Now internally, there are various structures of the heart that you should be aware of. First of all, we have what are called septa. Septa are the walls between the left and right chambers of the heart.
We have two septa. We have the interventricular septum and the interatrial septum. Now, the interatrial septum is the wall between the left and right atria, of course.
Interatrial septum between the atria. The interventricular septum is the wall between the left and right ventricle. Interventricular septum between the ventricles.
Four heart valves are also located in the heart internally. The function of the heart valves are to ensure proper blood flow through the heart and out of the heart. So these heart valves, think of them as one-way valves. They are going to make sure the blood flows in one direction, the proper direction, and not backwards.
So we have up here a heart valve between the right atrium and right ventricle. This is called either the right atrioventricular valve, also known as the tricuspid, and we have a heart valve between the left atrium and the left ventricle, known as the left atrioventricular valve, also known as a bicuspid valve or mitral valve. We have a valve here between the right ventricle and the pulmonary trunk. This is the pulmonary semilunar valve. And we have a valve here that you can only see just a little bit of.
This is a valve between the left ventricle and the aorta. It is the aortic valve. So if we talk about these internal heart structures by following the flow of blood, if we start with blood entering the heart from the systemic circuit, again blood from the systemic circuit enters the heart in the right atrium.
Besides the openings here from the superior and inferior vena cava, we also have this opening here called the coronary sinus. The coronary sinus is where blood coming from the coronary circulation enters the heart. We have this structure right here, this oval shaped patch of scar tissue is called the fossa ovalis.
The big parts here, the flaps of the tricuspid or right AV valve are known as cusps. So when the cusps are closed, they're all pushed up against each other and sealed so blood cannot go backwards from ventricle to atrium. But when the atrium contracts, the way the flaps are oriented is they can be pushed open and blood can push its way down from the atrium into the ventricle. So here is the right AV valve or tricuspid.
The flaps are called cusps. And these little long thin strands are called chordae tendineae. Chordae tendineae, they're like cords, right? The cords come from the cusps and go down and connect to these muscles here called papillary muscles.
Papilla means nipple. So these muscles are nipple shaped and this is what connects to the chordae tendineae. so that when the muscle contracts it pulls on the corti tendineae to make sure that the cusps of the valve don't flip up or backwards up into the atrium.
It holds them down so they don't get damaged. So through the valve into the right ventricle and then right ventricle contracts to start to force the blood out of the right ventricle. So as you can see here in the right ventricle, the blood flow gets pushed up this way. So this area right here that you see in the right ventricle is called the conus arteriosus.
It's this cone-shaped region of the right ventricle and it's where blood exits that right ventricle as it goes towards the pulmonary trunk. Once the blood goes through the pulmonary valve, Into the pulmonary trunk we begin the pulmonary circuit. At the end of the pulmonary trunk it splits into the pulmonary arteries which will deliver blood to each lung. Eventually, when the blood is returning from the lungs, it will return back to the heart through the pulmonary veins.
And from the pulmonary veins, that blood will then enter the left atrium. When the left atrium contracts, it forces blood through the mitral valve, also known as the bicuspid. or left atrioventricular valve, down into the left ventricle. Notice that also the mitral bicuspid or left AV valve, three names for the same valve, it also has cusps and corti tendineae that are connected to papillary muscles here to prevent them from prolapsing or going up, flipping up into the atrium. Now in both ventricles, the way the muscle is arranged.
So we have the papillary muscle, right? That's the part of the muscle here that looks like nipples, that's what we call it papilla. But then the other way that the muscle looks, it looks sort of like a honeycomb or a cave system, okay? It's the way the muscles are arranged on the inside of the ventricles. This muscle arrangement are what are known as trabeculae carnii.
Trabeculae carnii. trabecula in bones, or in the part of the spongy bone, looks like these little protrusions or ridges. So trabeculae in muscle, in the ventricles, are also these little protrusions or bands of muscle that you see here. So it's called trabeculae carnii. There's a special area of muscle right here that's only found in the right ventricle called a moderator band.
The left ventricle does not have a moderator band, just the right ventricle. And what happens is when the electrical signals are coming down the ventricles to stimulate the ventricles to contract, in the right ventricle that electrical activity goes through the moderator band right to the papillary muscle to get the papillary muscle to begin contracting. before the rest of the ventricle contracts so that we don't bend the cusps of the valve here upward into the atrium. That is not needed for the left ventricle, this is only a feature of the right ventricle, the moderator band.
Now once the left ventricle contracts, it will force blood up through the aortic valve up into the aorta. When the blood enters the aorta, It will go through the first portion of the aorta called the ascending aorta, then through the aortic arch, and then as it turns downward and goes behind the heart, down towards the rest of the body, that is the descending aorta. Along the way you will see many arteries, many large major arteries that branch off the aorta as it makes its way down the body. This view here is a real heart of course that's been sliced open so you can see what it looks like in real life and you can if you'd like you can compare them with the diagram here.
So we've zoomed in so we can look at heart valves we see you can't really see the cusps all that well that's up here but in this picture we can see the corti tendineae and we can see the papillary that it's anchored into. When you look at the atria, the right and left atria basically look the same. They have the same structure.
They have the same function. They both fill with the same amount of blood and they both, when they contract, have to squeeze the blood from them down into their respective ventricles. So they both are moving the same amount of blood in the same direction. for the same distance, atrium down to ventricles.
So their anatomy is very similar. Now, the right and left ventricles, on the other hand, they will hold the same amount of blood, and they have to pump the same amount of blood, but the right ventricle only has to create enough pressure when it contracts to move the blood from the heart to the lungs and back. The left ventricle, on the other hand, has to move that same amount of blood that the right ventricle does, but it has to move it to the rest of the body and back. So because of that, the ventricles do hold and pump the same amount of blood, But the left ventricle has to pump it a lot farther.
So the left ventricle has to be able to create about five times the amount of pressure that the right ventricle does to be able to move the blood a much longer distance through the body. We can see that reflected in its anatomy. You can see the wall of the right ventricle right here.
You see that it is thinner compared to the wall of the left ventricle. And also the way that the ventricles contract are a little bit different. So if we get a superior view of the heart, we sliced off the atria and we're looking down into the ventricles, right ventricle, left ventricle. During ventricular contraction, the right ventricle, when that contracts, it just contracts medially, medially only. This right ventricle, the wall of it contracts in a way that pushes it towards the left ventricle.
So that creates enough pressure to move the blood out. efficiently to the lungs and back. Now when the left ventricle contracts, the entire wall, the whole wall of the left ventricle, that contracts evenly like you're squeezing a water bottle. So the right ventricle just contracts this way towards the left ventricle, but the entire left ventricle contracts like you're squeezing a water bottle evenly on all sides and the left ventricle shortens.
The bottom of the left ventricle contracts upward. So you get squeezing from all sides on the left ventricle with thicker muscle. That creates about five times more pressure compared to the right ventricle.