Mary Baumann 10/13/2012
PSIO 485 exam 2 review
The Cardiac cycle: Brought about by electrical activity
• Resultant changes in blood flow
• Volume and pressure
Contraction and relaxation is related to their effects on valves which each take turns contracting
to empty and then relaxing to fill. Electrical signals pass down to the SAnode to initiate the
contraction (depolarization) of the muscle cell (mechanical stage). First electrical (potential
energy) and then mechanical.
Cardiac cycle consists of alternating periods of systole and diastole for the atria and ventricles.
Where the electrical signal travels down to depolarize both the atria, at the SAnode.As the atria
contract the ventricles fill while they are relaxing. The signal continues to depolarize the whole
atria until it reaches theAV node, where the atria push out the rest of the blood and relax.At this
point both the atria and ventricles are in diastole. Once theAV node initiates the Ventricles
begins to contract where blood is ejected out the pulmonary andAortic valves to the lungs and
body respectively. As the electrical signal travels down its concentration gradient and the
ventricles depolarize the atria are in repolarization.
This process reveals that the atria and ventricles are never at systole at the same time. They
only have a short period of time to relax together before the ventricles contract. Thus in the
Cardiac cycle the atria and ventricles go through separate cycles of systole and diastole. We will
never see contraction of the whole heart.
Blood flow through the heart during the cardiac cycle: changes in pressure and volume
As pressure increases in the atria when blood is filling, theAV will open. This is due to higher
pressures in the atria then in the ventricles putting pressure on the atrioventricular valves. Then,
after the pressure inside the ventricles starts to increase, pressure increases on the semilunar valves. When the pressure inside the ventricles is greater than the pressure in the aorta and
pulmonary arteries, then the semilunar valves will open up. Looking at how the volume of blood
can affect the pressure associated with the valves we can see that if we increase venous return,
and the amount of blood that fills up in the atria or ventricles we can increase pressure which
opens the valves much quicker. Thus the opening of the valves depend upon the amount of blood
that fills up the atria and ventricles.
As we mentioned earlier, there is a point when both the atria and the ventricles are relaxing. This
happens when both the two sets of valves are closed. This is called isovolumetric volume.At this
point there is no blood in moving in or out of the atria or ventricles, no increase or decrease in
pressure and or volume in the ventricles at this time.At a molecular level the sarcomeres (single
contractile unit) are not moving. This is the point where theAV node is delaying to allow the
heart to relax for a brief moment. This is called AV nodal delay.
The process we just described can also be thought of when theAV valve opens the volume in
the ventricles increases, and then when the semilunar valves open the volume decreases in the
As we become more familiar with the heart we will understand its basic principle which will in
turn allow us to expand on and know the complex principles. First we need to know the
sequential changes in pressure and volume that occur in the heart chambers during the cardiac
cycle. This begins with the basic principle where changes in pressure are seen before a change in
In Wiggers Diagram, ECG, chamber pressure, chamber volume, and heart sounds are all plotted
on the same scale to represent time line of the heart and its functions. The following is the order
at which the cardiac cycle occurs:
4. Volume 5. heart sound
Draw a electrocardiogram and name: early ventricular diastole, after ventricular
repolarization, early atrial diastole, ventricular systole, late ventricular systole, and atrial
From the beginning of the P wave, Left atrial pressure exceeds left ventricular pressure
which pushes theAV valve open allowing blood to passively fill in the left ventricle.
This increases the volume of the left ventricle slowly. This process is still during atrial
diastole; both chambers are relaxing. This increase in pressure in the atria is due to the
continuous inflow of blood from the venous system into the atria. Following the P wave
the atria leave diastole and role into systole, where depolarization brings about atrial
contraction pushing more blood into the ventricle. This step happens late in ventricular
diastole when the SAnode reaches threshold and fires. Contraction of the atria causes an
increase of atrial pressure and blood is actively forced into the ventricles to complete
filing. Left ventricular volume increase more rapidly during the active filling.Atrial kick
happens at the end of atrial systole and before ventricular systole. it contributes to 10%
of the volume in the left ventricle.
End of Ventricular diastole - include Ventricular systole, blood volume, end diastolic