- [Narrator] So here is a picture of the human heart. Now you notice, you see there's the reddish tinges. Those are muscle. But all of that off-whitish color you're seeing toward the top of it, that's all fat, believe it or not. Now a typical person is going to have fat around their heart. It's perfectly natural. So this is your heart and what it looks like if it were outside of your body. So we look at the heart as an artistic drawing here. What we've done is taken the heart and cut the front of it away. So you're looking inside from the front of the body. You have that interventricular septum. It's the structure in between the two chambers' lower heart. We then have three different layers of the heart, epicardium, myocardium, endocardium. Well endo, E-N-D-O, means inside. So endocardium, inside the heart. That's the layering that the blood will come into contact with. While the myocardium, myo meaning muscle, is the muscle of the heart. Almost the entire thickness of the wall you can see is muscle. That's your cardiac muscle. Then the epicardium. E-P-I, epi means outside or around. Epicardium is the ouside layer of the heart, helping it keep things closed and contained. So if you start with how the blood gets back to the heart, with the two vena cava. The superior vena cava and inferior vena cava. The superios vena cava drains blood from your head, your shoulders, your arms. While the inferior vena cava is going to drain blood from your legs and your torso. It enters into the chamber called the right atrium. From there, the blood goes down through this right atrioventricular valve, or right AV valve, into the right ventricle. Now that right AV valve, you might of heard of it as a tricuspid valve. It has two different names, either one works. So right AV valve or tricuspid valve. Once you're in the right ventricle, it's time to pump that blood out of the heart. But both the right atrium and right ventricle are oxygen-poor blood. If you're leaving the heart now, we're you going? Well you need to pick up oxygen. You're heading to the lungs. Through the pulmonary valve, or pulmonary semilunar valve, up through the pulmonary trunk and the right/left pulmonary artery. And that is not a typo, that is a blue artery. Because you remember, the arteries carry blood away from the heart. And this case, what we're carrying away just happens to be the low oxygen. It's going to the lungs. Once you reach the lungs, you pick up some oxygen. You come back in the left and the right pulmonary veins. And yes, these are going to be red veins. They enter back in the left atrium, which will then send the blood down through the left atrioventricular valve, which is anchored in place by the chordae tendineae. Those little strings or chords helps to hold the valve in place. And those chordae tendineae are anchored by the papillary muscles. Now I know I mentioned this over here in the left ventricle, but, the right ventricle has the same structures. The right ventricle still has the chordae tendineae, it still has the papillary muscles. But the difference between the right and left AV valves are how many flaps they have, how many openings. The right AV is known as the tricuspid, three flaps to the valve. While the left AV is called the bicuspid, meaning two flaps. Or maybe you heard of it as the mitral valve. There's a lot of different terminology of what it could be. So pick one, any one is fine. But left AV is usually the simplest to remember there. Once this left ventricle, with the oxygen-rich blood is ready to go, it pumps out the aortic semilunar. Up to the aorta and to the rest of the body. Either it goes up those three vessels up top, going to the head or neck and arms. Or it heads down the large aorta to the trunk and legs. So what's around this heart? We have the pericardium. This fibrous sack that's meant to hold the heart in and prevent it from over filling. We already said the three layers: epicardium, myocardium, endocardium. Remember the epicardium is the outside, mostly connective tissue. Myocardium is all that cardiac muscle. Endocardium is the epithelial, nice and smooth and thick. So how about these chambers? We've mentioned atrioventricles. Well we have two atria, a right and left atria. They're more towards the top, or superior, part of the heart. We'll have two ventricles. They're more powerful because the ventricles are what pump the blood out of the heart. We have four valves. Valves are meant to prevent backflow. We have the right and left atrioventricular valves, which if you notice I've put the other names up here. So the right atrioventricular can become also the tricuspid valve. Either of them works. The left atrioventricular is also known as the bicuspid valve, or the mitral valve. And then we have these two semilunar valves. The pulmonary valve and the aortic valve. They're named for where the blood is going. Pulmonary valve is going to the pulmonary trunk and to the lungs, which are referred to as your pulmonary circuit. While the aortic valve allows blood to pass through into the aorta. Hence the name, aortic valve. So here are these two circuits. I said that whole thing is one closed process you can't change. The two circuits are the systemic and pulmonary circuits. Lets look in the middle in that lightly shaded box. You can the heart, you can see both lungs. When the blood leaves the right side of the heart, the right ventricle, it pumps out the pulmonary trunk and pulmonary arteries into the lungs. It goes through gas exchange. It comes back into the lungs via the pulmonary veins into the left atria. That circuit going out of the heart and back to the heart is called the pulmonary circuit. But once that nice oxygen-rich blood is picked up it goes down to that left ventricle, which is going to pump it out either up to your head and arms or down to your torso and legs. But either way, it's one complete circuit. Now this pulmonary circuit is going to have several different steps. But it must start with the blood getting back to the heart. This is the oxygen-poor blood. So its coming back in to the superior and inferior vena cava into the right atrium. For the right atrium, it goes to that right AV valve into the right ventricle. The right ventricle is going to pump through the pulmonary semilunar valve into the pulmonary trunks. The pulmonary trunk with the pulmonary arteries head to the lungs. From the lungs, you come back in with oxygen-rich blood through the pulmonary veins into the left atrium. Through the left atrioventricular valve to the left ventricle. You've now completed your pulmonary circuit. Taking that oxygen-poor blood when it entered the heart, moving to the lungs, picking up oxygen, dropping of carbon dioxide. Going back to the heart in the left side now, getting down to the left ventricle. Well once you leave left ventricle, now you're in the systemic circuit. It's how you get oxygen delivered to the rest of your body. They travel from the left ventricle through the aortic semilunar valve, to the aorta. From here, the aorta starts branching and branching into arteries and arterioles. The further you get down the branching, the smaller the vessels get. Once you pass the arterioles, you're in the capillaries. The capillaries of your body's tissues. The capillaries then start to return the now oxygen-poor blood back into venules and eventually back into veins. Because once this oxygen-rich blood goes into the capillaries, you have gas exchange. The capillaries will allow oxygen to diffuse out into the tissues. It'll also allow carbon dioxide to come back into the blood. So we've started this return trip, venules to to veins, eventually the vena cava, into the right atrium. Well, we're back. That's it. From leaving the heart to returning to the heart. Now it's time for that blood to go into the pulmonary circuit again. So here's just a little pictorial view of what is going on. As you follow it through, the blue is trying to show oxygen-poor. The red is oxygen-rich. You can notice that once you come in through the superior/inferior vena cava, you go right atrium, right ventricle, pulmonary trunk, capillaries, the lungs. It comes back into the left atria, the left ventricle, out the aorta and into the systemic capillaries of the body. So one big continuous circle. The blood should never stop moving. It should constantly keep moving throughout this process.