Heart
For this lecture you will want to consult the diagrams of the heart,
heart conduction system, cardiac veins, and coronary arteries on the Human
Anatomy web page. You may also wish to view other diagrams from the internet.
The heart is located in the middle mediastinum - from sternum to vertebral column
Borders of the heart: Note that the heart lies on top of the diaphragm with its apex (tip) pointing to the left and somewhat anteriorly. Try to visualize this position and you will see that the heart basically lies on its right ventricle.
1. Apex – tip of left ventricle. Points inferiorly, anteriorly, and to the left. Lies at the 5th intercostal space.
2. Left border – Mostly left ventricle with some left atrium at the upper end. From left 5th costochondral junction to left 2nd intercostal space.
3. Superior border – great vessels enter/leave. A line drawn from left 2nd intercostals space to 3rd right costal cartilage.
4. Base – mostly right atrium. Points superiorly, posteriorly, and to the right.
5. Right border – right atrium. A line drawn from right xiphisternal articulation to middle of right 3rd costal cartilage
6. Inferior border – right ventricle and some left ventricle
Auscultation – Listening to heart sounds
The cardiac notch leaves a part of the fibrous pericardium uncovered by lung tissue. This area, on the left anterior chest, is called the area of superficial cardiac dullness. This is because it yields a dull note upon percussion (tapping with the finger).
There are several areas on the anterior thoracic wall where a person may place a stethoscope in order to hear various heart sounds. We will present 4 of the more common areas. As we proceed through this lecture we will find that there are 4 valves associated with the heart. These are; aortic semilunar valve, pulmonary semilunar valve, right atrioventricular valve (tricuspid), and left atrioventricular (mitral or bicsupid) valve. The functions of each of these valves will be discussed shortly.
To listen to the aortic semilunar valve the stethoscope should be placed just to the right of the sternum in the second intercostal space.
To listen to the pulmonary semilunar valve the stethoscope should be placed just to the left of the sternum in the second inteercostal space.
To listen to the right atrioventricualr (tricuspid) valve the stethoscope should be placed just to the left of the sternum in the forth intercostal space.
To listen to the left atrioventricular (mitral ot bicuspid) valve the stethoscope should be placed approximately 3 inches to the left of the sternum in the forth intercostal space.
If we were to remove the “breast plate” of an individual we would not see the heart between the two lungs. We would see a very tough, fibrous sac that contains the heart. This sac is known as the pericardium. The
pericardium has two portions;
1. fibrous pericardium - made of heavy connective tissue. Anchors the heart, prevents overdistension, provides a protective membrane
2. serous pericardium – thin, double layer
a. parietal layer – directly beneath the fibrous pericardium
b. visceral layer – also called epicardium. Attached to the muscles of the heart.
There is a potential space between the two layers of the serous pericardium known as the pericardial cavbity. This cavity contains pericardial fluid which is an ultrafiltrate of plasma. The fluid prevents friction between the two layers.
Pericardial fluid can build up and become life threatening. Inflammation of the pericardium is known as pericarditis. Advanced pericarditis may cause cardiac tamponade which is heart failure due to compression. The compression affects the return of blood to the left atrium by squeezing off the pulmonary veins.
To truly understand the anatomy of the heart it is best to first look at the functions of the heart. The heart, as we all know, functions to pump blood. But blood comes in two forms, oxygenated and deoxygenated. Oxygenated blood has been pumped through the lungs to pick up oxygen and now must be pumped out to the tissues of the body. Deoxygenated blood has returned to the heart from the tissues of the body and must be pumped to the lungs where it can release carbon dioxide and bind more oxygen. This means that the heart is essentially two pumps, one for pumping oxygenated blood and one for pumping deoxygenated blood. To see an animation of the blood flow through the heart chambers click on the picture of the heart and lungs on the Human Anatomy opening page.
The human heart (and all mammalian hearts) consists of four chambers, 2 atria and 2 ventricles. Atria act as receiving chambers as blood returns to the heart. They then pump the blood into the ventricles which act as pumps to force the blood out to its target areas. A closer look at the walls of these chambers shows that they are well designed for their specific functions. Additionally there are two small structures on top of each atria known as auricles.
Let us take a tour through the heart. We will start as deoxygenated blood entering the right atrium. There are two major entrances for deoxygenated blood entering the right atrium, the superior vena cava and the inferior vena cava. There are also two smaller entrances, the coronary sinus and the anterior cardiac veins. The majority of blood entering the right atrium comes via the inferior vena cava. Once inside the right atrium we can look around and see several interesting features. The posterior wall of the right atrium appears to be very smooth whereas the anterior wall appears to have a rough texture. The smooth posterior wall is known as the sinus venarum. Embryologically this was venous tissue that fused with the pumping heart chambers (after all, we had to connect the veins to the heart somehow). The rough muscles that are found on the internal surface of the anterior wall of the right atrium are known as musculi pectinati or pectinate muscles. On the external surface of the atrium these two areas are separated by a groove known as the sulcus terminalis. Internally they are separated by a ridge known as the crista terminalis. The wall between the right and left atria is known as the interatrial septum. In the right atrium we will see a depression in this wall known as the fossa ovalis. This is a remnant of a fetal structure known as the foramen ovale. Remember that in the adult the right side of the heart is dedicated to pumping deoxygenated blood to the lungs. In the fetus there is little need to pump blood to the lungs because there isn’t any oxygen there. Mom was supplying all of the oxygen that you needed via the umbilical cord. To help bypass the lungs we had an opening between the right and left atria, the foramen ovale. When the right atrium contracted some of the blood passed down to the right ventricle, but the majority of it passed through the foramen ovale into the left atrium. The last visible structure that we see in our tour of the right atrium is the right atrioventricular valve, also known as the tricuspid valve. It is through this valve that blood must pass in order to enter the right ventricle. Either term is acceptable but I would suggest that you use the more anatomical term of right atrioventricular valve rather than tricuspid valve. Although this valve does have three cusps the onus is on you to remember which side of the heart that would be on. If you go with right atrioventricular valve it would be almost impossible to get confused, i.e., it is the valve that can be found on the right side of the heart separating the atrium and the ventricle. The trip from the right atrium to the right ventricle is a very short one, approximately 1 inch. Therefore the muscular wall of the right atrium does not have to be very thick to generate enough force to move the blood.
The next stop on our tour is the right ventricle. Inside the right ventricle we can see that all of the walls appear to be made of a lattice of muscular tissue. This is called trabeculae carnae which literally means a lattice of meat. Arising out of the trabecular walls of this ventricle we will see some small, smooth projections known as papillary muscles. Coming from the tips of the papillary muscles and connecting to the cusps of the right atrioventricualr valve we will see several cords called cordae tendineae. The papillary muscles act to pull down on the cusps of the valve, NOT to pull the valve open but rather to prevent the valve from being blown (prolapsing) back into the atrium during ventricular contraction. The wall between the right and left ventricles is known as the interventricluar septum. We will see that it has several important anatomical features. Spanning across from the interventricular septum to the right side (margin) of the right ventricle we will see a specialized portion of the trabeculae carnae known as the septomarginal trabeculae (septum to margin = septomarginal, get it?). We will revisit the septomarginal trabeculae when we discuss the electrical conduction system of the heart. The walls of the right ventricle are considerably thicker than the walls of the atria. This has to do with the fact that the right ventricle must pump blood through the entire pulmonary circulation (vessels of the lungs). The last feature that we see before leaving the right ventricle is a smooth area leaving the top of the ventricle. This area is the conus arteriosus. This area is smooth because it acts as a funneling area to lead the blood into the pulmonary trunk. At this point the blood has been “pumped” and any roughness along the walls would slow the flow of the blood. At the end of the conus arteriosus we will find the pulmonary semilunar valve. This valve functions to prevent the back flow of blood from the pulmonary trunk to the right ventricle.
The pulmonary trunk (also known as the pulmonary artery) divides into the right and left pulmonary arteries which carry DEOXYGENATED blood to the lungs. Note here that we have arteries with deoxygenated blood. ALL arteries carry blood away from the pumping chambers of the heart, but they do not ALL carry oxygenated blood. Once in the lungs the pulmonary arteries quickly branch to supply roughly 3000 miles worth of capillary surface. This is necessary for efficient exchange of gasses. These capillaries then begin to unite to eventually form four pulmonary veins. The pulmonary veins carry OXYGENATED blood to the left atrium. Remember that in fetal life we did not need to send blood to the vessels of the lungs so much of the blood bypassed the right ventricle by passing through the foramen ovale. Some blood from the right atrium did pass into the right ventricle. This blood does not need to pass to the lungs for oxygenation so there is another bypass mechanism in place. In fetal life there is a duct, the ductus arteriosus, that connects from the pulmonary trunk to the aortic arch. Shortly after birth this duct will constrict and become a structure known as the ligamentum arteriosum
The left atrium has very few features of great concern to us. The walls of the left atrium are smooth. If we look up into the left auricle we will see pectinate muscles. At the bottom of the left atrium we see the left atrioventricular valve, also known as the bicuspid or mitral valve. Although all of these names for this valve are correct to make your life easier I would suggest that you use the term “left atrioventricular valve.” This term is completely descriptive and leaves little room for error. On the other hand, if I ask you about the bicuspid valve what clues do you have, other than pure memorization, that this valve is on the left side of the heart between the atrium and ventricle? The same is true of the term “mitral valve.” How many of you know why this valve was called the mitral valve? Even if I told you the history of the meaning it still would not help you learn the location of the valve. When the left atrium contracts the blood is pumped into the left ventricle. This is a minimal distance, about 1 inch. As with the right atrium, the muscular wall of the left atrium is not very thick.
Our tour will end with the left ventricle. The left ventricle contains the same structures that were seen in the right ventricle: trabeculae carnae, papillary muscles, and cordae tendineae. The walls of the left ventricle are the thickest of any of the chambers of the heart. The left ventricle is responsible for pumping blood to the entire body. This is quite a distance and requires a lot of force. Blood leaving the left ventricle passes through the aortic semilunar valve to enter the aorta. The aorta is a very large elastic artery. When the blood is ejected from the left ventricle into the aorta the aorta stretches slightly, ie., it is pressurized. This increased pressure corresponds to the systolic portion of our blood pressure. The elastic aorta then “squeezes” back down. This helps to further propel the blood through the vessels. This pressure corresponds to the diastolic portion of our blood pressure. Blood passes upward in the very short ascending aorta before reaching the aortic arch. The aortic arch continues as the descending aorta. There are two branches of the ascending aorta: the right and left coronary arteries. These will be discussed shortly. There are three branches of the aortic arch. From proximal to distal these are: the brachiocephalic trunk, left common carotid artery, and the left subclavian artery.
The blood supply to the heart itself is via the two coronary vessels and their branches. The first branch of the ascending aorta is the right coronary artery. This artery circles around the right side of the heart in the groove between the right atrium and right ventricle. Along the way this artery sends muscular branches to the anterior surface of the right ventricle. Just before passing around the right edge (margin) of the heart there is a branch known as the marginal branch that supplies the right margin of the right ventricle. The right coronary artery then circles around to the posterior surface of the heart where it joins another artery, the circumflex coronary artery, to form the posterior interventricular artery. The second branch of the ascending aorta is the left coronary artery. This vessel is only about 1 inch long. The left coronary artery divides to form the anterior interventricular artery and the circumflex coronary artery. The anterior interventricular artery, as its name implies travels along the front of the heart overlying the interventricular septum. From here it sends muscular branches to the anterior surfaces of both ventricles. The circumflex coronary artery travels around the left side of the heart and usually joins with the right coronary artery to form the posterior interventricular artery. Along the way the circumflex coronary artery sends off various muscular branches to the left ventricle. There are other vessels associated with the heart but we will not discuss them at our level of anatomy.
The venous drainage from the heart tissue travels through various cardiac veins. All of the venous drainage from the heart tissue drains into the right atrium, as does all venous blood from the rest of the body as previously described. The majority of the anterior heart and the left portion of the left ventricle drain into the great cardiac vein. This vein then circles around the left side of the heart and heads toward the right atrium where it will empty into the coronary sinus. The coronary sinus in turn empties into the right atrium. As it circles around the left margin of the heart the great cardiac vein is joined by the marginal vein. The small cardiac vein drains the right margin of the right ventricle and the circles around the right margin of the heart to join the great cardiac vein immediately before it empties into the coronary sinus. The posterior side of the heart is drained by the middle cardiac vein which also drains into the great cardiac vein. The upper portion of the right ventricle of the heart does not drain toward the coronary sinus but rather drains directly into the right atrium via 2 or 3 anterior cardiac veins.
Like all muscle cells, contraction of cardiac muscle involves an electrical depolarization of the cellular membrane which results in muscle contraction. The read out of this electrical activity is known as an EKG.
To understand what the various deflections of an EKG represent it is necessary to understand the electrical wiring of the heart.
In the upper right region of the right atrium there is a small collection of cardiac muscle cells known as the sinoatrial (SA) node. These cells are designed to have a slow leak of potassium ions. When enough ions have leaked from the cell the electrical balance of the membrane has shifted enough that we see a depolarization of the membrane. This is followed by a contraction of the cells which, through some physiology that we will not discuss, resets the membrane potential. Because of this well timed leaking and subsequent “firing” of the cells in the sinoatrial node it is often referred to as the pace maker of the heart. The wave of depolarization follows across the atria following internodal tracts. This causes the atria to contract from the top down thus forcing the blood through the atrioventricular valves into the ventricles. In the lower left wall of the right atrium we have another specialized collection of cardiac cells known as the atrioventricular (AV) node. This node acts when it is stimulated by the depolarization reaching it from the internodal tracts. At this point we have to change the scheme of things. We CANNOT simply continue the depolarization from the top down because blood is ejected from the top of the ventricles. This means that we must have a mechanism whereby the electrical impulses can travel down to the apex of the heart and thus cause contraction from the bottom up. This is accomplished by running a special band of conductive fibers from the AV node over to the interventricular septum. This is known as the interventricular bundle (Bundle of His). Once in the interventricular septum the fibers split into right and left bundle branches which will carry the impulse down around the apex of the ventricles and back up around the margins of the heart. Here we will see smaller branches penetrating into the cardiac tissue. These branches are known as conduction myofibrils (Purkinje fibers).
In the right ventricle we see another specialization that was previously talked about, the septomarginal trabeculae, or moderator band. If the right ventricle were to contract equally upward from the bottom the major force of the blood would be directed directly against the right atrioventricular valve. This would no doubt result in a prolapsed valve. To direct the main flow off blood away from the valve and into the conus arteriosus we need the right wall of the right ventricle to begin contracting slightly before the rest of the ventricle. This is accomplished by sending some of the electrical impulse through a “shortcut” over to the right wall. This shortcut is the septomarginal trabeculae.
For the following information your assignment is to find an EKG (electrocardiogram) on the internet and identify the following and tell what each represents.
P wave
QRS complex
T wave
P-R interval
ST segment
The following changes in an EKG indicate some form of pathology. Can you explain these?
P wave – upward deflection – depolarization of atrial fibers through stimulation of the sinoatrial node. Missing or abnormal P wave indicates dysfunction of the sinoatrial node.
P-R interval – prolonged interval suggests a conduction delay in the atrioventricular node.
QRS Complex – upward deflection – depolarization of the ventricles. When abnormal generally indicates ventricular problems. An enlarged R spike usually means enlarged ventricles.
ST Segment – is depressed when the heart receives insufficient oxygen and is elevated in acute myocardial infarction.
T Wave – ventricular repolarization – altered T waves my mean an arteriosclerotic heart.
1. patent foramen ovale. This condition occurs when the foramen ovale fails to close after birth. Deoxygenated blood from the right atrium dilutes the oxygenated blood in the left atrium.
2. interventricular septal defect. In this condition there is an opening in the wall between the two ventricles. This results in deoxygenated blood from the right ventricle diluting the oxygenated blood of the left ventricle.
3. patent ductus arteriosus. In this case the ductus arteriosus fails to atrophy and become a ligamentum arteriosum. Again deoxygenated blood dilutes oxygenated blood. This time at the aortic arch.
4. tetrology of Fallot . This condition is a series of 4 different birth defects, hence the term tetrology. We will only consider one of these defects here. In this defect the aorta arises from both the right and left ventricles. This results in an extreme mixing of deoxygenated and oxygenated blood.