Cardiac Cycle

Cardiac Cycle


– In this video,
we’ll be talking about all the events
associated with blood flow through the heart in
one complete heart beat, the cardiac cycle. Just a couple of quick
definitions before we begin. First systole,
systole is contraction of the heart muscle. So when you see systole or
systolic, think contraction. Diastole is relaxation. So when you see the term
diastole or diastolic, think relaxation of
the heart muscle. In a single heartbeat, you
have to consider both the atria and the ventricles. So you have atrial systole
and you have atrial diastole. But you also have
ventricular systole and ventricular diastole. Note that these overlap. The atria contract while
the ventricles are relaxed. And the ventricles contract
while the atria are relaxed. It’s a continuous cycle. So it’s a bit
arbitrary as to where you set time zero for
a single heartbeat. But here we’re
looking at time zero as the beginning
of atrial systole. Note that ventricular
systole does not begin until the atria
are done contracting. Now let’s bring the
heart into our picture. Here, we can see what’s
happening to the heart throughout the cardiac cycle. First, we have atrial systole. The atria contract, squeeze down
to force the last bit of blood down into the ventricles. Both the right and left atria
contract at the same time. Next, in B and C, you
have the atria relaxing and the ventricles
beginning to contract. Both ventricles
contract simultaneously. Remember that the AV valves
close when the ventricles begin to contract so blood
does not get pushed back into the relaxed atria. The first phase of
ventricular systole is called the
isovolumetric phase. The ventricles are contracting,
but they haven’t quite created enough pressure yet
to open the semilunar valves and eject blood
into the arteries. So since nothing is
ejecting, there’s an equal, a steady
amount of volume, of blood volume
in the ventricles. And that’s why you get iso– which means equal–
volumetric contraction phase. The second phase of ventricular
systole is the ejection phase. In this phase, pressure
in the ventricles is high enough to force
the semilunar valves open and eject blood into the
pulmonary trunk and the aorta. As ventricular diastole
begins, the relaxation phase, the semilunar valves close as
blood falls back against them. The AV valves open a little
bit later in the phase. And blood drains into
the atria from the veins and down into the ventricles
through the AV valves. This is a slightly different
view of the cardiac cycle. It starts at a
different point in time which, of course, doesn’t
matter because we’re talking about a cycle. So like I said,
it’s arbitrary as to where you set time to zero. I’m showing this
diagram to you because I like how it illustrates the
different phases in ventricular systole and diastole. So starting at the
beginning here, we have late
ventricular diastole. Blood is passively draining from
the atria into the ventricles. About 80% of blood enters
the ventricles this way. The last little
bit, the last 20% happens during atrial
systole, when the atria contract and force that blood down. After the atria relax, the
ventricles start to contract. There is a really brief, that
isovolumetric contraction phase, that lasts just for a
moment before the semilunar valves open and
blood is ejected into the aorta and
the pulmonary trunk. After the ventricles
relax, there’s a momentary phase when the
AV valves are still closed, before the pressure of blood
accumulating in the atria forces them open. Then we’re back to the
passive ventricular filling phase again. So this wraps around back to
passive ventricular filling. This diagram shows how
the mechanical events of the cardiac cycle match up
to the electrical events shown on the electrocardiogram. Remember that the P wave
is atrial depolarization. Notice how atrial systole
follows pretty much on the heels of that P
wave, just slightly after. It’s the polarization
that the P wave represents that stimulates,
that triggers contraction of the atria. Then you have the QRS complex,
ventricular depolarization, which stimulates contraction
of the ventricles. Ventricular systole
starts in about the middle of the QRS complex. The T wave represents
repolarization of the ventricles, which
allows the ventricles to relax. So the T wave putting the
ventricles into ventricular diastole. If you can work
through this graph and understand
what it’s showing, you will understand the
events of the cardiac cycle. This graph just shows everything
that’s happening in the heart during a single heartbeat,
a single cardiac cycle. Time is on the x-axis. So you can pick a
single point in time and look up the graph
to see everything that’s happening at that moment. Let’s pick it apart
and see what it shows. At the top here, you have the
electrical events on an EKG. We already talked
a bit about how the EKG relates to the
mechanical events in the heart. So let’s set this
bit aside for now. On the bottom here, we have
left ventricular volume. You see passive filling up
to this point labeled 1, then atrial ejection up to 3, OK? So notice how there’s
a little bit of blood, but then there’s just that last
little bit getting squeezed in when the atria contract. This takes us to the
point of maximum blood volume in the left ventricle,
called the end diastolic volume or we’ll be referring
to it as EDV. Between the end diastolic
volume and point 6 down here, we have ventricular contraction,
ventricular systole. This is when blood is being
ejected out to the aorta. There is still a small amount
of blood left in the ventricles at the end of
ventricular systole. This is your end
systolic volume. Once the ventricles relax,
passive filling begins again and you start seeing
the blood volume in the left ventricle going up. Now let’s take a look
at the pressure graph. We’ll start with
the left atrium. That’s the line in blue here. With the left atrium, you see
just a little rise in pressure as the atria contract. See how this corresponds
to the active filling phase in the left ventricle? There’s not much going
on with this line. There’s just a little
bit of a blip in pressure when the left AV valve
closes and when the left AV valve opens again. Now let’s take a look at
pressure in the left ventricle. This one’s a little
more interesting. In the left ventricle– remember
we’re looking at pressure now. So you have a little
blip as blood, as the atria contract and
blood is being pushed down into the left ventricle. But when you really see pressure
rising in the left ventricle is when ventricular
systole begins. That’s what’s happening
with this big mountain. See how that corresponds to
ventricular systole up here? And see how it corresponds
to the drop in blood volume as blood is ejected
out to the aorta? Pressure falls as
the ventricles relax. The last line on this
pressure graph is the aorta. And that’s in black. In the aorta, you see a
slight decrease in pressure until the ventricles
start to contract. At that point, blood is being
ejected into the aorta. You see a rise in
aortic blood pressure. It’s kind of disappearing
behind the ventricle line here. Starts to come back down
again as the ventricles relax. There’s a little blip in
aortic blood pressure here. It’s called the dicrotic notch. What happens is that the
aorta is a very elastic, thick-walled artery. So those elastic walls
get stretched as blood is ejected into the aorta. And then they snap back a little
bit as the ventricles relax. So this little blip, this
little rise in blood pressure is from the elastic
walls of the aorta recoiling back and
causing just a blip of an increase in pressure. So this is a complicated graph. There’s a lot going on here. But if you break it down,
take it piece by piece and see how those pieces
relate to each other, it’s not nearly so intimidating. After studying this
video, you should be able to distinguish between
the different forms of systole and diastole. You should be able to explain
what’s happening in the heart during the cardiac cycle and
relate these events to blood flow through the heart,
the electrical activity of the heart, by
which I mean the EKG, and pressure changes in both the
heart’s atria and ventricles, and the aorta.