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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.
| 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. |
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.
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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.
| 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 pericardial sinuses:
 | transverse pericardial sinus |
| oblique pericardial sinus |
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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.
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. |
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| 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. |
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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 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. |
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.
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| 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. |
When the heart is viewed from
the back, the most obvious structure lying in the coronary sulcus is the
coronary sinus.  |
Internal Structure of the Chambers of the
heart
| 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.
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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).
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| 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
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| 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.
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Heart Valve Positions
| . 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.
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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
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Innervation of the Heart
| 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.
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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.
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Potete
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