Anterior Mediastinum

Before studying the heart, I would just like to mention the anterior mediastinum and what its contents are. This part of the mediastinum contains connective tissue and fat, as well as, a few blood vessels, maybe a lymph node or two and, sometimes, the lower end of what used to be the thymus. It also contains the anterior folds of the pleura, the costomediastinal folds.

The Pericardium

The heart and its pericardium make up the contents of the middle mediastinum. The left and right phrenic nerves and their adjacent arteries (pericardiacophrenic) lie to the left and right of the pericardium and anterior to the roots of the lungs.
A diagram of the pericardium and its reflections

As you can see, the parietal and visceral pericardium are continuous. This continuity takes place at the points where the major blood vessels enter and leave the heart.

You should also be aware that the parietal pericardium has two inseparable parts, an outer fibrous part and an inner smooth part, the serous part.

The potential space between the visceral and serous parietal pericardium is the pericardial cavity. I call this a potential space because in life there is only a single layer of fluid between the two layers.

In clinical cases when air is introduced into the pericardial cavity you have what is known as a pneumopericardium. This might occur in penetrated wounds to the thorax.

When blood fills this space, you have a hemopericardium. Since the pericardial cavity is a closed space and if it is filled with blood, the heart can no longer work and the condition is fatal if not recognized and treated. The term for this syndrome is cardiac tamponade. This happens after long term cardiac pathology when the walls become very thin and weak. The heart virtually blows out and enters the pericardial cavity.

Pericardial Sinuses

Within the pericardial cavity, at the points where the visceral and parietal pericardia are continuous with one another, small chambers or sinuses are located. In this diagram, the heart has been removed and you are looking toward the posterior wall of the pericardial cavity. Although not labeled, you should be able to identify the superior and inferior venae cavae, the left and right pulmonary veins and the ascending aorta and pulmonary trunk.
The pericardial sinuses:
  • transverse pericardial sinus
  • oblique pericardial sinus

For those of you who are studying the cadaver, the transverse pericardial sinus can easily be reached by sticking your finger between the superior vena cava and the ascending aorta and pulmonary trunk. This sinus is a leftover from heart development in the embryo.

Again, for those of you who are studying the cadaver, and the heart is still in place, slide two or three fingers under and behind the heart until they reach a dead end. Your fingers are now in the oblique pericardial sinus.

The Heart

For those of you studying the cadaver heart, the problem of orientation invariably crops up. How can you tell the anterior from the posterior heart surfaces? My technique may not work for everyone but what I do is first identify two small ruffled appendages of the left and right auricles. Once I can identify these, I then know that they will always point to the front of anterior part of the heart. These cannot be observed from the back. Once you are oriented, take a look at the various surfaces and borders of the heart.
The anterior surface of the heart is also known as the sternocostal surface for obvious reasons. Notice the ruffled edges of the left (LA) and right (RA) atria. These are the ones to use for orientation. Since we are looking at the anterior surface of the heart, they can be seen.

Note that the anterior surface shows parts of each of the four chambers of the heart:

  • right atrium (RA)
  • left atrium (LA)
  • right ventricle (RV)
  • left ventricle (LV)

Also note the three borders of the heart:

  • right border (1) made up of the right atrium
  • inferior border (2) made up of right atrium, right ventricle and left ventricle
  • left border (3) made up of the left ventricle
When you view the heart in a radiograph, these borders will be obvious.

Finally learn the great vessels of the heart and their relationships to one another. I usually learn the vessels from the view of blood circulating through the heart, starting with the return of blood from the body to the heart.

  • superior and inferior venae cavae
  • pulmonary trunk and left and right branches
  • pulmonary veins (usually 4 in number but this can vary
  • ascending aorta

The last item to identify is the remains of the embryonic connection between the pulmonary trunk and aortic arch, the ligamentum arteriosum. At one stage in development, the lungs were not functional so the blood was shunted into the arterial system at this site. Oxygen exchange in the embryo occurred at the placenta and not the lungs.

The adjacent figure shows the posterior aspect of the heart. Its identifiable features include:
  • right atrium (RA)
  • left atrium (LA)
  • right ventricle (RV)
  • left ventricle (LV). Notice that most of the left ventricle is posterior.

The left and right ventricles make up the diaphragmatic surface of the heart. This part rests on the fibrous part of the diaphragm.

The left atrium makes up the so-called base of the heart. When the body is in the supine position (lying on its back), the heart rests on its base and the apex of the heart (the tip of the left ventricle) projects up and to the left. The same three borders are seen from the back of the heart:

  • right (1)
  • inferior (2)
  • left (3)
When the vessels are removed from the heart, certain sulci (grooves) can be seen and separated the various chambers of the heart.
From the anterior view of the heart, the anterior interventricular and coronary sulci can be seen (the darker brown areas). The anterior interventricular sulcus separates the right and left ventricles. The anterior part of the coronary sulcus separates the right atrium from the right ventricle.
From the posterior view of the heart, the posterior part of the coronary sulcus and the posterior interventricular sulcus can be seen. From this view, the coronary sulcus can be seen to separate the left and right atria from the left and right ventricles. The posterior interventricular sulcus separated the right ventricle from the left ventricle and if followed inferiorly, it can be seen to be almost continuous with the anterior interventricular sulcus.

Coronary Arteries and Cardiac Veins

The heart muscle is supplied by the coronary arteries which are direct branches of the ascending aorta, so the heart muscle gets the freshest blood possible. The heart muscle is drained by the cardiac veins. Most of the venous drainage is by was of the coronary sinus into the right atrium. A small amount of blood drains directly into the right atrium by way of the anterior cardiac veins.
The heart is supplied by two major coronary arteries, the right and left.

The left coronary divides into the anterior interventricular and circumflex branches almost immediately after it arises from the left side of the ascending aorta. The anterior interventricular lies in the anterior interventricular sulcus and is also known as the left anterior descending artery. The circumflex branch lies in the coronary sulcus and forms an anastomosis with the right coronary in the posterior part of this sulcus.

The anterior interventricular artery is the one most often involved in coronary occlusions and is often the one that is bypassed in bypass cardiac surgery.

The right coronary lies in the coronary sulcus and gives rise to an important branch immediately after leaving the ascending aorta. This is the anterior right atrial branch which gives rise to the important nodal artery. This artery supplies the sinoatrial node (SA node) or pacemaker of the heart. When this vessels loses its ability to supply the node, a person usually needs to have a pace maker placed in their thoracic wall to take the place of the original pace maker.

The right coronary continues in the coronary sulcus, giving a branch along the right inferior border of the heart called the marginal artery.

Finally the right coronary gives rise to the posterior interventricular (or descending) branch, and then anastomoses with the circumflex artery from the left coronary.

When the heart is viewed from the back, the most obvious structure lying in the coronary sulcus is the coronary sinus. This sinus receives most of the venous blood from the heart and empties into the right atrium. Its tributaries are the small cardiac vein, the middle cardiac vein and the greater cardiac vein. There is a small vein that arises along the left side of the left atrium just beneath the lower left pulmonary artery (called the oblique vein. This vein is a remnant of the embryonic left superior vena cava.

The arteries seen in the back of the heart are the circumflex coronary artery, the terminal part of the right coronary artery and its posterior interventricular branch.

Internal Structure of the Chambers of the heart

The right atrium has a forward extension into its auricle. This space is lined by ridges of muscle called pectinate muscles and are not shown in the diagram.

Starting with the right atrium, the internal structures are:

  • fossa ovalis
  • openings of the superior (SVC) and inferior (IVC) venae cavae and the coronary sinus opening (CS) The entrance of the inferior vena cava and coronary sinus may be covered with small valve leaflets (valve of the interior vena cava and valve of coronary sinus).
  • The SA node is located at the junction or the superior vena cava with the right atrium. The AV node is located in the lower part of the interatrial septum near the opening of the coronary sinus.
The right atrium and right ventricle communicate with each other by way of the tricuspid valve. As the name implies, it has three leaflets. As you observe the chamber of the right atrium, notice the following:
  • chordae tendineae, attaching the free border of the valve cusps (leaflets) to either papillary muscles (PM) or directly to the wall of the heart chamber. Papillary muscles are only found in the ventricles of the heart.
  • the rough lining of the ventricular wall is called trabeculae carneae, because of their meaty appearance
  • the anterior papillary that has an attachment to the interventricular wall known as the septomarginal trabecula or the moderator band.
Blood leaves the right ventricle and passes through the pulmonary trunk to the lungs. Oxygenated blood returns to the left atrium of the heart from the lung through the pulmonary veins. The left atrium doesn't have much to talk about. There is an extension into the small auricles which have pectinate muscles in its walls.

The left atrium communicates with the left ventricle through the mitral or bicuspid valve. Just as in the right ventricle, the valve cusps or leaflets connect to the papillary muscles (PM) by way of chordae tendineae. The inner walls of the left ventricle is thrown into folds of trabeculae carneae just as in the right ventricle. Note, in particular that the left ventricular has a much thicker wall than the right ventricle. If the heart is not too diseased, this is how you can tell the difference between the two ventricles.

Note the interventricular septum (IVS) between the two ventricles.

Blood leaves the left ventricle through the ascending aorta and is then sent to body organs and tissues.

Heart Valve Positions

This diagram is a special dissection that shows the four heart valves and their relationship to one another. The view is from the top after the left and right atria have been removed.

Start with the right atrioventricular valve (tricuspid valve). It has an anterior (A), posterior (P) and septal (S) cusp.

The left atrioventricular valve (mitral valve) has an anterior (A) and a posterior (P) cusp.

The pulmonary and aortic valves are both tricuspid. During embryonic development, these two vessels were one. With rotation of the heart and a separation of the single channel, the pulmonary trunk ends up anterior and the ascending aorta ends up posterior. The original contained four primitive valve cusps, an anterior, left and right and a posterior. The left and right valves were divided during the separation so that a left and right ended up in both the pulmonary (or anterior channel) and the ascending aorta (or posterior channel). In the adult, this development results in an anterior displaced pulmonary trunk with an anterior and a left and right cusp, while the posterior displaced ascending aorta has a left and right coronary cusp and a posterior cusp. The coronary cusps are named because the origins of the left and right coronary arteries are found lateral to these cusps.

Innervation of the Heart

The strength and frequency of the heart beat is controlled by the autonomic nervous system. Both parasympathetic and sympathetic parts of the autonomic nervous system are involved in the control of the heart.

The sympathetic fibers arise from segments T2-T4 of the spinal cord and are distributed through the middle cervical and cervico-thoracic (or stellate) ganglia and the first four ganglia of the thoracic sympathetic chain. The sympathetic fibers pass into the cardiac plexus and from there to the SA node and the cardiac muscle. The effect of the sympathetic nerves at the SA node is an increase in heart rate. The effect on the muscle is an increase in rise of pressure within the ventricle, thus increasing stroke volume.

The vagus provides the parasympathetic control to the heart. The effect of the vagus at the SA node is the opposite of the sympathetic nerves, it decreases the heart rate. It also decreases the excitability of the junctional tissue around the AV node and this results in slower transmission. Strong vagal stimulation here may produce AV block.

The heart also has an internal nervous system made up of the SA (sinuatrial node) and the AV (atrioventricular) node. The AV bundle (His) leaves the AV node near the lower part of the interatrial septum and splits over the upper part of the interventricular septum into a left bundle branch (LBB) and a right bundle branch (RBB). The cardiac muscle is then supplied by branches of the two bundles.
  • SVC - superior vena cava
  • IVC - inferior vena cava
  • AO - aorta
  • RA - right atrium
  • RV - right ventricle
  • LV - left ventricle
  • CS - opening of the coronary sinus

Pleura   Posterior Mediastinum

Table of Contents for Thorax
Practice Examination