Now, there are several ways to describe the anatomical position of the heart. The heart is located in a midspace or partition between the two pleural cavities surrounding our lungs. This compartment between the two pleural or lung cavities is known as the mediastinum. or mediastinum. The heart is also situated above or superior to the sheet-like breathing muscle known as the diaphragm.
Now two-thirds of the heart's mass lies left of the midline of the body. The midline runs through the middle of the breastbone or sternum. Therefore, the midline of the body is also known as the mid-sternal line.
Therefore, the bone that covers and protects the anterior aspect of the heart is the sternum or the breastbone. And the bone that covers and protects the posterior aspect of the heart is known as the vertebral column. So we can also describe the heart as being behind or posterior to the protective sternum.
We can also describe the heart as being anterior or in front of the vertebral column. Now, the point of maximal intensity, also known as the PMI, is a particular point on the chest wall where the beating or contraction of the heart is. best heard and best felt because this is where the heart beats closest to the anterior surface of the chest wall. Now pinpointing the location of the PMI is often of important diagnostic value, particularly in physical cardio exams. Auscultating or listening to heart sounds at the PMI can potentially identify abnormal heart sounds known as murmurs.
The identification or auscultation of heart murmurs at the PMI can potentially indicate underlying heart disorders or abnormal functioning of heart structures such as valves. There are two major anatomical regions to the heart. Now, the broad superior end of the heart that is attached to major blood vessels, such as the aorta, the superior vena cava, the pulmonary arteries, this is known as the heart's base, which constitutes the widest or broadest part of the heart. Now, the other major anatomical region of the heart is termed its apex. The apex is the narrow inferior end of the heart that projects into the left lung.
The apex of any anatomical structure or organ constitutes its narrowest point. Going from interior to exterior, or deep to superficial. Let's walk through the various layers of the heart wall.
We will start with the layer that is immediately adjacent to the heart chamber, which periodically fills and empties with blood, known as the endocardium. Endo translates to in or inner. So this layer constitutes the innermost layer of the heart wall.
It is comprised of a specialized type of tissue known as endothelium. Endothelium is simple squamous epithelium that lines the internal surfaces of blood vessels such as arteries, veins, capillaries, as well as lymphatic vessels, and as we see, the heart chamber itself. Endothelium can be found in these three locations, blood vessels, lymphatic vessels, and the heart itself. There is also underlying connective tissue associated with this simple squamous epithelium, and that connective tissue type happens to be the packaging tissue known as loose areolar connective tissue. This is the type of connective tissue that underlies epithelial tissue.
Loose areolar connective tissue can be found distributed underneath. epithelial tissue. Now making our way externally, the next layer of the heart wall we arrive at is the myocardium. Myo itself translates to muscle.
So this is indeed the muscular layer of the heart wall that is loaded with cardiac muscle cells. So the myocardium contains a high concentration of cardiac muscle cells. The next layer over is the surface of the heart itself, which is covered by the visceral layer of the serous pericardium. The visceral layer is one member of the serous pericardium.
The other member of the serous pericardium is known as the parietal layer. Together, The parietal and the visceral layer of the serous pericardium make up or comprise the dual or double layered membrane known as the serous pericardium. The parietal layer of the serous pericardium lines the wall of the heart cavity, while the visceral layer of the serous pericardium directly covers the surface of the heart organ. Found situated between the visceral and the parietal layer of the serous pericardium is the fluid-filled pericardial cavity, which contains lubricating pericardial fluid to absorb the shock and impact of the visceral and parietal layers gliding against one another during the process of contraction.
So by definition, a serous membrane is always comprised of two layers or two members, an inner visceral member and an outer parietal member. There is an alternate term that we can assign to the visceral layer of the serous pericardium, and that term is epicardium. The visceral layer directly encloses and surrounds the heart organ itself, hence the term epi. which denotes around or upon. Since the term viscera is derived from organ, the visceral layer of the serous pericardium will always be intimately associated with the organ itself, and the parietal layer of the serous pericardium will always line the wall of the pericardial cavity.
Moving one layer past the the parietal layer of the serous pericardium, we finally arrive at the fibrous pericardium, which constitutes the outermost heart wall layer known as the fibrous pericardium, which is comprised of dense connected tissue and acts as an additional covering beyond the serous pericardium. Here we have a visual display. of the fibrous pericardium.
The purpose of the fibrous pericardium is to prevent excessive increases in the size or enlargement of the heart. Bear in mind, this outermost fibrous pericardium is not a part of the serous pericardium. The serous pericardium, as we established earlier, is exclusively comprised of only the two layers or members known as the visceral and parietal pericardium.
The visceral layer of the serous pericardium can be simply abbreviated as visceral pericardium. The parietal layer of the serous pericardium can simply be abbreviated as the parietal pericardium. Again going from interior to exterior, the layers of the heart wall will always appear in the same sequence.
Again, immediately adjacent to the heart chamber is the innermost layer known as the endocardium. External to the endocardium is the very muscular myocardium responsible for the muscular contractions of the heart. Directly enclosing or surrounding the surface of the heart organ itself is the visceral pericardium, a.k.a. the epicardium. External to the visceral pericardium or epicardium is the fluid-filled pericardial cavity. External to the pericardial cavity is the parietal pericardium.
And finally, representing the most outer structure, the outermost structure of all, the dense connective tissue-filled fibrous pericardium. This brings us to the first checkpoint question of this lecture recording. Now there are four chambers in total that comprise the heart. Two smaller upper chambers known as the two atria. The heart is also comprised of two larger lower chambers known as the two ventricles.
The two upper atria are dubbed the receiving chambers of the heart. as they are responsible for receiving blood from the pulmonary as well as the systemic circulation. The pulmonary circulation corresponds to the blood coming from the lungs, and the systemic circulation corresponds to blood from the entire body.
Now the right atrium of the heart. receives blood from the systemic circulation, whereas the left atrium of the heart receives blood from the pulmonary circulation. Going forward, RA will represent the right atrium, and the abbreviation LA will represent the left atrium. These two atria are separated by a wall or partition known as the inter...
atrial septa. In the left-hand image, we have a frontal or coronal section of the heart. This is the view along which the four chambers are visible. In the figure on the right-hand side, we have an anterior view of the heart.
We can see here that the two atria are covered anteriorly by an ear-shaped appendage. This ear-shaped appendage is known as an oracle. So this is the right oracle or the oracle of the right atrium.
This other ear-shaped appendage is the left oracle or also known as the oracle of the left atrium. The function of these oracles is to increase the volume of blood that each of the atria can hold. This brings us to the two lower chambers of the heart, known as the right ventricle and the left ventricle, which going forward can be abbreviated as RV and LV, respectively.
The two lower ventricles are often described as the discharging or ejecting chambers. This is because the ventricles are responsible for ejecting or sending blood, discharging blood, into the pulmonary and systemic circulation. Now specifically, the right ventricle is responsible for discharging. or expelling blood into the pulmonary circulation of the lungs, whereas the left ventricle is responsible for ejecting or sending blood into the systemic circulation of the entire body. We can also see that the two ventricles are also separated by a wall or partition known as the interventricular septum.
So septum translates to wall. or partition. Now on the anterior surface of the heart, there are external anatomical landmarks that provide a rough guide to the location of underlying cardiac structures. For instance, the groove known as the anterior interventricular sulcus provides an external guide to the deeper underlying interventricular septum.
So a surgical incision through the anterior interventricular sulcus or groove would effectively sever the right ventricle from the left ventricle. There is an additional groove on the surface of the heart known as the coronary sulcus, which marks the separation between the upper atrium and the lower ventricle. Here we have what is known as the right coronary sulcus, which is a groove situated between the upper right atrium and the lower right ventricle. There's also a corresponding left coronary sulcus on the remaining left side of the heart. This left coronary sulcus is a groove that marks the separation between the upper left atrium and the lower left ventricle.
Before we trace the pathway of blood through the heart, let's familiarize ourselves with the respective structures of the heart and their corresponding abbreviations. We first have what is known as the superior vena cava, abbreviated as SBC. This is a large prominent vein that conducts oxygen-poor or deoxygenated blood used by the upper body and empties it into the right atrium of the heart. The inferior vena cava is also another large prominent vein that conducts oxygen-poor or deoxygenated blood used by the upper body and empties it into the right atrium of the heart. deoxygenated or oxygen-poor blood used by the lower body and empties those oxygen-poor blood contents into the right atrium as well.
Going forward, we will also be using blue to denote deoxygenated or oxygen-poor blood. We will also use red to denote oxygen-rich or well-oxygenated blood. There are valves within the heart itself that separate the upper atria from the lower ventricles.
On the right side of the heart, we have what is known as the tricuspid valve, which going forward we will abbreviate as TV. This valve separates the upper right atrium from the lower right ventricle. On the left side of the heart, we have what is known as the mitral valve, which we will abbreviate as MV. Now, the mitral valve is also alternatively known as the bicuspid valve. These valves are named after the number of cusps.
The mitral valve, or the bicuspid valve, separates the upper left atrium from the lower left ventral. Now a memory device or mnemonic to help us recall and recollect that the tricuspid valve separates RA from RV and the bicuspid or mitral valve separates LA from LV is the mnemonic going from right to left, tri before you bi. Now because the tricuspid and the mitral valves mark the separation between the upper atria. and the lower ventricles, they are classified as atrioventricular valves.
Now we have another set of valves that actually separates the lower ventricles from the major blood vessels branching or emerging from the heart. Those valves are known as the semilunar valves, of which there are two types. The pulmonary semilunar valve, and the aortic semilunar valve. Going forward, we will abbreviate the pulmonary semilunar valve as PSV and the aortic semilunar valve as ASV. Here we can see that the pulmonary semilunar valve or the PSV marks the separation between the right ventricle and the major artery or blood vessel known as the pulmonary arteries.
The ASV, also known as the aortic semilunar valve, separates the left ventricle from the major blood vessel that is known as the aorta. The aorta is characterized by a very prominent arch. Now, in terms of the structural layout of the pulmonary arteries, there are two pulmonary arteries. There is the left pulmonary artery, which going forward we will abbreviate as LPA.
There is a right pulmonary artery, which going forward we will abbreviate as RPA. And Both the right and left pulmonary arteries diverge from a common union point known as the pulmonary trunk. It's important to note that the pulmonary semilunar valve is directly continuous with the pulmonary trunk, which then diverges into the respective left and right pulmonary arteries.
The left... pulmonary artery will conduct or convey blood to the left lung and the right pulmonary artery will conduct blood to the corresponding right lung. We can think of the semilunar valves as the entryway to the major blood vessels of the heart. The pulmonary semilunar valve is the gateway to the pulmonary trunk. The aortic semilunar valve is the gateway or entryway to the prominent aorta.
The last remaining structure to account for are the left and right pulmonary veins, which going forward we will abbreviate as left PV and RPV. The left pulmonary veins are responsible for conducting blood from the left lung returning to the heart. The right pulmonary veins are responsible for conducting blood from the right lung returning to the heart. Both the left and the right pulmonary veins empty blood contents from the lungs into the left atrium. Let's chronicle the pathway of blood through the right side of the heart.
First, deoxygenated blood will enter the right atrium, right, by way of the SVC and IBC. This deoxygenated blood will cross the tricuspid valve into the right ventricle. The right ventricle will pump this deoxygenated blood across the pulmonary semilunar valve into the pulmonary trunk and then the respective...
right and left pulmonary arteries towards the lungs where this deoxygenated blood can become re-oxygenated. Let's chronicle the events on the left side of the heart. The now newly oxygenated blood is brought back to the heart or returned to the heart by way of the right and left pulmonary veins which both empty into the left atrium of the heart.
Now, blood in the left atrium will flow across the mitral valve into the left ventricle. The left ventricle will then pump blood across the aortic semilunar valve into the aorta. The aorta will then distribute this oxygenated blood to all parts of the body. Now, there are general rules.
or principles that govern how blood vessels function or operate. By and large, veins deal exclusively with oxygen-poor or deoxygenated blood. This is where we use the blue color coding to denote the presence of veins.
This is known as the oxygenation principle governing veins. Arteries almost exclusively conduct oxygen-rich or well-oxygenated blood. This is where we utilize the red coloration to denote the presence of arteries. This is the oxygenation principle that governs arteries. In summary, the general oxygenation principle is that veins carry deoxygenated blood and arteries carry oxygenated blood.
Now, veins and arteries are also governed by a directional principle. As we can see in our schematic here of the three types of blood vessels, the vena system, or the veins, largely conduct blood towards the heart. And here we can see that the arterial system, or the arteries, conduct blood away from the heart.
This is the directional principle that governs blood vessels. Veins conduct blood towards the heart, and arteries conduct blood away from the heart. As with every...
rule or principle, there will always be an exception. In our animation of the heart, we can see that the right and left pulmonary veins are denoted by a red color to signify they conduct well-oxygenated blood. The pulmonary veins are the only example of veins in the body that carry well-oxygenated blood.
Pulmonary veins violate the oxygenation principle because they carry well-oxygenated blood instead of de-oxygenated blood. Note that the pulmonary arteries are also denoted by a blue color to signify they conduct de-oxygenated or oxygen-poor blood. The pulmonary arteries are the only example in the body of an artery that carries de-oxygenated or oxygen-poor blood.
Keep in mind that the pulmonary veins and the pulmonary arteries only violate the oxygenation principle. They do not violate the directional principle. Pulmonary veins return blood to the heart, so pulmonary veins still obey and satisfy the directional principle of veins conducting blood towards the heart.
The pulmonary arteries still satisfy and obey the directional principle where arteries conduct blood away from the heart. Note that there are a series of smaller blood vessels that encase and surround the outer surface of the heart itself. These blood vessels belong to the coronary circulation of the heart. These blood vessels belong to the heart's coronary circulation, therefore they are termed coronary blood vessels.
Like any working muscle in the body, the heart too requires its own constant blood supply to carry out all of its various tests. This is where the coronary blood vessels come in to exclusively service the heart and the heart only. So the coronary circulation is the exclusive blood supply of the heart muscle itself. Coronary arteries violate the directional principle.
This is because the coronary arteries function in delivering oxygen rich blood towards the heart muscle itself for its use. Therefore, it violates the directional principle because arteries, as we saw earlier, generally conduct blood away from the heart, not towards. However, it still obeys the oxygenation principle.
So these are still arteries that carry oxygen-rich blood. Now, once the heart has used up and exhausted the oxygen to carry out and perform its tasks, the blood is now rendered oxygen-poor and must be initially carried away. by what are called coronary veins.
One of the major examples of coronary veins is the coronary sinus located in the posterior aspect of the heart. This is a vein that carries this oxygen poor blood used by the heart initially away from it. Eventually, this deoxygenated blood will return to the right atrium. But initially, it is carried away from the heart, but eventually it will be emptied back to the right atrium of the heart to join the rest of the deoxygenated blood from the upper and lower body. The sinus earned its name from its relatively widened or dilated structure, so because of the brief period where coronary veins carry.
oxygen-poor blood away from the heart, coronary veins do violate the directional principle of veins. This brings us to our next checkpoint question of this lecture recording. Now the walls of the upper atria are lined with a set of muscles known as pectinate muscles.
These muscles are found lining the inner walls. of the upper atria. And we can see here that they are raised into prominent parallel ridges resembling the teeth of a comb. This is where pectinate muscles earn their name due to their comb-like appearance.
Their function is to allow the atria to increase the force of their contractions without the need for for the heart to increase in mass or size. So in other words, we can maximize the force of atrial contractions without having to increase the overall size of the heart muscle. These are the blood vessels that enter into the right atrium. We have the superior vena cava entering into the right atrium. We have the inferior vena cava entering into the right atrium.
Recall that the large vein in the posterior side of the heart also empties oxygen-depleted blood used by the heart muscle, eventually back to the right atrium as well. Here is the opening of the coronary sinus into the left atrium. the right atrium of the heart. And the only vessels that have entry points into the left atrium are the right and left pulmonary veins. And here are the right and left pulmonary veins.
Along a posterior view, we will be able to see both of their entries into the left atrium of the heart. Note that the exclusive blood vessels serving the heart known as the coronary arteries are situated in the grooves or the sulci on the external surface of the heart. Here we have what is called the right coronary artery, which is located in the right coronary sulcus.
We can also see the left coronary artery, which is an artery again exclusively serving the heart muscle. We can see that it is located in the groove between the left atrium and the left ventricle. So the left coronary artery is situated in the left coronary sulcus.
Here we have another coronary blood vessel known as the anterior interventricular artery. We can see that it is located in the large groove known as the anterior interventricular sulcus. Keep in mind that this is an anterior view of the heart, so we are viewing the heart in anatomical position. So we must reverse our left and right accordingly. Now in a posterior view of the heart, keep in mind that we lose anatomical position.
So therefore, the right structures of the heart are now aligned with our right, and the left heart structures are now aligned with our left. Here is... a magnified view of that widened or dilated coronary sinus. This happens to be classified as one of the coronary veins of the heart.
We also have a posterior interventricular artery, which is located in a groove known as the posterior interventricular sulcus. Now, the upper atria were exclusively lined by pectinate muscles. In the lower ventricles, we will observe two types of muscles that are entirely exclusive to the lower ventricles.
Those two muscles are the trabeculae carnii as well as the papillary muscles. These are the two muscles exclusive to the lower ventricle. Now, trabeculae carnii is the muscle that is derived from the Latin terms for fleshy or meat-like. hence the term carniate and trabeculae is derived from the term for beam-like or needle-like. recall the trabeculae of spongy bone.
In other words, these muscles resemble fleshy beams, needle-like beams. They are quite thin in comparison to the thicker papillary muscles. So here we have the finer and more delicate trabeculae carniae.
We can also observe the thicker and more pronounced papillary muscles. They are more mound-like and noticeably thicker compared to the finer trabeculae carnii. Right here are the mound-like papillary muscles.
Papillary muscles will project into the cavity of the ventricles, the space of the ventricles. The trabeculae carni are more closely adhered to the walls of the ventricles. So whereas trabeculae carni tightly hug and line the walls of the lower ventricles, the papillary muscles project into the space of the ventricles, the ventricular cavities.
Now, both papillary muscles and trabeculae carni serve a similar function. They both utilize... a pulley system in which a series of contractions and relaxations help the heart pump more efficiently. And both trabeculae, carni and papillary muscles also prevent leakage or regurgitation of blood. We refer to this as the prevention of backflow.
We establish that blood should flow through the heart. in a top-down trajectory from the upper atria down into the lower ventricles. Backflow is the leakage from the lower ventricles into the upper atria.
Essentially, backflow is blood being conducted or flowing in the wrong direction. Now, blood regurgitation or leakage can lead to . interruption of unidirectional blood flow, which means ventricles could very well have inadequate blood being pumped towards the systemic circulation to oxygenate tissues or inadequate blood being conveyed to the lungs for re-oxygenation. This is why backflow or the regurgitation of blood should be prevented. Here we can see the papillary muscles are directly attached to what are effectively string-like cords.
These are known as chordae tendineae, which translates to tendinous cords. These are commonly known as heart strings. So we can see that they have a direct attachment to the papillary muscles, and the papillary muscles are connected to the trabeculae carnii.
So in a way, the trabeculae carnii have an indirect association to the chordae tendineae. When the papillary muscles and the trabeculae carnii undergo relaxation, this yields some slack in the chordae tendineae, which allows the corresponding atrioventricular valve. either the tricuspid or the mitral valve, to effectively open and allow blood flow from the upper atria down into the lower ventricles. Now when the papillary muscles and the trabeculae carnii undergo contraction, this will effectively tug and pull on the tardia tendineae, in turn completely sealing off. the atrial ventricular valve.
This prevents any regurgitation or backflow of blood from the lower ventricles back up into the atria. Now, it's imperative that each time the right ventricle undergoes contraction to expel blood into the pulmonary circulation, and each time the left ventricle undergoes contraction to expel blood into the systemic circulation, the respective tri-cuspid and mitral valves should remain closed, completely sealed and closed to prevent the regurgitation of blood back up the atria. This is because the forces of contraction can cause blood flow to become so turbulent that blood can easily leak back up the atria.
So again, it's imperative that during right and left ventricular contraction, the tricuspid and the mitral valves are completely sealed off by the cordae tendineae, the papillary muscles, and the trabeculae, trabeculae carniae. These three structures must work in tandem to prevent the regurgitation or backflow of blood from the lower ventricles back up the atria. This brings us to our final checkpoint question of this lecture recording.