In today's video with the help of the cadavers here in the lab, we're going to take a look
at the heart and discuss exactly how it's able to contract dependably over the course
of your lifetime.
As in somewhere around two and a half billion times over the course of your lifetime.
That is the definition of a workhorse.
It's going to be a bumpy one.
Let's do this!
[Intro]
The heart is unique in the fact that it's capable of self-excitation.
Meaning that it doesn't require input from the brain in order to contract.
Now, the brain is still definitely involved, but it's important to note that the heart
can contract on its own if it needs to.
In fact, if I were to take a heart from a living person and place it here on this table,
it would then contract on its own for around three to four minutes before succumbing to
oxygen deprivation.
Inside of the heart are specialized cells called pacemaker cells which are modified
versions of normal cardiac cells.
Now, there's a group of these cells located in what's called the right atrium of the heart
near the entrance of a gigantic blood vessel called the superior vena cava.
Now, these group of cells are called the Sinoatrial Node or SA node for short.
Now, the SA node is capable of depolarizing which will cause the neighbouring cardiac
cells to contract while also initiating cascade of coordinated muscular contraction to go
through the heart.
Put simply, the SA node is the heart's natural pacemaker causing the two atria to contract
first and then the two ventricles.
Under normal resting conditions, SA node causes the heart to be anywhere between 60 and 100
beats per minute although depending on the individual and the situation, that can definitely
change and still be considered normal.
However, this contracting cascade happens so quickly that it needs to be slowed down
so that atria can push all of the blood into the ventricles before the ventricles contract.
This is the job of the next group of specialized cells called the Atrioventricular Node or
the AV node.
Now, the AV node is going to be located in the wall that separates the left and right
atria called the Interatrial Septum.
The AV node delays the impulse for around a tenth of a second and then that conductive
cascade can continue down into the ventricles.
Interestingly, if the SA node fails, the AV node can take over acting as the new pacemaker.
The only problem is it's at a significantly slower rate of around 40 to 60 beats per minute
but hey, at least the heart is still beating.
The impulse next travels to another bundle of specialized cells called the Bundle of
His.
Now, it may look as though it should be pronounced "his" instead of "hiss".
It's actually named after Wilhelm His Jr. who's the first to discover that it physically
linked the atria to the ventricles but the Bundle of His is located at the base of the
interatrial septum and then it's going to divide into two descending branches.
These branches, unsurprisingly, are called the left and right bundle branches.
The bundle branches are going to be located in the interventricular septum which separates
the left and right ventricles.
The left bundle branches then going to further separate into what's known as the Anterior
and Posterior Fascicle but we now have an impulse that's travelling down towards the
most inferior aspect of the heart and it's then once again going to split to what are
known as the Perkinje Fibers.
Perkinje Fibers are specialized cells that have lots of mitochondria which are vital
for creating energy that the heart will definitely need.
Now, they're just underneath the epithelial layer of the heart called the Endocardium.
The endocardium is the innermost layer of the heart and it comes into direct contact
with the blood itself.
Now, it's the job of these Perkinje Fibers to transmit that impulse to the contractile
heart cells inside of the myocardium or that muscular layer of the ventricles.
It's important to know that myocardial contraction occurs at the tip of the heart known as the
apex and from there will then move superiorly.
This allows for the blood to be ejected in a directional manner.
Imagine if the ventricles were just contracting everywhere all at once, it would cause the
blood to just go in every direction imaginable dramatically reduce its efficiency.
Alright, so now that we understand how the signal gets from the SA node down to the Perkinje
Fibers, let's see exactly why this is useful in medicine.
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Alright, let's get back to it.
Whether you've seen it in the hospital first hand or through the various medical shows
and movies, we're all familiar with the electrocardiogram or ECG.
The ECG which is the exact same thing as an EKG is a highly effective method at determining
the heart's electrical activity.
Each point on an ECG tells you something about where the heart is during its conductive cycle
as well as how strong or weak that conductivity is.
The first blip on the ECG is what's known as a P wave and it represents the SA node
depolarizing as well as the muscle tissue inside of the atria themselves.
Picture this - an impulse is spreading through the atria getting them ready to contract.
You'll then notice a straight line segment between the P and Q waves.
This represents the atria actually contracting, thereby pushing blood into the ventricles.
The next three waves are usually combined to form what's known as the QRS Complex.
It's easily the biggest spike on an ECG, so it probably comes at no surprise but it's
going to be covering a lot of cardiac real estate.
The QRS complex is the depolarization of the ventricles.
Starting at the interventricular septum and then spreading into the entirety of the ventricular
myocardium.
This is where the impulse is travelling from the AV node to the Bundle of His down the
left and right bundle branches to the Purkinje fibers and then spreading into the muscle
fiber of the heart itself.
Now, the segment between the S and T waves represents the period of time that the ventricles
are actually contracting, again, pushing blood upwardly from the apex towards the lungs and
to the rest of the body.
The T wave represents the ventricles repolarizing.
Essentially preparing themselves to become depolarized again.
Now, interestingly, the atria also repolarized except it happens at the same time as the
QRS complex.
So, it gets drowned out in the ECG and you just simply can't see it.
We then find ourselves back at the beginning waiting for the SA no to kick things off so
that we can see the P wave once again.
Now that we understand exactly what's happening under normal conditions, let's talk about
abnormal conditions or specifically what are known as heart arrhythmias or what most would
just refer to as an irregular heartbeat and the reason why I say it like that is because
heart arrhythmias are actually pretty complex and they encompass many other issues besides
just conductive problems such as with the SA node, AV node, or others.
In fact, Jonathan and I plan on doing future videos discussing and detailing many of these
heart arrhythmias.
So for now, I'm going to be pretty broad and relatively brief.
Tachycardia is generally considered to be a heart rate in excess of 100 beats per minute.
However, there's a ton of nuance to this because anyone who's exercising is going to have a
heart rate over 100 beats per minute meaning that tachycardia doesn't mean you have a heart
problem.
However, if you're sitting on the couch, you know, watching some Curiosity Stream and rocking
a heart rate of 180, that probably demands some attention.
You see, tachycardia can happen in the ventricles or in the atria and depending on which one,
you're going to get different symptoms and consequences and depending on which one it
is, you'll see something different on the ECG.
Bradycardia is essentially the exact opposite with a heart rate of less than 60 beats per
minute.
Again, though, highly-fit individuals can have a resting heart rate of less than 60
beats per minute and Adults when they're sleeping can also have a heart rate of less than 60
beats per minute and both are obviously considered to be okay.
But for certain individuals, it can be a precursor for more intensive heart conditions and in
some cases, may necessitate an artificial pacemaker to take over for the heart's natural
pacemaker.
Fibrillation tends to be one of those heart conditions that a lot of people are familiar
with.
You could think of it as an almost random but definitely disorganized contraction of
the heart.
Again, it can happen in either the atria or the ventricles and both of them will come
with their own unique set of consequences ranging from mildly concerning to almost a
guaranteed fatality.
Thanks for watching everyone.
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