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An Introduction to the Cardiovascular System

We believe knowledge is power especially when it comes to your health.  As such, we feel one of our jobs is also to bring you resources, educational content, and tools to help you learn more about cardiovascular health. In today’s post, we share a great video put on by the Strong Medicine YouTube channel. It’s an informative deep-dive into the human vascular system, and may help you not only understand yourself better, but also help you make informed decisions about your healthcare.


Video Transcript:

Hello, I’m Eric strong, and I’m a clinical associate professor of medicine at Stanford University, and this is the first video in this series on the cardiovascular system. This will provide an overview of the heart and blood vessels, starting from the basics, in order to provide a broad common knowledge base for all viewers before we dive into individual topics. By the end of this video, you will be able to describe the overall function of the cardiovascular system, identify the major anatomic structures of the heart, including the coronary arteries, and the primary components of the conduction system. You’ll also be able to describe the cardiac cycle (including the definitions of sisterly and ghastly), list and describe the different types of blood vessels, and describe the histological structure of the heart.

In short, the function or purpose of the cardiovascular system is to pump blood around the body. But why? The simple answer is the delivery of oxygen. Deoxygenated blood, which is blood that’s relatively lacking in oxygen, is pumped from the heart to the lungs, where the oxygen is picked up. The now oxygenated blood returns to the heart, where it’s pumped to the rest of the body, at which point the oxygen is offloaded within other organs and tissues. Oxygen is an essential part of cellular respiration. This is the process by which cells generate energy, storing ATP from glucose and other molecules containing chemical potential energy. But, the movement of oxygen is just part of the story! Blood carries a lot more than that! For example, waste products, most notably carbon dioxide, is a byproduct of cellular respiration, which the heart must pump back to the lungs to be exhaled. Blood carries other metabolic waste products to the liver, such as bilirubin and lactate for processing, and in some cases, elimination from the body. For example, in bile, it carries urea to the kidneys to be excreted in the urine. Blood transports electrolytes, glucose, fatty acids, and it also transports hormones like insulin and cortisol, which allow one part of the body to send signals to another. It carries essential components of our immune system, like white blood cells and proteins, such as antibodies and cytokines. So, while it’s true that the circulation of oxygen is the single most important function of the heart, it is far from the only necessary one.

To understand how the pumping mechanism works, let us first focus on the anatomy of the heart itself. The heart is a surprisingly difficult structure to represent with a two dimensional picture. No matter what angle or cross section is viewed, at least one notable structure will be hidden behind others, kind of like a map of the world. There is no two dimensional view that accurately represents both the relative size and location of the chambers. I’ll start with this view, but I’ll switch around to other images as needed. On the most basic level, the hearts physical structure consists of four chambers, surrounded by muscular walls for valves, a handful of so called “great vessels”, and a number of minor vessels. To give a lay of the land, the path that blood takes as it returns from the body in its deoxygenated state. First, blood arrives in the chamber called the right atrium; it can reach the right atrium through one of three paths. All blood returning from above the diaphragm enters via the superior vena cava. of blood returning from below the diaphragm enters via the inferior vena cava, and most blood returning from the heart muscle itself enters the right atrium via this much smaller vessel called the coronary sinus. From the right atrium, blood travels through a valve opening called the tricuspid valve into the right ventricle from the right ventricle. It travels through another valve called the pulmonary valve into the pulmonary artery which carries the still the oxygenated blood to the right and left lungs, where carbon dioxide will be offloaded and oxygen will be picked up. The oxygenated blood returns to the heart via one of four pulmonary veins, bringing it to the left atrium. From the left atrium it travels through the mitral valve into the left ventricle. From the left ventricle, blood then exits the heart through the aortic valve, which is hidden behind the pulmonary valve in this particular view, and then it travels via the aorta to the rest of the body. This sequence of steps from the vena cava to the order is separated into two segments by the lung, with the right side of the heart containing deoxygenated blood, and the left side of the heart turning oxygen into blood. The right and left atria are separated by a relatively thin inter atrial septum, hidden here behind the right ventricular outflow tract, while the left and right ventricles are separated by the relatively thick and muscular interventricular septum. As a side note, while almost all diagrams of the heart show the right heart in blue and the left heart and red, deoxygenated blood is not literally blue, but rather more of a maroon color. Also, going back to my previous comment about how no single view of the heart is perfect, this view makes it appear that the right heart is larger than the left, when in reality, the left is larger than the right, as the left heart sits somewhat posterior to the right when viewed from this angle. If we look at a cross section of the heart taken along this plane, we get a better idea of the relative volume and shape of the ventricular chambers. The large round, thick walled one is the left ventricle, and the smaller crescent shaped one is the right ventricle. The left ventricular wall needs to be much thicker because it pumps against much higher pressures than the right one does. Back to this view, let’s discuss the valves a little more. The valves consist of leaflets that allow movement of blood in only one direction. All of the valves normally have three costs, except the mitral valve which normally has two. Each valve belongs to one of the two sides of the heart, right versus left, and each has one of two structures, atrial ventricular versus semilunar. The cusps of the two atrioventricular valves are attached to fibrous threads called tendinous cords. These are, in turn, attached at the other end to the papillary muscles, which are conical projections from the walls of their respective ventricles. The purpose of the quarter tendon and the papillary muscles is to prevent backwards movement of the valve cusps during the high pressures generated when the ventricles contract, which otherwise could result in the retrograde movement of blood from the ventricles backwards into the atria.

Semilunar valves are named because their costs are reminiscent of half moons are concave when viewed from above. In this classic cross sectional view of the heart, known as the parasternal long axis view, we can see this significant difference in structure between the mitral valve, with some of its core de and one of the papillary muscles and the adjacent smaller semilunar aortic valve. One final point to make about the valves is that although it can be hard to appreciate in most two dimensional views, the four valves sit in close proximity in roughly similar plane to one another. This plane is composed of fibrous tissue that includes fibrous rings around the valve openings. These rings are stiff, and they help to maintain the valve shape, and also provide structure to which the valve cusps can attach more genuinely. This fibrous tissue functionally acts as a skeleton of the heart. It also is non conductive of electricity, which is critical for proper regulation of the heart rhythm, as it funnels electrical signals between the atria and ventricles to a relatively small location called the AV node, where it can be more easily regulated. Although I’ve already mentioned them individually, to review, let me list the eight great vessels. These are the superior and inferior vena cava, which bring blood from the body to the right atrium, the main pulmonary artery, which bifurcates into the right and left pulmonary arteries, which traveled to the right and left lungs respectively. The aorta which is the main artery of the body from which all other arteries branch, except the pulmonary artery, it’s often divided into four parts, the A sending an order, which then loops up and posteriorly into the aortic arch, which continues downward and becomes the descending thoracic aorta coursing posterior to the heart. Once the aorta crosses below the diaphragm, it’s known as the abdominal aorta. And last for pulmonary veins, which bring blood from the lungs to the left atrium. And this brings us to the conduction system. While we are Think of the heart as primarily a mechanical pump, there needs to be some mechanism that tells each chamber when to contract and when to relax. And that mechanism is the conduction system, which delivers electrical signals to different chambers at different times. electrically, impulses normally originate within a specialized tissue called the sinoatrial node, SA node, or sometimes just sinus node. It’s located in the superior posterior aspect of the right atrium. The frequency of these impulses is governed by a balance between the sympathetic and parasympathetic divisions of the autonomic nervous system. The sympathetic nervous system tells the SA node to fire more frequently, while the parasympathetic nervous system tells it the fire less frequently. In normal healthy adults, the balance between these two results in a resting heart rate between 50 and 90 beats per minute. electrical impulses fired from the SA node travel throughout the two atria, triggering their contraction and the ejection of blood through the AV valves into their respective ventricles. The electrical signal will next reach the atrial ventricular or AV node. Within the inferior part of the inter atrial septum. Well, the SA node can be thought of as the pacemaker of the heart, the AV node is more of a gatekeeper. As mentioned, it’s the sole location in which electricity can normally pass from the atria to the ventricles, and it conducts relatively slowly, providing time for blood to move from the contracted atria into the relaxed ventricles. After holding up the signal by 100 or so milliseconds, or a 10th of a second, the AV node, let’s continue through a band of conducting fibres called the his bundle that divides into right and left bundle branches, which traveled to the right and left ventricles respectively. The right and left bundles terminate in a network called the purkinje fibers, which rapidly deliver the signal to the ventricles. The overall consequence of this system is the rapid and nearly simultaneous contraction of the right and left ventricles in a wavefront that begins inferiorly at the cardiac apex and travels upwards towards the valves, which helps to increase the efficiency with which the heart ejects blood. See the coronary arteries. That is the arteries that supply the heart muscle itself, we need an exterior view.

And again, the limitations of two dimensions make it impossible to see all of them at once. Coming off the aorta immediately above the aortic valve are two coronary arteries. One is the right coronary artery or RCA, which predominantly supplies blood to the right ventricle, and usually to the essay and AV nodes of the conduction system. The other is the left coronary artery, more commonly known as the left main coronary artery, which quickly bifurcates into the left anterior descending artery or LED, which supplies the anterior interventricular septum and anterior wall of the left ventricle and the left circumflex artery, which wraps around the hind to supply the lateral wall of the left ventricle. coronary circulation contains a number of anastomosis or distal connections between these three arterial territory’s that allow for some redundancy of blood supply in the event that one artery becomes obstructed. Also, there are many normal and pathologic anatomic variations to the coronary circulation, most notably the normal variations in how blood is supplied to the posterior interventricular septum and the inferior wall of the left ventricle. As I mentioned, the heart is a difficult organ to accurately represent anatomically in two dimensions. So therefore, I’m going to show you what it looks like in three dimensions. So you can get a better idea of how different cardiac structures relate to one another. Here is the exterior of the heart, we can start with the great vessels and we can immediately see that several are in very close proximity to one another. Here’s the SVC or superior vena cava. Here is the aorta, which loops upwards and posterior into the aortic arch. And here’s the main pulmonary artery, which bifurcates into the left and right pulmonary arteries. We can also see the four pulmonary veins as they enter the left atrium on the posterior side. Another structure apparent from the outside one which I have not discussed yet, are these funny shaped things which extend a little over the surface anatomist call these the articles of the right and left atrium, but most clinicians refer to these as the right and left atrial appendages. These are important particularly the left atrial appendage because these outpouching are region of relative spaces for blood, and therefore they can be the site of blood clot formation, particularly during abnormal heart rhythms in which the atria do not contract, such as one called atrial fibrillation. On the outside, we also see the coronary vessels. This one here is the right coronary artery, which supplies the right ventricle. The left main coronary artery is hidden behind the pulmonary artery, as is its bifurcation. But here’s the left anterior descending artery supplying the anterior wall, the left ventricle, and around the back is the left circumflex artery supplying its lateral wall. I had mentioned that there were a large number of variations to coronary anatomy, including which arteries applies blood to the inferior wall, the left ventricle.

In this case, if we follow this muscle right here, backwards, we’ll see that it originates from the right coronary artery. So therefore this is called right and dominant circulation. The last thing to note on the outside is this blue vessel

right here, which at its most distal end is known as the coronary sinus, which drains blood from the heart itself into the right atrium. Now let’s take a look inside. A few things to note here. First are the valves specifically how close together they are, particularly the aortic and the mitral valves, these weight extensions, or the cord a tendon a, we can also better appreciate the difference in size and shape between the two ventricles. the right ventricle, which is down here, inferior to the tricuspid valve is relatively small and moon shaped on the left ventricle is slightly larger and a little bit more around. We also can see here just superior to the tricuspid valve is the entrance to the of the coronary sinus where the coronary sinus drains into the right atrium. On the inside surface of the ventricles, we can note this mesh of muscular bundles here. These are known as Dr. Becky lations. And it’s actually not known what function they serve.

The cardiac cycle is a sequence of all events within the heart that occur from one heartbeat to the next. Each cycle is triggered by the SA node firing an electrical signal. This signal triggers Do you right and left atrial to contract, squeezing blood through the already open tricuspid mitral valves into the right and left ventricles. This period of atrial contraction is formerly referred to as atrial sisterly, but it’s more commonly referred to as the atrial kick. After the electrical signal reaches the AV node, and undergoes the brief AV delay, allowing time for filling of the ventricles during the atrial kick, the signal propagates through the historic Kenji system and triggers ventricular contraction. As we’ve already discussed. The rapid increase in pressure within the ventricles causes the AV valves to quickly snapshot triggering the first heart sound any classic lub dub of the heartbeat, this first heart sound is known as s one and the intraventricular blood is injected through the now open semilunar valves. After a brief period of time, the ventricles relax again. At this point, there is higher pressure in the pulmonary artery and aorta than in the right and left ventricles respectively. So the semilunar valves snapshot, resulting in the second heart sound known predictably as s two. At this point, the ventricles have emptied much of their blood and relax to the point that the pressure and the atria are now higher than in the ventricles, and so the AV valves open once again, and this pressure gradient drives the passive filling of blood into the ventricles even before the next SA node impulse triggers atrial contraction, and a repeat of the cycle. The period of the cycle during which the ventricles are contracting is called ventricular sisterly, or usually just Sicily. And the period of the cycle during which the ventricles are relaxing is called ventricular diaster Li or just diastolic. confusingly, the timing of ventricular diaster Li includes atrial sisterly, which is why atrial sisterly is usually referred to as the atrial kick or sometimes just atrial contraction and blood. One is moving from the atria to the ventricles throughout all Gastly so both before and during atrial contraction.

final point about the cardiac cycle, sisterly is always shorter than ghastly. Although the relative fraction of the cardiac cycle each takes up is dependent on the heart rate. As the heart rate increases, dastardly shortens much more so than sisterly such that at extremely fast rates, they can be almost equal in duration.

Up until now, I’ve just been discussing the heart, but the heart is only one half of the cardiovascular system and the other half is the blood vessels, which are the conduits through which blood is pumped around the body, moving from the heart to the peripheral tissues, and from the peripheral tissues back to the heart. There are five basic types of vessels. Starting from the heart, all blood leaves the left ventricle via the aorta. From the aorta branch off large and medium sized arteries, which are thick walled with significant elastic tissue and smooth muscle, allowing them to handle high pressure. arteries divide into smaller arterioles, which are the site of greatest resistance to blood flow through the circulation. And materials eventually divide and divide again into microscopic networks of extremely small vessels called capillaries, which are so small as to be lined by a single layer of endothelial cells. Sometimes, the capillaries are where gas exchange between blood and peripheral tissues actually occurs. Although the capillaries are tiny, there are billions of them in the body, such that their net cross sectional area is many times that of the aorta or of any other level of blood vessel. As a consequence, the capillaries are where blood travels the most slowly through the circulation, which is of course helpful for the exchange of gas, nutrients and waste. After the capillaries, blood next moves to the venules, which are analogous to the arterioles and from there to the veins. In contrast to arteries, veins are relatively thin walled and lack much elastic tissue, resulting in significant distensibility. This sensibility allows veins to act as a reservoir for blood volume. arteries and veins are typically named for either the part of the body they travel through, or which organ they bring blood to or away from. And they often exist as matching pairs. For example, the left renal artery and left renal vein or the right femoral artery and right femoral vein. There are of course, numerous exceptions. In addition to the blood vessels, there are also conduits called lymphatics lymphatic vessels are responsible for returning interstitial fluid that is extra vascular fluid that surrounds the cells and sits in the connective tissue back to the bloodstream. lymphatic capillaries merge into ever larger vessels until eventually most feed into a conduit called the thoracic duct, which empties into large veins of the thorax. The last topic I’ll discuss is histology and the microscopic structure of the heart. From a histological perspective, the heart is composed of three layers. First is the thin inner endocardium, partly composed of endothelial cells similar to those that line the blood vessels, the endocardium also forms the lining of the heart valves. Immediately beneath the endothelial is the sub endocardium, composed of loose connective tissue, and is also the location of the purkinje fibers. The next major layer is the thick myocardium, which is the heart muscle, and which is composed predominantly of cardiomyocytes. cardiomyocytes, or cardiac muscle cells, each contain long myofibrils that contain a repeating microstructure called sarcomeres, which are the fundamental contractile units of the cell. Within the sarcomeres are filaments of two proteins called actin and myosin, whose ATP and calcium dependent interaction is responsible for myocyte contraction. cardiomyocytes containing high density of mitochondria, necessary for the continuous production of energy containing ATP. The cells are connected to one another via porous bridges called intercalated discs, which allow for the free movement of electrolytes, which is necessary for rapid transmission of electrical signals from one cell to the next through the muscle tissue. This rapid transmission of signal is necessary for coordinated contraction. The overall process by which an electrical signal results in myocyte contraction is called excitation contraction coupling.

external to the myocardium is the pericardium. The pericardium is a fibrous here’s structure that encases the heart and the roots of the great vessels. It consists of a tough outer fibrous pericardium, which is tethered to the diaphragm in the sternum, and which keeps the heart in place. And there’s a inner smooth Cirrus pericardium that’s folded over on itself to make two separate layers containing a potential space between them. The outer of these serious layers is the parietal pericardium, which is fused to the fibers pericardium. The inner of the serious layers is the visceral pericardium, which lies on the myocardium and is sometimes referred to as the epicardium. Yes, the terminology is frustrating to keep straight. The potential space between the two serious layers is called the pericardial space, and is normally much thinner than it is in this image. It contains a thin film of fluid that allows the heart to beat in a near frictionless environment. However, in certain pathologic states, this space can significantly enlarge due to the pathologic excessive accumulation of fluid, negatively impacting cardiac function.

To summarize what I’ve discussed so far, the cardiovascular system can be very broadly considered a collection of nine functional components, five in the hearts and four in the periphery and pathologic conditions of the system can usually be mapped to just one of these components. For example, the heart is made up of the valves, which are affected by valvular, heart disease and an infection called endocarditis. The myocardium which is affected by heart failure and mild carditis, the pericardium which is affected by pericardial, effusions, and pericarditis, the conduction system which is affected by a very diverse collection of arrhythmias or abnormal heart rhythms, and the coronary arteries, which are involved in coronary artery disease, colloquially known as heart disease, and which is responsible for the conventional types of heart attacks. The blood vessels can be subdivided into arteries, including arterioles, which are impacted by peripheral artery disease in the veins including venules, which are impacted by a condition called venous insufficiency. The capillaries which are rarely the primary site of pathology, but which can be the site of manifestations of sepsis, and lymphatics, which cause a condition called lymphedema when obstructed when approaching a patient with an undiagnosed disease. I find this framework to be helpful in considering and categorizing different diagnostic possibilities within the cardiovascular system, keeping in mind that the framework somewhat excludes congenital heart disease, which can involve just one component like a bicuspid aortic valve, which usually spans multiple functional components, which transcends this framework altogether. That concludes this introductory overview of the cardiovascular system. If you found it helpful, please consider liking and sharing it.

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