Download The Cardiac Cycle

The Cardiac Cycle
We will begin the cycle during mid to late ventricular diastole, when the heart is completely
relaxed. We will focus on the left side of the heart; remember that pressure differences are also
occurring on the right side (they are a lot less, however).
1.
Mid to late Diastole
The pressure in the atrium is
increased and greater than the
ventricular pressure because
blood is returning to the atria.
This increased atrial pressure
forces the AV valves open and
the left ventricle fills passively
as the atrial blood drips into the
ventricle (flow from higher to
lower pressure). This dripping
fills the ventricle to 70% of its
capacity!
2.
Atrial Systole
The atria contract and rising atrial pressure pushes blood
into the ventricles through the open AV valves This atrial
contraction "tops off" the ventricles, adding another 30 %
of the total volume of blood to the 70% of the volume that
has passively "dripped". The ventricles now contain the
maximum amount of blood that they will contain (about
130 ml). This is the end diastolic volume (EDV).
3.
Early Ventricular Systole
The ventricles contract isometrically, generating tension and
a rise in ventricular pressure. This is the period of
isovolumetric contraction. As the pressure inside the
ventricles rises above that in the atria, the AV valves shut
(LUB!) No blood can flow because the semilunar and AV
valves are both closed.
4.
Late Ventricular Systole
Once pressure in the ventricles exceeds the pressure in
the aorta, the semilunar valve opens and blood flows into
the aorta. This is the start of ventricular ejection. About
60 - 70 ml of blood is ejected. This ejected blood is
called the stroke volume. Ventricular pressures then
rapidly fall and aortic blood begins to flow back towards
the semilunar valves. This closes the valve (DUP!) and
the aortic pressure slightly increases as the elastic aortic
walls recoil, forming the "bump" in pressure that is called
the dichrotic notch. The amount of blood remaining in
the ventricle when the semilunar valve closes is called
the end systolic volume (ESV).
5.
Early Ventricular Diastole
All the heart valves are now closed and the ventricles are
relaxing. Ventricular pressure is still higher than atrial
pressure. This is the period of isovolumetric relaxation.
When the ventricular pressure falls below the atrial
pressure, the AV valves open and blood "drips" into the
ventricles. Both the atria and the ventricles are now in
diastole, but the ventricular pressures continue to fall.