202203 Cardiac Cycle Made Easy

Cardiac Cycle
Made Easy!
By: LESLIE SAMUEL
Interactive-Biology.com
March 2022 edition
Support Interactive-Biology by becoming a patron:
https://www.patreon.com/interactivebiology

Cardiac Cycle Made Easy!
Video published on March 24, 2022
Watch the video here.
Ever wonder what happens in a heartbeat? What happens inside our heart when we hear the
‘lub-dub’ sound? When I say cardiac cycle, I’m talking about everything that happens from the
beginning of one heartbeat to the beginning of the next. So we’re dealing with an entire
heartbeat.
Introduction
Let’s look at the cardiac cycle diagram. I know, it looks hard, but it isn’t really. Here’s why it
seems so difficult. Because when your professor teaches you about the cardiac cycle, you see
this complicated diagram right here.
It has a TON of details and it looks kinda scary. Let’s not do it like that. Let’s break it apart, and
build it together.
3

What happens in a Cardiac Cycle?
Let’s start with blood coming back from the body. Blood enters the heart first through the atria.
On the left side, we have blood coming back from the lungs and on the right side, we have
blood coming back from the rest of the body.
When the atria contract, they push blood into the ventricles. And then when the ventricles
contract, they push blood out of the heart. Those are the two contractions that cause blood flow.
If this seems unfamiliar or unclear for you, head on to my article on How Blood Flows Through
the Heart and you’ll get a more detailed explanation.
The Cardiac Cycle Diagram Explained
The first place we’re going to look at is the electrocardiogram. We’re going to start here because
this shows the electrical signals that are responsible for the heart beating.
4

Atrial Systole
The first thing we see is the P wave. This shows the depolarization of the atria. That’s the
electrical signal traveling through the atria. When that happens, that causes the atria to contract.
To see that, we’re going to look at a different line on this graph, the one that shows you the atrial
pressure.
5

What do you expect to see right after the depolarization of the atria? Well, that’s the signal. It
tells the atria to contract, and if the atria contracts, you would expect to see an increase in atrial
pressure. Right? Of course. If you squeeze something, the pressure increases. And that’s
exactly what we see here – right after the P wave. Makes sense. I love it.
Now, there’s a fancy word that goes along with this and that’s systole. When you see systole,
think contraction. And here, the atria are contracting, so we have Atrial Systole. Simple.
What happens to the blood when the atria contract?
Well, it can only go one place. It’s gonna go through the atrioventricular valves – the one-way
valves between the atria and the ventricles, and it’s gonna go into the ventricles. So what would
you expect to see happen to the ventricular volume? You’d expect an increase in ventricular
volume because you’re filling it with blood. And that’s exactly what we see right here.
6