The heart is a muscular organ just slightly bigger than a person’s loosely clenched fist. It is located in the thorax - more specifically, between the two lungs, in a space called the mediastinum. The heart is covered by a tough membrane called the pericardium, that separates the heart from the other structures in the mediastinum. This position allows the heart to do its job, which is to pump oxygen-rich blood to the entire body, and to send poorly oxygenated blood to the lungs, where gas exchange takes place. The heart is made up of four chambers, and as a whole can be functionally divided into the right heart, made up of the right atrium and the right ventricle, and a left heart, made up of the left atrium and left ventricle. And both atria have pouch-like protrusions called auricles, which can increase their capacity when needed.

Ok, now, poorly oxygenated blood from our bodies and tissues enters the right atrium through the superior vena cava and inferior vena cava. From there, blood passes into the right ventricle, which pumps into the pulmonary trunk, on a voyage towards the lungs and is considered part of the pulmonary circulation. On the other hand, after gas exchange takes place in the lungs, oxygenated blood returns from the lungs through the four pulmonary veins, which drain into the left atrium. Then, oxygenated blood goes in the left ventricle and from there, it’s pumped into the aorta so that it reaches the whole body and is considered part of the systemic circulation.

So, looking at it in three dimensions, the heart looks like an upside down, tipped-over pyramid with four sides, a base that’s mostly posterior and an apex, or tip, that points anteriorly and slightly to the left.

On an anterior or posterior view in two dimensions, the heart is shaped like a trapezoid, so it has a superior and inferior as well as a right and left border. It is important to understand what comprises the borders of the heart because the heart is rotated to the left on its longitudinal axis within the mediastinum, so the apex is directed more postero-laterally.

The superior border is formed by the left and right atria. Along the superior border, from right to left, there’s the superior vena cava, which enters the right atrium. Then there’s the ascending aorta, which emerges from the left ventricle and then curves posteriorly to form an arch, called the aortic arch. Finally, there’s the pulmonary trunk and its two branches, the right and left pulmonary artery, with the right pulmonary artery going under the aortic arch. The inferior border is almost horizontal and is made up mainly by the right ventricle and a part of the left ventricle. The right border is made up by the right atrium and it’s between the superior and inferior vena cava and finally, the left border is mainly made up by the left ventricle and a part of the left auricle. Now, pay a bit of attention here, because the end of the inferior border, along with the lower part of the left border make up the apex of the heart, which represents the tip of the left ventricle. Some of these features can also be identified on a postero-anterior chest X-ray. The heart silhouette is between the lungs, and the right border, made up by the right atrium, as well as the left border, made up by the left ventricle and part of the left auricle, can be clearly seen. Above the left auricle, we can identify the pulmonary artery and the aortic arch. The superior and inferior borders of the heart aren’t as easy to see as the right and left borders.

Now, let’s identify the structures in an anterior and posterior view. On an anterior view, we can identify several structures, starting with the four chambers and ending with the great vessels. From right to left, there’s most of the right atrium located superiorly and inferior to it, the right ventricle. They are separated by the coronary sulcus, also called the atrioventricular groove or AV groove for short. Then there’s the auricle of the left atrium and the left ventricle, which are also separated by the coronary sulcus. The right ventricle and left ventricle are separated by the anterior interventricular sulcus. Moving on, you can actually see parts of the great vessels on an anterior view. So, from right to left, there’s part of the superior vena cava, which opens in the superior part of the right atrium. Then there’s part of the inferior vena cava, which also opens in the right atrium, but the opening can only be seen on a posterior view. Then there’s the ascending aorta, which emerges from the left ventricle, and then arches towards the back to form the aortic arch. The branches of the aortic arch can be seen as well and from right to left, they are: the brachiocephalic trunk which branches into the right subclavian artery and right common carotid artery, the left common carotid and the left subclavian artery. Finally, medial to the left auricle, there’s the pulmonary trunk which emerges from the right ventricle and branches into the left and right pulmonary arteries, with the right pulmonary artery going under the aortic arch. Interestingly enough, the superior portion of the proximal left pulmonary artery is connected to the inferior surface of the aortic arch by the ligamentum arteriosum. Even though this fancy-termed element is just a fibrous band now, back during embryological development it used to connect the aortic arch and the pulmonary trunk as an adaptation of fetal circulation to intrauterine life.

Now, on a posterior view, we can see the left atrium and the left auricle, most of the left ventricle, as well as the right atrium and right ventricle. As before, the atria and ventricles are separated by the coronary sulcus, but now the right and left ventricle are separated by the posterior interventricular sulcus, also called the crux cordis. In the upper portion, above the coronary sulcus, there are the great vessels. The left and right pulmonary veins which bring oxygenated blood in the left atrium, and the left and right pulmonary arteries which emerge from the pulmonary trunk and bring deoxygenated blood to the lungs above them, the aortic arch and the superior and inferior vena cava. It is also important to visualize that the most anterior portion of the heart is the right ventricle, and the most posterior portion of the heart is the left atrium, which lies directly anterior to the esophagus.

Time for a pop quiz! What are the borders of the heart and what structures form them?

Very well, let’s get back at it. Now let’s have a look at each chamber of the heart, starting with the right atrium. This is a right antero-lateral view of the heart where we dissected the right atrium in order to see its contents. The right atrium has a smooth thin-walled posterior part, called the sinus venarum, where both superior and inferior venae cavae, as well as the coronary sinus open. Then there’s a rough muscular anterior part formed by the pectinate muscles. Externally, the smooth and rough parts of the atrium are separated by a vertical shallow groove called the sulcus terminalis. Internally, these structures are separated by a vertical ridge called crista terminalis. Inferiorly, you can also see the right AV orifice, also known as the tricuspid orifice, which is where the blood from the atrium goes in the ventricle. Now, the superior vena cava enters through the superior part of the right atrium, which is roughly at the level of the third costal cartilage. The inferior vena cava opens inferiorly almost parallel to the superior vena cava, roughly at the level of the fifth costal cartilage. Finally, the coronary sinus opens between the tricuspid orifice and the inferior vena cava orifice. Now, the right and left atrium are separated by the interatrial septum, which has a depression called the oval fossa, or fossa ovalis, which is a remnant of the oval foramen, or foramen ovale, that connected the atria during embryological development. In fetal life, oxygen rich and nutrient rich blood travels directly into the left atrium from the right atrium via the foramen ovale to bypass the pulmonary circulation.

Let’s stay on the right track and move on to the right ventricle. Just like before, we’re going to look at an anterior view of the heart where we dissected the right ventricle. The right ventricle has an inflow part, where blood gets inside the ventricle and outflow part through which blood leaves the right ventricle. The inflow part has muscular elevations called the trabeculae carneae and the outflow part is formed by an infundibulum called the conus arteriosus, located superiorly and leads to the pulmonary trunk. So far so good. Now, the right ventricle receives blood from the right atrium through the tricuspid orifice, which is posterior to the body of the sternum at the level of the fourth and fifth intercostal space. The tricuspid orifice is surrounded by a fibrous ring that’s a part of the fibrous skeleton of the heart, keeping its shape constant.

And in order for blood to flow from the right atrium to the right ventricle at exactly the right time during each heartbeat, there’s a tricuspid valve attached to the tricuspid orifice; and it’s called the tricuspid valve because it has three cusps or leaflets that open and close during systole and diastole: an anterior one, a posterior one and a septal one. The base of each cusp is attached to the fibrous ring, while the free edges of the cusps are attached to tendinous cords called the chordae tendineae. These chordae tendineae anchor the leaflets to the papillary muscles, which are projections of the right ventricle. The anterior papillary muscle is the largest one and arises from the anterior wall of the right ventricle. Its chordae tendineae attach to the anterior and posterior cusps of the valve. Then there’s the posterior papillary muscle that arises from the inferior wall of the right ventricle. Its chordae tendineae attach to the posterior and septal cusps. Finally, there’s the septal papillary muscle which arises from the interventricular septum and its chordae tendineae attach to the anterior and septal cusps. This smart and complicated mechanism is mainly meant to block the backflow of blood from the right ventricle to the right atrium during systole, which is when the heart contracts to pump blood out of the ventricles.

Not done with the right ventricle, hang in there. The interventricular septum or IVS separates the two ventricles and forms a part of the wall of each ventricle. It’s made up of muscular and membranous parts. Now, the muscular part of the IVS is mostly part of the left ventricle, so it’s two to three times thicker than the rest of the right ventricle. The membranous part is a thin membrane that’s part of the fibrous skeleton of the heart and it’s located superiorly and posteriorly. On the right side of the IVS, the septal cusp of the tricuspid valve attaches to the middle of the membranous part.

Finally, in the right ventricle there’s also a muscular bundle that crosses from the inferior part of the IVS to the base of the anterior papillary muscle. This is called the moderator band or the septomarginal trabecula, and it helps the right ventricle contract properly by making the anterior papillary muscle contract at the same time as the other two papillary muscles.