Valves and the Electrical System – Anatomy of the Heart

00:00 So, here are the four heart valves. The ones in blue are the right sided ones of course, and the ones in red are the left sided ones. Let’s take the one at top of the diagram because that’s the front of the heart and that’s the tricuspid valve. Tricuspid means three cusps, three components and you can see that, right? You can see that there are three parts to the tricuspid valve. So, the blood is now passing through the tricuspid valve into the right ventricle and then there is going to be ven-… right ventricular contraction, squeeze and the blood is going out the pulmonary artery. Look at the blue valve to the left- that’s the pulmonary artery valve and you will notice it’s open because the right ventricle is squeezing blood through it. That blood goes to the lungs as we talked about, gets oxygenated, get… picks up oxygen, comes back through the pulmonary veins to the left atrium and then passes through the valve- the red circled valve on the right hand side of the diagram. Notice that valve only has two cusps. So, it is the only valve with two cusps. The other three valves have three cusps - the tricuspid valve, the pulmona-… pulmonic or pulmonary valve, the aortic valve, all have three cusps. But the mitral valve, which is between the left atrium and the left ventricle, has only two cusps. In fact, it resembles its name for the Bishop’s mitre which is the crown that the Bishop wears in the Catholic Church which basically has just two sides to it, resembling a little bit the mitral valve.

01:36 Once the blood is in the left ventricle, the left ventricle contracts and the blood goes out the aortic valve- that’s the one you see right in the center and you will notice it’s also open. So, what we are seeing here is right and left ventricular systole- that is squeeze, systole is squeeze and you see that the pulmonary valve, pulmonic valve and the aortic valve are open and blood is flowing respectively into the pulmonary artery through the pulmonic valve and into the aorta through the aortic valve.

02:05 Now, I mentioned before that the heart has an electrical system. Indeed it’s the electrical system which is the trigger for mechanical contraction. Without the electrical system, the heart muscle will not contract. It’s each… heart cell responds to the electrical activity by contracting. So, where does this electrical activity start? It starts at the top of the atrium, there is a pacemaker- an automatic pacemaker which can be influenced by adrenaline circulating or by nerves from the brain, it can accelerate or it can decelerate depending upon a variety of conditions which we will be talking about later. But, in any case, the impulse starts spontaneously, passes through a number of little fibers in the atrium into what you see that little bulb there, that’s the AV node, it pauses there for a little bit. Why does it pause? Of course, you can’t have the atria and the ventricle contracting at the same time. And if the impulse traveled rapidly through, you would have them contracting at the same time and the blood wouldn’t be going anywhere. So, there is a certain pause while the atria finish their mechanical contraction, and then the electrical activity passes down from that AV node, also called the Bundle of His, down into the branches that are in the ventricle and at that point, the ventricular muscle contracts. You can see below it a… a diagram of the electrocardiogram. The first wave is called the P wave- that’s atrial contraction and the big deflection is called the QRS- that’s the ventricular contraction and you can even see the heart sounds in there with atrial contraction and ventricular contraction, you can see the first and second heart sounds. In fact, we record the electrical impulse passing through the heart with something called the electrocardiogram.

03:56 I think most of you are well aware of this. I am going to talk a lot more about this when we come to the diagnostic lecture, but in any case, here is a diagram that shows you six electrocardiographic leads, they are taken in the frontal plane, that is this plane right here, but they are taken from different angles. They are a little electrical biopsy from different angles around the heart. And we put all of this information together, as we will talk about later, it helps us to diagnose specific forms of heart disease, it also tells us a lot about how well the wave of depolarization is passing through the heart- are there abnormalities in the electrical conduction system? Or are there abnormalities in some of the minerals in the blood, for example, potassium?