oxygenated blood, we’ll cover the exception later in this
video.
Artery walls consist of three layers of tissue.
Arteries also have the thickest, most muscular walls of all the blood vessels because the
pressure in them is quite high.
Some arteries you might have heard of are the aorta, and also the carotid arteries which
run up each side of your neck to supply blood to your brain.
The major arteries split into minor arteries, and then into even smaller ones known as arterioles.
Veins carry blood back to the heart, meaning they USUALLY carry de-oxygenated blood.
Again, we'll cover the exception later on.
I find this one easier to remember, because the word “vein” is similar to the spanish
and french verb “venir” meaning “to come”.
Veins, like arteries, have 3 layers of wall tissue, but do no not need to be as muscular
because the pressure in them is much lower.
However, because the pressure in them is low, they need valves to prevent back flow.
A vein you’ve probably heard of is the jugular vein, which runs down the right side of your
neck to return blood from the head back to the heart.
The major veins split into minor veins, and then into even smaller ones known as venules.
Capillary “beds” are groups of 10-100 tiny blood vessels that branch out from arterioles.
Red blood cells can fit through them single file.
Because they are so small and have such thin walls, oxygen, carbon dioxide and nutrients
can easily diffuse in and out of them through their walls.
The heart doesn’t directly interact with capillaries; to reach the body’s capillary
beds, the blood must first pass through a series of increasingly smaller blood vessels.
The heart pumps blood through the body in a repeating process called the Cardiac Cycle.
During the Cardiac Cycle, the chambers of the heart rhythmically contract (systole)
and relax (diastole).
During systole, the heart pushes blood out through its major vessels, and the relaxation
of diastole allows the chambers to refill.
This is much like an ordinary pump that you might use to inflate a bicycle tire.
The blood follows a very specific path through the heart and encounters a number of important
structures along the way.
This pathway is important because it ensures that only oxygen rich blood is delivered to
all your tissues; we’ll notice a few fail-safe mechanisms along the way.
De-oxygenated blood is funneled back to the heart through a blood vessel called the vena
cava.
It has two branches, superior (from the upper part of the body) and inferior (from the lower
part).
The de-oxygenated blood enters the heart’s right atrium.
Before we go any further, here’s a quick reminder that, whenever we’re discussing
anatomy, we always refer to the PATIENT’S right, not our own right.
This is why a lot of diagrams appear “backwards” at first glance.
When the right atrium contracts, it pushes blood into the next chamber, the right ventricle,
through a one-way valve called the tricuspid (sometimes also called the atrioventricular)
valve.
The valve has flaps that seal tightly to make sure the blood can’t flow backwards.
The opening and shutting of the valves is what makes the LUB-DUB sound of a heartbeat.
This blood passes through a second valve called the pulmonary valve and into a blood vessel
called the pulmonary artery.
Even though this pair of blood vessels is carrying de-oxygenated blood, the flow travels
away from the heart, so it’s an artery, not a vein.
Once the blood reaches the lungs, gas exchange occurs.
Carbon dioxide diffuses out and oxygen diffuses in.
The blood is now considered oxygenated, and changes from dull red to bright red.
This is also a good time to mention that de-oxygenated blood is not actually blue; it’s more of
a maroon color.
However, I’m showing it as blue here so it’s easier to see the difference especially
if you’re watching on a small screen.
The oxygenated blood returns from the lungs via the pulmonary veins.
Again, these are flowing back towards the heart, so these blood vessels are veins, even
though the blood in them is oxygenated.
The blood re-enters the heart in the left atrium.
When the atria next contract, the blood is pushed through the mitral valve into the left
ventricle.
This valve has two flaps, so it’s sometimes also called the “bicuspid” valve.
Finally, the left ventricle contracts and forces blood out into the aorta through the
aortic valve.
Remembering the order of the valves can be a challenge, so here's a trick...
T.P.M.A.
Tricuspid, Pulmonary, Mitral, Aortic or Toilet Paper My...
Arms!
Notice how muscular the walls of the left ventricle are, even compared to the right
ventricle.
The right ventricle only has to be powerful enough to drive the blood to the lungs, but
the left ventricle has to have enough power to pump blood all over the body.
The exterior of the heart also has several important blood vessels.
The coronary arteries are one of the first branches off the aorta.
They supply oxygen to the heart muscle.
If the blood flow through the coronary vessels is cut off, oxygen can't reach the heart muscle
tissue.
This is when the heart muscle starts to die.
We call this an "acute myocardial infarction" or a "heart attack".
So, how does the heart know when to beat?
Cardiac muscle cells have the ability to contract autonomously, but their activity needs to
be coordinated in order to be effective.
The heart has two “nodes” that coordinate the harmonious activity of the muscles, much
like how a DJ might get a crowd to dance to a beat.
The first node, known as the sinoatrial or SA node is located in the wall of the right
atrium.
Electrical charges from the sinoatrial node cause the two atria to contract together as
a pair.
The impulse then travels to the atrioventricular node which is between the right atrium and
right ventricle.
It then travels on down the walls of the ventricles via a series of specialized fibers, allowing
the ventricles to contract.
This second process includes a tiny delay, which allows the atria to fully empty their
contents into the ventricles before the ventricles contract.
In short, after the SA node gives the first command, the AV node repeats it and tells
the crowd to drop the beat.
This coordinated electrical activity is what you can detect and record using an EKG or
“electrocardiogram”.
Hopefully, if you ever see one, it’s not a flat-line.
That pretty much wraps up our review of the heart and circulatory system.
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