Basic Anatomy of the Heart

I am sure that we all have had basic cardiac anatomy, but review never hurts. For the new students, this will be a great starter to understanding the heart, and for those old-timers like me, we can refresh and possibly remember something we had forgotten.

Position of Heart in Chest
Position of Heart in Chest

The heart is a hollow organ, about the size of your fist, that lies in the mediastinum. (the mediastinum is the area between the lungs. It holds the heart, great vessels, trachea, esophagus and other structures, and goes from the sternum to the vertebral column). Typically it weighs about 11 ounces or 250 to 350 grams. If you measure it, it will be about 5 inches long (12cm),3½ inches wide (9cm) and 2½ inches thick (6cm).

We learned that the walls of the heart were anterior, posterior, septal, inferior and lateral. When we speak of those, we are referring to the ventricular walls from inside the heart. But on the outside of the heart, we also have surfaces. These terms refer to what position the outer part of the heart is facing or touching.

The heart has five basic surfaces-

  • Diaphragmatic surface/Inferior surface
  • Posterior surface
  • Anterior surface
  • Right lateral surface
  • Left lateral surface

The diaphragmatic surface is the lower portion of the apex. It is made up of the right and left ventricles, but mostly the left. (remember, the apex is formed by the left ventricle) it rests on the diaphragm (the muscle between the lungs and abdomen that helps the lungs expand so we can breathe). You may also hear this surface being referred to as the inferior surface.

The posterior surface covers the base of the heart. This will include the left atrium, part of the right atrium, and the proximal portions of the inferior and superior vena cava and the pulmonary veins.

The anterior surface is the portion of the heart that is under the sternum, and although the left ventricle and part of the right atrium are considered anterior as well, the majority of the heart that lies under the sternum is the right ventricle. This is because the heart sits tilted in the chest.

Then the lateral surfaces; the left lateral surface faces the left lung and the right lateral surface faces the right lung within the mediastinum.

Surfaces of the Heart
Surfaces of the Heart

 

The heart is covered by a double walled sac called the pericardium. The reason for this covering is to act as protection from trauma and infection. The tough outer layer is called the fibrous parietal pericardium and is fibrous in nature. It attaches to some of the structures around the heart, such as the diaphragm and sternum by ligaments to help keep the heart in it’s place.

The inner layer of the pericardium, the serous pericardium, is actually made up of two layers. These layers are called the parietal and visceral. The parietal layer lays next to the fibrous parietal pericardium and the visceral layer lays next to the heart muscle itself . The visceral layer is also known as the epicardium.   This is a small amount of fluid between the visceral and parietal layers that helps keep the heart movement from causing rubbing in the pericardial sac.

pericardial layers

On a side note, the reason we have pain in our shoulder and neck when we are having chest pain is because the phrenic nerves pass through the pericardium as they travel downward toward the diaphragm.

So when you are trying to remember the layers of the pericardial sac, maybe this association will help, The outer most layer is fibrous and tough-so it is the fibrous parietal layer. We know the serous layer is inside the fibrous layer. When you think of serous, you think of fluid, right? So we know there needs to be two layers in the serous layer to hold the fluid. The parietal layer will lay next to the fibrous parietal. Parietal goes with parietal. and when you think visceral you think inside, for the non medical, gut wrenching emotion- so the visceral layer will be the most inside layer.

Now the walls inside the heart are made up of three layers; endocardium, myocardium, and epicardium. The endocardium lines the inside of the heart. It is made up of epithelial cells that covers all the surfaces of the heart, the walls and valves,  chordae tendineae and papillary muscles. Since the endocardium is part of the innermost layer, it is also part of the arteries, veins, and capillaries. This creates a closed circuit system for the heart.

The middle layer is called the myocardium, it is the strong muscle of the heart that does the work of the contraction. As pressure increases in the heart, this muscle will work harder to push the blood out and will get thicker. Just like if you lift weights; the more weight you lift the bigger your arm muscles get. The myocardium is divided into two halves. The innermost half is called the subendocardium and the outermost halve is the subepicardium.

The heart’s outermost layer is the epicardium (this is also the visceral pericardium) It contains  blood capillaries, lymph capillaries and nerve fibers along with fat tissue. The main coronary arteries lay on the epicardium before they dive into the myocardium to give the heart it’s blood supply.

Now when you are trying to remember the sub layers of the myocardium, just remember to associate. The subendocardium is next to the endocardial layer and the subepicardium is next to the epicardial layer.

heartwall

The heart walls are made up of muscle fibers, which are made up of many muscle cells (myocytes). These fibers are striated, which makes them look a lot like a skeletal muscle, but myocytes are involuntarily controlled where as skeletal muscles are controlled by the voluntary (somatic) system. These striations are made up of proteins and alternate between being thick and thin. The thicker portions are called “A” bands and the thinner ones “I” bands. Coming out of the cell membrane are tubes, called T (transverse) tubules. The impulses travel down from the cell’s surface into the cell through the T tubules. These, along with long branching cells (intercalated discs) help join the cells together.

Calcium, potassium and sodium cross the cell’s membrane through openings called channels or gap junctions.The exchange of these ions changes the charge in the cell causing it to contract.

The heart has four chambers, two on the right and two on the left. It has four valves, one between each chamber and one leaving each chamber, and lots of vessels coming in and leaving the heart.

There are two filling chambers, the right and left atria, and two pumping chambers, the right and left ventricles.

The right atrium receives blood that is oxygen depleted from the body by way of the inferior and superior vena cava. It also receives the deoxygenated blood from the heart itself from the coronary sinus. ( The coronary sinus lies between the inferior vena cava and the tricuspid valve. There is a small remnant of tissue at the opening called the thebesian valve) The inferior vena cava collects the blood from the lower half of the body and feeds it into the right atrium on the lower portion  of smooth tissue left over from the fetal heart called the sinus venosus. The eustachian valve (a remnant of fetal circulation) guards the mouth of the inferior vena cava.  You might see it on echo, and if you do, be sure to get a good view that clearly shows it at the mouth of the inferior vena cava to prove that it is not a thrombus. Usually your subcostal view will be the best for this.

The inferior vena cava feeds into the lower portion of the right atrium and the superior vena cava is demonstrated by the arrow
The inferior vena cava feeds into the lower portion of the right atrium and the superior vena cava is demonstrated by the arrow

Feeding into the upper portion of the sinus venosus is the superior vena cava. (The arrow on the left of the heart is pointing it out.)

It collects the deoxygenated blood from the brain and upper extremities and brings it back into the heart.

Along the edge of the sinus venosus, is a ridge called the christa terminalis. This ridge separates the smooth portion of the right atrium (sinus venosus) from the rough muscle that surrounds the rest of the atrium and into the right atrial appendage. This rough muscle is called pectinate muscle.

The purpose of the pectinate muscles are to act as kind of a volume controller , they provide extra stretch in the right atrium when there is extra volume.

The deoxygenated blood that has filled the right atrium will cause the pressure in the right atrium to rise. The right ventricle is practically empty at this point because it has been squeezing the blood out and into the pulmonary artery. Without the volume in the ventricle the pressure will drop. When the pressure in the right atrium get higher than the pressure in the right ventricle, the tricuspid valve will be forced open and the majority of the blood that was in the right atrium will be sucked into the right ventricle. Remember, the right atrium is constantly receiving blood, so a point will come where the the pressure between the atrium and ventricle will begin to even out and at that point the atrium will contract to push out the rest of it’s volume into the right ventricle.

The tricuspid valve lies between the right atrium and right ventricle. Therefore it is known as one of the atrioventricular valves (AV valves). It has three leaflets, an anterior (or infundibular), posterior(or marginal) and septal (or medial) leaflets.  The anterior leaflet is the largest and lies next to the right ventricular outflow tract (or infundibulum). The posterior leaflet is the next largest leaflet and is in the posterior position in relation to the ventricle. The third leaflet (septal or medial) is the smallest and is attached to the septum and a fibrous area that forms a triangle between the tricuspid, aortic and mitral valves, called the right and left trigones. The septal leaflet is not very mobile because of the way it is attached, so even though the tricuspid valve has three leaflets, it functions like it only has two.

If we only had the leaflets, when the right ventricle contracts, the great pressure it generates would push the leaflets back into the atrium. To prevent this from happening, the leaflets are attached to muscles called papillary muscles by fibrous strings called chordae tendinea. Since we have three leaflets, we need three papillary muscles. They are named the anterior, posterior and septal papillary muscles because of the leaflets they serve. The leaflets connect to the wall by way of a fibrous ring, known as the tricuspid valve annulus. And the fragile corners where the leaflets join together are called the commisures. These are similar to the corners of your eyes or mouth.tricuspid valve

The right ventricle is triangular in shape and is about 1/3 the size of the left ventricle. Positionally, it sits more anterior than the left ventricle. The pressures on the right side of the heart are lower than the pressures on the left side of the heart. But the same amount of blood is ejected from the right ventricle as the left ventricle with each beat. As the right ventricle contracts, the pressure in the ventricle becomes higher than the pressure in the pulmonary artery, which will cause the pulmonic valve to open allowing the blood to be ejected into the pulmonary artery and to the lungs to become oxygenated.

The pulmonic valve is one of the semilunar valves because the cusps are shaped like little moons. It has three cusps which are named the right, left and septal cusps. The right and left cusps are attached to the septal wall and face the right and left aortic valve cusps. The anterior cusp faces the anterior surface of the heart and away from the aortic valve.

The pulmonary artery allows the blood to flow from the right ventricle through the pulmonic valve and to the lungs. The main pulmonary artery is about 3 cm wide and 5 cm (There are 2.54 cm in an inch). When it reaches the level of the aortic arch (about at the level of the 5th cervical vertebra) the artery then branches off into the right pulmonary artery (RPA) and left pulmonary artery (LPA) which travel to their perspective lungs.

Side Note: Actually the left pulmonary branch is a continuation of the main pulmonary trunk.

Oxygenated blood is returned to the left atrium by way of four pulmonary veins. They are named the right upper (or superior) right lower (or inferior) left upper( or superior) and left lower( or inferior). in concordance with which lung it came from and it’s position where it enters the left atrium.

(When you think of veins and arteries, people tend to think that arteries always carry oxygenated blood and veins always carry deoxygenated blood. This is not a true statement. Arteries carry blood away from the heart and veins carry blood toward the heart. The pulmonary ARTERY carries deoxygenated blood while the pulmonary VEINS carry oxygenated blood. Think Artery-Away to help you keep it straight.)

The blood flows from the pulmonary veins to the left atrium during ventricular systole. The left atrium, just like the right atrium, is smaller than the ventricles, more rounded in shape, with thinner less muscular walls. As the left atrium fills, it the pressure in the left atrium rises. When the pressure in the left atrium gets higher than the pressure in the left ventricle, the pressure pushes the mitral valve open and the blood is sucked into the left ventricle.

The mitral valve (or bicuspid valve) lies between the left atrium and the left ventricle. This makes the mitral valve an atrioventricular valve (or AV valve). It is comprised of two leaflets, the anterior mitral valve leaflet, which faces the anterior surface of the heart, and the posterior mitral valve leaflet, which faces the posterior surface of the heart. The anterior mitral valve leaflet is larger than the posterior mitral valve leaflet, and lies closer to the aortic valve. When you are performing your echos, use you aortic valve to help you get your bearings on which leaflet is anterior or posterior. Each leaflet has three scallops. On the anterior mitral valve leaflet they are named A1, A2,and A3. On the posterior mitral valve leaflet,they are named (you guessed it) P1,P2,and P3. There are commisures at the corners which makes the valve look like it is smiling when you view it from the ventricular side. As with the tricuspid valve, the mitral valve apparatus is made up of the leaflets, the annulus, the chordae tendenea, and two papillary muscles; the anteriolateral papillary muscle and the posteriomedial papillary muscle.

mitral valve 2

The left ventricle is ellipsoid in shape (or bullet shaped) and has much thicker, muscular walls than the right ventricle. Because the left ventricle is the main pump, the pressures on the left side of the heart are much higher than on the right side, allowing the left ventricle to pump the oxygenated blood throughout the body. As the left ventricle contracts, the pressure in the ventricle becomes higher than the pressure in the aorta, which will cause the aortic valve to open allowing the blood to be ejected into the aorta, to the rest of the body, in order to supply the cells with oxygen and nutrients. As with all the walls of the heart, the left ventricular walls are made up three layers, the endocardium, the myocardium and the epicardium.

The aortic valve is made up of 3 semilunar cusps. The right and left coronary cusps coincide with the right and left coronary arteries. The non coronary cusp does not have a coronary artery and rests next to the inter-atrial septum. Just past the cusps, the aorta widens to form a “root”. This aortic root is known as the sinuses of Valsalva. The right and left sinuses of Valsalva gives rise to the right and left coronary arteries. As the sinus of Valsalva narrows again, it joins the ascending aorta at what is known as the sinotubular junction. Past this junction marks the beginning of the ascending aorta.

sino (2)

After the blood circulates throughout the body, giving oxygen and nutrients to the cells and picking up the waste and carbon dioxide. It travels back to the right atrium by way of the inferior vena cava and superior vena cava, leaving our cycle complete.