 Now, we talk about the heart, the pumping organ. Heart of human beings is four chambered as all the mammals do have. It have two upper chambers called atria and two lower chambers called the ventricles. Heart of a human being is the size of a clenched fist. If you make it like this, it's of that size. All of these four chambers have the lumens. Between the atria and the ventricles, there are valves which are separating the atria with ventricles with atria. And we know that from the vessels that heart is giving away one large vessel, one large artery, the aorta. And the other one is the inferior vena keva and superior vena keva which it is receiving from the body. Heart have two types of valves. One group is called the semi-lunar valves which are present in the pulmonary artery and the pulmonary vein which actually separate the pulmonary artery and the aorta with the chambers of the heart. And the other group is called the atrioventricular valves, the valves which are present between the atria and the ventricles. So, we call them atrioventricular. Now, there is a right side of the heart and there is a left side of the heart. The right side of the heart actually receives the blood from the body from the vena keva and then send it towards the lungs through the pulmonary artery to let it oxygenate. The left side of the heart on the other hand receives blood from oxygenated blood from the lungs and then send it to the aorta. Now, because we know that lungs are close to the heart and the other body tissues are far away. So, the left side of the heart which have to send blood towards the body is more strong. Its valves are more thick in comparison to the right side because right side have to receive blood from the body and it have to send it towards the lungs which are close. So, the left part of the heart is more stronger. This is the reason that we normally feel the heart is present more on the left side because left side is more strong and it is more strongly contracting all the time. Let us have a look on the structure of human heart. Look at the diagram. On the right side for the ease of understanding, the blood is shown in blue because it is deoxygenated. On the left side, the blood is shown in red because it is oxygenated. The upper chambers are called atria, right atrium and the left atrium. The lower chambers are the ventricles, right ventricle and the left ventricle. You can observe that the wall of the left ventricle is more thick in comparison to the wall of the right ventricle. Now, we look at the vessels which are, first of all we look at the veins which are giving, which are taking blood from the body and returning to the heart. In the blue, on the right side, you can see inferior vena keva and superior vena keva. Both are returning blood and they are opening inside the right atrium. They open into the right atrium and give blood to the right atrium. Then on the other side, look at the left atrium. That left atrium is receiving blood from by the pulmonary vein. So, pulmonary vein is actually opening inside the left atrium. It is giving oxygenated blood from the lungs to the left atrium. Left and right atria contract at the same time. It means that both atria are filled at the same time. The right atrium is filled by deoxygenated blood from the inferior and superior vena keva. But the left atrium is filled with the blood coming from the lungs to pulmonary vein. Then both of the atria contract at the same time. When they contract, the atrioventricular valves, they open and they allow the flow of blood towards ventricles. On the right side, you can see the atrioventricular valve. This is called a tricuspid valve because it has three flaps. But on the left side, you can see a valve between the left atrium and the left ventricle is called a bicuspid valve because it has two flaps. So, when the atria are filled with blood, they contract and they release their blood inside the ventricles because of opening of the atrioventricular valves. The tricuspid valve and the bicuspid valve. When the ventricle is filled, ventricle contracts and both ventricles, again just like the atria, both ventricles contract at the same time. Left ventricle, as you can see in the diagram, is connected to the aorta, the aortic arch. Before the aortic arch, the major vessel which is called aorta. So, left ventricle pushes the blood with pressure into the aorta. And you can see there is a valve between aorta and the left ventricle. This valve is called a semi-lunar valve. When the ventricles contract, then the AV valves, the atrioventricular valves, both, they are closed because as I said, both of the ventricles will contract at the same time. And both of the AV valves, they are closed. When they close, they produce a sound. And this sound we call a lub sound. You see, the hard sound is lub-dub, lub-dub, lub-dub. The lub sound is actually the sound of closing of this valve. And then the dub sound is the result of the closing of the valve of the semi-lunar valve, the other valves. So, this is the reason that when your doctor want to assess that whether your heart valves are working correctly or not, they listen through a stethoscope to the sounds of your heart. And then they guess that due to the sounds that your valves are correctly working or maybe there is a problem in the valves. Now, we come back to the right atrium. As you can see in the right ventricle that when the ventricle is filled just like the left ventricle, ventricles at the same time contract. Through right ventricle, blood goes towards the pulmonary artery. That is, this is the artery which is taking blood from the, taking this deoxygenated blood from the ventricle, right ventricle towards the lungs. And in the lungs, this blood is oxygenated and then again it is returned back to the heart through the pulmonary vein. Now, actually the right atria, so I just simply repeat the whole process that is, the right atrium is filled by the inferior and superior vena keva. And the left atrium is filled by the blood coming through the pulmonary vein from the lungs. In the right atrium, there is a deoxygenated blood, in the left atrium there is oxygenated blood. And then both atria contract at the same time, the atria ventricular AV valves are open and blood goes to the ventricles. Then the ventricles contract, when they contract then the AV valves are closed. And blood and the semilunar valves which are present, one in the aorta and the other in the pulmonary vein, they are open. And blood moves or it is thrown towards the aorta from the left ventricle and towards the pulmonary vein from the right ventricle. And when the ventricles relax again, then the semilunar valves they are closed. This prevents actually back flow of the blood towards the ventricles. Now, we just described the structure of the heart. We go ahead to talk about the heart beat and the cardiac cycle. The cardiac cycle or the heart beat, cardiac cycle is actually a cycle that starts from filling of the atria, then filling of the ventricles and contraction of the ventricles to push the blood towards the lungs or the body. This whole process from the filling to still sending is called a cardiac cycle. During cardiac cycle, when the atria are filling, then all the muscles of the heart are relaxed. Atria are not contracting, ventricles are also not contracting. We call this situation a diastole. Then first of all, when atria are filled, they contract. We call it atrial system, that is atria contracting. Then ventricles are filled due to atrial contraction. We call it ventricular systole. And atrial systole is of less power, is less strong in comparison to the ventricular systole because ventricles have to send blood towards far away tissues or towards lungs. But atria just have to send blood to the ventricles, which are just very close, just attached to each other. We look at a diagram to make the cardiac cycle more clear. Step one, atrial and ventricular diastole. You can see all the muscles are relaxed. And in this condition, at this time, the airy walls, they are open. And the semi-lunar walls, they are closed. And you can see that this condition, and this is the time when the atria are filling. When the atria are filling, blood is returning from the body and the oxygenated blood is coming back from the lungs in the atrium, in the left atrium. This is called diastole. This lasts for 0.4 seconds. Then atria are filled. And when atria are filled, they contract. This is called atrial systole. And this is ventricular diastole because the ventricles are still not contracting. They are relaxed. This stage lasts for only 0.1 seconds. It means that atria contract for only 0.1 seconds. Then the ventricles are filled with blood due to atrial contraction. Then the ventricles contract, as you can see in the third part. This is called ventricular systole. And this is atrial diastole because atria are now relaxed. Ventricles, they are now contracting. During this time, blood is pushed from the ventricles towards the aorta. And blood is pushed from the left ventricle to the aorta and from the right ventricle to the pulmonary vein. At this time, as you can see in the diagram that AV valves, tricuspid and bicuspid valves are closed. In the semilunar valves, they are open in the aorta and in the pulmonary vein. This lasts for about 0.3 seconds. And then all of the chambers of the heart, they are again relaxed and come back to the diastole. This whole process is called a cardiac cycle. And this cardiac cycle makes one heartbeat. We observe one heartbeat. One heartbeat actually consists of this whole cycle. And we know that human heart contracts for about 72 beats a minute. Normally, when we are exercising or we are doing some other activity, this may increase or sometimes may decrease. But mostly, it is 72 times per minute. So, this was about the cardiac cycle. Now, we look at the electrical activity of the heart, which is behind all this contraction and relaxation. Let us have a look on a diagram. We may have heard of a pacemaker. It is a pacemaker in the heart. If it gets damaged, an electrical organ is placed in it, which is called an artificial pacemaker. Look at this diagram. In yellow colour, it is showing the pacemaker in its connections. Heart. Heart muscles are involuntary. These are not in our conscious control. These are automatically contracting and relaxing as it is required by the body. This is due to electrical system present inside the heart. Heart has certain nerves. We can say nervous system, which is shown in yellow in this diagram. On the top of the right atrium, where it is connected to the venakiva, is close to the venakiva, there is a thick group of muscles, which is called sinoatrial node. We also call it a pacemaker. This is a group of muscles, which actually can generate an impulse or electrical activity itself. When it generates an impulse, then this impulse passes towards the oracles, and this impulse is generated by the pressure of blood from the venakiva towards the oracles. This impulse is generated, spread through the oracles and the oracles they contract. Actually, oracles is the other name for atria. So the atria contract. Then slowly, this impulse come or this electrical activity comes down towards another bundle, which is called an AV bundle. That is atrio-ventricular bundle. Here it is, we can say accelerated. Its magnitude is increased and then it moves towards the ventricles. When its magnitude is increased, when it reaches the ventricles, it creates a more strong impulse and more strong contraction. Due to this reason, heart always continue to contract and then relax. So this is the pacemaker. The sinoatrial node is also called sometimes the pacemaker. When it generates impulse, it spreads through the atria and then its magnitude is increased and then it enters the ventricles and contracts the ventricles as well. So this was about the electrical activity of the heart.