We began talking about this in lab... a cardiac cycle is when first the atria contract, then the ventricles contract, then everything relaxes to get ready to start over.  Right?  We talked about how if you watched a single RBC travel through the heart, you'd see it go through the right side, then the lungs, then the left side, and then the body.  But that although that is the path, the heart doesn't contract the two sides separately.  Both the left and right atria contract together, and then the left and right ventricles contract together.  Right?

    On this page, you will learn a little more about how the heart goes through this cardiac cycle.  There are some word you will need to be more familiar with in order to really understand this.  The two words I want you to think about now are pressure and volume. 

    First, let's work on the word pressure.   We all have some passing familiarity with pressure as a force (not just as in studying-pressure!).  But as a force, pressure is something to be reckoned with.   One way I can begin to address this issue with you is through example.  Let's start with a balloon.  When you try to blow up a balloon, it can be rather difficult.   You have to blow hard enough to produce enough force to get the walls of the balloon to start stretching.  The force you are applying when you start blowing is air pressure.  When you exert enough air pressure to surpass the elastic force that keeps the balloon walls unstretched, the balloon starts to expand.  If you continue to supply air pressure, the balloon continues to expand.  If you continue to supply air pressure, you will even force the walls of the balloon to explode!  This example with the balloon is only an example of pressure, and does not imply anything about how the heart deals with pressure.  Certainly, the heart doesn't expand or explode with pressure.  It has to be strong enough to withstand the pressure of the blood, like a garden hose withstands the pressure of water running through it when you water your lawn.

    The pressure that we will be dealing with here is blood pressure, not air or water pressure.  You are familiar with the notion of measuring blood pressure.  But we normally measure blood pressure externally, by putting a cuff over the arm and measuring the blood pressure within the arteries in the arm.  We will be discussing blood pressure within the heart, and within many different blood vessels as we go through this unit and then the next.  Not just the blood pressure that you record from the arm.

    Another word that you have to think about is volume.   We are used to measuring the length of things.  We are also used to measuring the area of things (like, this table is 3' x 5', or 15 ft²).  But often students start getting lost when we talk about volume.  With blood, a fluid, it is impossible to really talk about amounts of it using length of area measurements.  Do you follow that?  I mean, what would a 4 inch amount of blood look like?  That is meaningless.  So, instead we use volume measurements.  When you cook, you use volume measurements like cups and tablespoons.  In science we use metric measurements of volume like liters (L) and milliliters (mL).  You already know that we have about 5 L of blood in each of our bodies.  That should be something that you can picture based on liter amounts you are familiar with-- like sodas in their 2L bottles.  There are 1000 mL in one L, so there are 5000 mL of blood in one human body.  Every time a ventricle squirts blood out, less than 100 mL of blood is what is actually squirted out.   Those beakers you were using to clean up around the lab were 400 mL beakers... and there are 10, 100 mL amounts within a 1 L bottle of liquid.  Another way to think about it is if you had to pour 20 cups of soda from one 2 L bottle, how much would be in one cup?  100 mL.

Now back to the main topic-- the cardiac cycle...

    Let's start talking about a cardiac cycle by considering what the heart is like when it is totally relaxed-- no contractions in any of cardiac muscle are occurring when the heart is totally relaxed.  In this condition, the atrioventricular valves (A-V valves) are open.  As blood returns to the heart from the lungs (into the L atrium) or the body (into the R atrium), it enters the atria but then just continues straight into the ventricles.  As long as the A-V valves are open, the blood just spills all the way through.  Certainly, some blood gets caught in the atria, because they are chambers and can fill, but most just drains on through (as can be seen in the ventricular volume trace in Figure 15.17, and is explained on the web page that describes that figure).

    Consider all the valves during the relaxed period of the heart.   The A-V valves are both open.  These only close under pressure.  The semilunar valves work the opposite way.  During the relaxed period of the heart, the semilunar valves are both closed.  These valves only open under pressure.  The pressure that causes the A-V valves to close and that causes the semilunar valves to open is generated by the contraction of the heart muscle.  I have tried to represent this in the following table:

    Now, let's start at the beginning of a cardiac cycle.  The blood has been entering the relaxed heart.  At some point, the atria begin to contract.  This is called atrial systole.  Actually, any contraction of heart chambers is called a systole.   This forces the last bit of blood from the atria into the ventricles.  But there is still no change in the valves.

    Shortly after atrial systole, ventricular systole begins.  With ventricular contraction, an increase in ventricular pressure occurs.   In order to understand this, we need to go back to an example of pressure.  Think about a garden hose.  You know how when you cover the end of it with your finger, the water sprays out farther?  That's because the same amount of water is pushing to get out of the hose whether your finger is covering the end or not.  If your finger is there, the end is smaller, yet all that water still has to exit.  So the water pushes out through the smaller hole, due to water pressure, but the pressure causes it to go faster so that it all still gets out in the same amount of time; this causes it to go farther.  You can also think about the balloon... if you blow up a balloon and tie it, it won't pop on its own.  But now if more pressure is applied, like by you sitting on it, it will pop.  When the ventricle starts to contract, the space within the ventricle gets smaller, and this squeezes the blood.  Therefore, there is an increase in blood pressure.

    As soon as the blood pressure starts to rise, even the tiniest bit, that causes the A-V valves to close.  You see, the way that the A-V valves are designed, when blood pushes up against them from inside the ventricles, they are forced shut.  In fact, because they are forced shut under pressure, they close like a door slamming.  Therefore, when you listen to the heart, you can hear them close.   The sound they make is the first sound of the doublet you hear.  People describe the heart as making a "lubb dupp, lubb dupp, lubb dupp, lubb dupp, etc." sound.  The closing of the A-V valves is the lubb.

    We left off with the ventricles beginning to contract.  As their pressure increases, not only are the A-V valves closed, but the semilunar valves are forced open.  Now blood starts to flow out of the ventricles, and into either the pulmonary trunk (from the R ventricle) or the aorta (from the L ventricle).

    So far, we have seen atrial systole, then ventricular systole.   Shortly after the ventricles have just started their contraction, the atria are beginning to relax.  This relaxation is called atrial diastole.   Any relaxation period of the heart chambers is referred to as a diastole.  Then, after much of the blood has left the ventricles, the ventricular diastole begins.  As the ventricles begin to relax, the semilunar valves shut.  When they are all the way relaxed again, the A-V valves re-open.

    When the semilunar valves shut, that makes the second sound... the "dupp."  Again, the shutting of valves is like the slamming of a door.   Only the shutting makes a sound.

    Your book uses this figure to discuss atrial systole and atrial diastole.  I have only included it here to point out a few specific details.

    First, notice how the internal volume of each atrium is smaller during systole than diastole.  This should help you imagine how the blood gets squeezed out into the ventricles.  Secondly, during ventricular systole, the atria start and continue relaxation.  In the bottom half of the diagram you can see how the ventricles are squirting while the atria are filling.  Finally, this simple schematic shows a little bit how the two types of valves differ enough for one to open under pressure, and the other to close under pressure.

    Once the ventricles are totally relaxed, there is a period where no contraction is occurring, and the only thing that is happening is that the heart is re-filling with blood for the next go-around.  This totally relaxed state is part of the diastole.

    So, a cardiac cycle can be described as systole followed by diastole.  To be precise, you have to pick one kind of systole or diastole.  So, you can say that a cardiac cycle is one ventricular systole followed by one ventricular diastole.  Or, less commonly used, you can say it is from one atrial systole to one atrial diastole.  Got it?

© 2011 STCC Foundation Press
written by Dawn A. Tamarkin, Ph.D.