-To continue dealing with the heart we have to talk about the cardiac cycle. How the heart contracts and relaxes. Because your heart never really takes a break. But it must take a short break. So even though you're heart is beating 60, 70, maybe even 80 times a minute, you still need to make sure that the heart beats, that the muscles can relax for a moment. Not long, but a very quick relaxation. So we'll start with the atrial systole. Systole is a term referred to when the muscle of the heart is contracting. So in atrial systole, both atria contract pushing blood down into the ventricles, helping to fill the ventricles up. Well if blood's going from the atria down to the ventricle, the AV valves must be open. Now both the pulmonary and aortic semilunar valves are closed. The blood is not leaving the heart yet. It's just accumulating inside the ventricle. In ventricular systole, well that means we have contractions. Both ventricles contract. Now the ventricles contract, they're trying to generate force to push the blood out of the heart. Well they're pushing it out of the heart that means the aortic and pulmonary semilunar are opened, allowing the blood to leave. But the atrial ventricular, or the AV valves, are closed. They're preventing the backflow of blood back up into the atria. Diastole, that's relaxation. Both you'll have atrial diastole and ventricular diastole. When this occurs, the semilunar valves are closed. All the ventricles and all the atria are relaxed, slowly, passively filling with blood. So here we show the movement of blood to the heart. First off, we'll start with systole, that's contraction. You notice it only starts in about point one seconds. So there's not very much time during atrial systole. So this is when the atria contract, pushing blood into the ventricle. Then we go into ventricular systole. This lasts about point three seconds, or so. At this point, the ventricles are contracting, pushing blood out to the semilunar valves, out through the aortic and pulmonary trunk. As you can see both the right and left AV valves are closed, preventing backflow. Then we end up with diastole, the relaxation. Both atria and ventricles relax allowing for the blood to flow passively down in the ventricles. You have to say, hold on... Point one plus point three plus point four, that only equals point eight seconds. That's as long as normal heart beats. Because the average heart beat's up in the 70s somewhere. Some people are in the 80s, some people are in the 60s. Sure, no problem. But if you average everyone out, it's about in the 70s. So that means that point eight seconds is how long each one of those beats would take to get into the 70s per minute. I'm sure you've all heard of this lub-dub, lub-dub, lub-dub. It's the heart sound. It's the sound of the heart closing the heart valves. The first sound, the quote lub sound is when both the AV valves close. Now these AV valves are going to close when the ventricles are contracting. When the ventricles are in systole. Well the second sound, the dub sound is when both semilunar valves close. Both the pulmonary and aortic snap shut. That can only happen once the ventricles stop contracting. So this lub-dub sound is the AV opening and the closing. The dub is the semilunar closing. So it's constant back and forth. Which valve is open? Which valve is closed? So the first ones to close are the AV, making sure the blood moves forward. The second ones to close are the semilunar, preventing blood from coming back into the heart. But sometimes we end up with problems, heart murmurs. Now heart murmurs are caused when the blood flow is disrupted. It could be disturbed somehow, it might be a defective valve. It might be the valves are not formed properly. There might not be a lining. They might not be closed... You get the idea. Something is wrong with the valve. So if blood flow is being disturbed, it's being disrupted, it's creating an extra sound. That is your heart murmur So how does the heart beat? Well we have this whole entire conduction system. Your heart has it's own intrinsic, or within, electrical system. The first piece of this five part puzzle is the SA node. The SA node is your cardiac pacemaker. It's what starts the heartbeat. What's kind of weird is this works by itself. So you can actually be holding a heart that was just removed from an organism. That heart would keep beating for a short time. The SA node just keeps firing. It eventually stopped beating because of a lack of nutrients, a lack of particles to feed it. The AV node? Well, the AV node is simply a relay station. It's meant to take the signal from the SA node and pass it down the line to the AV bundle. Now the AV bundle is going to be receiving the signal and carrying it down to the ventricles, the Purkinje fibers. So we have our SA node, AV node, AV bundle and Purkinje fibers. But you're thinking, hold on. He said five. There is a fifth one. It goes up where the AV bundle is, as well. So between the AV node and the Purkinje fibers you actually have two different components that help to relay the information. So here we go. We start with the SA node, the sinoatrial node. Send signals to both atria. Then the AV node, ventricles carry the information down to the AV bundle. Then here's the new one, bundle branches. So the AV node, the bundle branches are carrying information down towards the ventricles where the Purkinje fibers are going to contract. Purkinje fibers are a stimulating contraction of the muscle to cause the blood to leave the ventricles. To be able to look at these electrical impulses we look at an electrocardiogram. An electrocardiogram, or ECG, a lot of times more commonly known as an EKG, is meant to track the electrical activity of the heart. To track when the SA node fires and caused a stimulation, a contraction. Now the average healthy individual has a pretty typical pattern to their ECG. They have three main features called a P wave, QRS complex, and T wave. The P wave is when the atria are being stimulated, the atria are contracting. QRS complex, now that's talking about when the ventricles are being stimulated, when the ventricles are contracting. And the T wave is the end of electrical activity. It's the point where the heart needs to rest momentarily, before the next P wave occurs. Now these EKGs can basically detect a few different problems; things like arrhythmias, ventricular fibrillation. So you can tell what's going on and give the medical personnel a better idea of how to help the individual. So here's how an EKG runs. You can see one up top. A lot of times you have all these different wires all over the heart, as well on the arms and the legs trying to measure an electrical signal. Letter B you can see is the normal ECG. You have a little P wave, then a QRS spike, and a little T wave. Another P wave, QRS and T wave. Now that's typical. But the one on the bottom, we've got a little problem here. That's called ventricular fibrillation. If you notice, it does not come close to matching the normal one. In ventricular fibrillation, the ventricles are just rapidly contracting. On, off. On, off. On, off. In no real pattern. In ventricular fibrillation that's really dangerous. You better get yourself to a hospital very quickly. Otherwise this could potentially kill you. Because the heart, the ventricles specifically, are beating erratically. The blood is not adequately being sent to the rest of the body, impairing function of all the rest of the organs. So normal EKG, those typical blips you see there. But one example of a problem is ventricular fibrillation.