Anatomy of the heart

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Introduction0:00–1:45

The heart is a muscular organ just slightly bigger than a person's loosely clenched fist. It's 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 can take 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.
Both atria have pouch like protrusions called oracles, which can increase the capacity when needed. Ok.
Now, poorly oxygenated blood from our bodies and other 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 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.

Structure of the heart1:45–2:25

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's 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 posterior laterally. The superior border is formed by the right and left atria along the superior border.

Borders of the heart2:25–3:58

From right to left. There's the superior vena cava which enters the right atrium.
Then there's ascending aorta which emerges from the left ventricle and occurs posteriorly to form an arch called the aortic arch.
Finally, there's the pulmonary trunk and it's 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 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 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 the posterior 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.

Anterior view of the heart3:58–6:07

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 inferiorly to it, the right ventricle, they're separated by the coronary sulcus, also called the atrioventricular groove or a V groove for short.
Then there's the oracle 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 toward the back to form the aortic arch.
The branches of the aortic arch can also be seen 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 turned element is just a fibrous band. Now, in fact, 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.

Posterior view of the heart6:07–7:06

But now the right and left ventricle are separated by the posterior intraventricular sulcus in the upper portion above the coronary sulcus.
There are the great vessels, the left and right pulmonary veins which bring oxygenated blood into 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's also important to visualize that the most anterior portion of the heart is the right ventricle and the most posterior part of the heart is the left atrium which lies directly anterior to the esophagus.
And time for a pop quiz, what are the borders of the heart and what structures form them? Very well.

Quiz7:06–7:17

Let's get back at it. Now, let's have a look at each chamber of the heart starting with the right atrium.

Right atrium7:17–9:08

This is the right anterior lateral view of the heart where we dissected the right atrium. In order to see its contents, the right atrium has a smooth, thinwalled posterior part called the sinus erum where both superior and inferior vena cava, 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 the crista terminalis inferiorly.
You can also see the right A V orifice, also known as a tricuspid orifice, which is where the blood from the atrium goes into 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 intra atrial septum, which has a depression called the oval fossa or fossa ovalis, which is a remnant of the oval foramen or foreman 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 into the right ventricle just like before we're going to look at an anterior view of the heart where we dissected, the right ventricle.

Right ventricle9:08–10:02

The right ventricle has an inflow part where blood gets inside the ventricle and the outflow of heart through which blood leaves the right ventricle.
The inflow of heart has muscular elevations called the trabecula carne. And the outflow of heart 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 to the tricuspid orifice which is posterior to the body of the sternum at the level of the 4th and 5th intercostal space.
The tricuspid orifice is surrounded by a fibrous ring that's part of a 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.

Tricuspid valve10:02–11:27

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 a fibrous ring. While the free edges of the cusps are attached to tendinous cords called the chordae tendinae.
These chorda tendon anchor the leaflets to the papillary muscles which have projections of the right ventricle. The anterior papillary muscle is the largest one and arises from the anterior wall of the right ventricle.
It's chordae tendinae attached to the anterior and posterior cusps of the valve. Then there's a posterior papillary muscle that arises from the inferior wall of the right ventricle.
It's chordae tendinae attached to the posterior and septal cusps And finally, there's a septal papillary muscle which arises from the intraventricular septum and chordae tendinae attached 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.
Ok. We're not done with the right ventricle.
Hang in there. The interventricular septum or IBS separates the two ventricles and forms a part of the wall of each ventricle.

Interventricular septum11:27–12:28

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 2 to 3 times thicker than the rest of the right ventricle. The membranous part is a thin membrane that's part of the fibro skeleton of the heart.
And it's located superiorly and posteriorly on the right side of the IBS, 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 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.
So to put all these structures into perspective, let's briefly talk about blood flow. The right atrium contracts when the right ventricle is empty and relaxed.

Blood flow through the right heart12:28–12:58

So blood from the right atrium passes through the tricuspid valve and enters the right ventricle. Finally, blood then leaves the right ventricle through the pulmonary valve, which is located at the level of the third costal cartilage and enters the pulmonary trunk where blood is sent to the lungs for oxygenation.
Ok. Now, let's pause for a heartbeat and see if you can identify the structures that make up the right atrium and ventricle.

Quiz12:58–13:10

Ok. If you didn't get them, all right, no worries, we can go left.

Left atrium13:10–14:20

Now, starting with the left atrium, the left atrium is slightly thicker than the right atrium. And along with the left auricle and the pectinate muscles within it forms a large part of the base of the heart.
The rest of the left atrium is smooth. The four pulmonary veins, namely the left, superior, left, inferior, right, superior and right inferior pulmonary veins enter the smooth part of the left atrium.
Posteriorly. Unlike other structures of the heart, the pulmonary veins don't have any valves and blood flows through them and into the left atrium freely.
Now, on the intraatrial septum, there's a semilunar depression indicating the floor of the oval fossa with a surrounding ridge being the valve of the oval fossa, which as we mentioned before used to represent a communication between the atria during embryonic development.
Finally, inferiorly there a left avi orifice or mitral orifice through which blood flows from the left atrium into the left ventricle.
Finally, let's look at the left ventricle which forms the apex of the heart. Since the pressure inside the left ventricle is higher, its walls are 2 to 3 times as thick as those of the right ventricle.

Left ventricle14:20–15:22

The left ventricular walls have more but finer trabeculae Carney than the right ventricle. It also has a smooth walled nonmuscular part which is called the aortic vestibule, which leads to the aortic orifice and aortic valve.
The aortic valve is located posterior to the left of the sternum at the level of the third intercostal space. The mitral orifice is surrounded by the mitral valve which is located posterior to the sternum at the level of the fourth costal cartilage.
The mitral valve is also referred to as a bicuspid valve as it only has two cusps anterior and posterior compared to the three cusps of the tricuspid valve.
Each of the cusps are attached to the papillary muscles through chordae tendinae which allow the cusps to resist the high pressure during heart contraction.
Now that we're done with the heart chambers, we'll now take a look at the pulmonary trunk and aorta blood flows from the right ventricle to the pulmonary trunk via the pulmonary semilunar valve.

Semilunar valves15:22–17:29

While blood flows from the left ventricle to the ascending aorta via the aortic semilunar valve. These valves each have three cusps called the semilunar cusps.
The pulmonary valve has an anterior a right and left cusp. While the aortic valve has a posterior right and left cusp.
Unlike the tricuspid and mitral valves, the semilunar valves are smaller and don't have chordae tendinae to support them.
Instead, during ventricular relaxation, blood is forced backwards into the heart where the cusps catch this reverse blood flow and snap clothes like an umbrella caught in the wind.
The cusps edge come together to close the semilunar valves and support each other as their edges meet to prevent backflow.
The edge of each cusp is thickened where they meet forming the lul and at the apex of the angulated free edge, they form the nodule.
During ventricular contraction, the semilunar valves open as they are pressed towards the walls of the pulmonary trunk and aorta superior to each semi linear cusp.
The walls of the origins of the pulmonary trunk and the aorta are a bit dilated forming a sinus where blood gets in to prevent the cusp from sticking to the wall of the vessel when they open.
And it also helps them close back up. The sinus formed above the right aortic cusp is the origin of the right coronary artery.
And the sinus formed above the left aortic cusp is the origin of the left coronary artery, both of which nourish the heart muscle during ventricular relaxation.
When the semilunar valves are closing, the backflow of blood towards the heart fills these sinuses. And the force of this backflow is what allows the left and right coronary arteries to fill.
Now, before going any further, let's take a look at the conducting system of the heart, which consists of nodal tissue that automatically generates and transmits impulses that produce coordinated contractions of the heart muscle.

Conducting system of the heart17:29–19:02

Effectively turning the heart into a pump. One special nodal tissue is a sinoatrial or sa node located anterior laterally at the junction of the superior vena cava and right atrium near the superior end of the sulcus terminalis.
The essay node is the pacemaker of the heart and it establishes the basic heart rhythm. Then there's the atrioventricular or AV node, which is a smaller bit of nodal tissue located in the posterior inferior region of the intraatrial septum near the opening of the coronary sinus.
The way it goes is that the essay node initiates an impulse that's conducted in the atria, making them contract the impulse then spreads to the A V node.
And afterwards, the A V node spreads the impulse to the ventricles to the AV V bundle. Finally, at the junction of the membranous and muscular part of the IV S, the AV bundle spreads into a right and left branch that further stimulates the IV S and then the respective ventricles through smaller branches called the purkinje fibers.
This way, the ventricles can contract effectively. And that's the basic electrophysiology of how the heart coordinates its contractions.
So blood can leave the heart and reach every organ in the body. In addition to the conducting system of the heart.

Cardiac plexus19:02–20:45

The heart is supplied by the cardiac plexus, which is most commonly found on the anterior surface of the bifurcation of the trachea, posterior to the ascending aorta and the bifurcation of the pulmonary trunk.
The cardiac plexus has both sympathetic and parasympathetic fibers as well as visceral afferent fibers for the reflux of and nociceptive or pain sensing fibers of the heart.
The fibers of the cardiac plexus run along the coronary vessels and conducting system. Sympathetic innervation is from presynaptic fibers with cell bodies found in the superior 5 to 6 thoracic segments of the spinal cord and postsynaptic sympathetic fibers with cell bodies in the cervical and superior thoracic paravertebral ganglia of the sympathetic trunks.
Postsynaptic fibers travel across cardiopulmonary splanchnic nerves and the cardiac plexus ending in the essay and A V nodes.
And they also have connections with the terminations of parasympathetic fibers on the coronary arteries. Sympathetic innervation is needed to increase heart rate.
The force of heart contractions and increase flow through the coronary vessels to support the increased activity. These actions are crucial if you're caught in the wrong neighborhood and are running from a pack of kittens, parasympathetic innervation which slows the heart rate and kicks in after you've escaped.
All those kittens derives from presynaptic fibers of the vagus nerves, postsynaptic parasympathetic cell bodies are located in the atrial wall and the intraatrial septum near the sa and AV nodes and along the coronary arteries.

Review20:45–22:55

All right, as a quick recap. The heart is a muscular organ located in the mediastinum or thorax between two lungs.
The heart has four chambers, two atria separated by the intraatrial septum and two ventricles separated by the intraventricular septum.
The right atrium receives deoxygenated blood from the superior and inferior vena cava and it sends it to the right ventricle through the tricuspid valve, which has three cusps that attach to the papillary muscles via chordae tendinae.
Then the blood passes to the pulmonary sem lunar valve and reaches the lungs via the pulmonary trunk and its two main branches, the left and right pulmonary arteries, the left atrium receives oxygenated blood from the four pulmonary veins and then sends it to the left ventricle through the mitral valve.
Also attached to papillary muscles via chordae tendinae. From the left ventricle blood is pumped into the aorta through the aortic semilunar valve, reaching the rest of the body.
The heart has a conducting system made up of the sa node located anterior laterally at the junction of the superior vena cava and right atrium.
The A v node located in the posterior inferior region of the intraatrial septum. Now, on an anterior view, you can see the following structures, the coronary sulcus, the anterior interventricular sulcus, the superior inferior vena cava, the ascending aorta and the pulmonary trunk as well as the ligamentum arteriosum.
On a posterior view, you can see the posterior intraventricular sulcus left and right, pulmonary veins, left and right, pulmonary arteries, the aortic arch and the superior and inferior vena cava.
The heart is innervated by sympathetic and parasympathetic nerve fibers from the cardiac plexus, which is most commonly found on the anterior surface of the bifurcation of the trachea.