So now that we've discussed the
electrical events that happened in the heart,
let's look at the mechanical
events that happened in the heart.
First, look at a little bit of vocabulary.
Systole is going to refer to the
period where the heart is contracting.
Diastole is going to refer to the
period where the heart is relaxing.
And then we have our cardiac cycle.
This is gonna refer to blood flow through
the heart during one complete heartbeat.
In a heartbeat, you start with atrial systole and diastole
and then you go to ventricular systole and diastole.
This cycle is going to represent a series
of pressure and blood volume changes
and these mechanical events can follow the electrical
events that you see on an electrocardiogram.
There are three phases to the cardiac cycle.
So following the left side and starting
with total relaxation, let's take a closer look.
The first phase is going to be ventricular filling
which is going to start at mid to late diastole.
So at this point, the pressure is low and about 80%
of the blood is just passively flowing from the atria
through the atrioventricular valve
into the ventricles from the atria.
At this point, the semilunar valve is closed.
Next we have atrial depolarization.
This is going to trigger atrial systole which is going
to be that P wave portion of the electrocardiogram.
During this portion, the atria are going to contract and it's
gonna push that remaining 20% of blood into the ventricle.
Our end diastolic volume is going to refer to the volume of
blood in the ventricle at the end of ventricular diastole.
So after this, the depolarization
is going to spread to the ventricles
and this is reflected on our
electrocardiogram as the QRS wave.
The atria will finish its contraction and return
to diastole, and as it is returning to diastole,
ithe ventricle is now beginning systole.
The second mechanical event
of the heart is ventricular systole.
During this event, the atria is relax
and the ventricles begin to contract.
This leads to a rising ventricular pressure
causing the atrioventricular valves to close.
This is going to occur in two phases.
First, we have the isovolumetric contraction
phase in which all of our valves are closed.
This is followed by the ejection phase where the ventricular
pressure exceeds that of the pressure in the large arteries
forcing the semilunar valve open.
The pressure in the aorta is about 80 mmHg and so
the pressure in the ventricle must exceed this 80
in order for SL valve to open and the blood to pump
out of the ventricle into systemic circulation.
So at this point, we have our end-systolic volume.
This is the volume of blood that
remains in the ventricle after systole.
The third mechanical event of the heart
is going to be isovolumetric relaxation.
So following ventricular repolarization, which
we see with the T wave on our electrocardiogram,
the ventricles are relaxed and the atria relaxed in filling.
Backflow of blood in the aorta and in the pulmonary
trunk is gonna cause the semilunar valves to now close.
This causes what's known as the dicrotic
notch which is a brief rise in the aortic pressure
as the blood is going to rebound off of those
closed valves so it's gonna be like a little blip
where the pressure goes up briefly.
Then the ventricles are totally closed
chambers and thus it is isovolumetric.
Once the atrial pressure exceeds the
ventricular pressure, the AV valves open again
and we start the process all over again.