The Heart

Reading resources: Moore, Clinical Anatomy
pp 54 - 67

Objectives:
After this laboratory you should be able to:
1. Identify the great vessels of the heart
2. Identify the pericardial sac and describe its components
3. Demonstate the oblique and transverse pericardial sinuses
4. Identify the following arteries of the heart: right coronary,
    left coronary, right marginal, anterior
    interventricular (anterior descending), posterior interventricular,
    circumflex
5. Identify the following veins of the heart: great cardiac,
    coronary sinus, middle cardiac, anterior
    cardiac, small cardiac
6. Identify the characteristic anatomical features of the four
    chambers of the heart.
7. Describe the surface projection of the heart on the anterior thoracic wall.
8. Name the components of the conducting system
9. Compare the route of blood flow through the adult and fetal heart (Netter, 217)
 

The pericardial sac has been exposed in the previous dissection of the thorax. Make transverse incisions superiorly and inferiorly. Connect these incisions by a vertical incision. Extend these incisions to completely expose the heart. Note the components of the pericardium: visceral pericardium, parietal pericardium, fibrous pericardium, serous pericardium.
 
Identify the transverse sinus, behind the aorta and pulmonary artery. Identify the oblique sinus posterior to the heart. It is formed by the reflection of parietal pericardium onto the heart as visceral pericardium (epicardium).

Note the surface projection of the heart. The surface projection of the apex of the heart is in the 5th intercostal space in the midclavicular line. The left border is indicated by a line drawn from the apex to the 2nd intercostal space, a little more than 1 cm from the sternum. The right border of the heart can be drawn from the 3rd costal cartilage to the 6th costal cartilage. The inferior border passes horizontally from this point across the xiphisternal junction.

 Identify the great vessels of heart

Heart 1. Brachiocephalic  Trunk 2. Left Common Carotid 3. Left Subclavian Artery 4. Vagus 5. Arch of Aorta 6. Left Pulmonary Vein 7. Left Ventricle 8. Right Ventricle 9. Right Atrium 10. Superior Vena Cava

Before removing the heart identify in situ the following structures: aorta, pulmonary trunk, ligamentum arteriosum, left atrium, right atrium, left and right ventricles, apex, superior and inferior vena cava, atrioventricular (coronary) groove, anterior interventricular sulcus (Netter, 201).
 
Removal of the heart:
Identify the ascending aorta and pulmonary trunk. Transect these at the level of the transverse sinus. Cut the superior vena cava 2 cm above the right atrium. Note the azygos vein as it enters the superior vena cava. Cut the inferior vena cava as low as possible within the pericardial sac. Next identify the four pulmonary veins and cut them about a cm away from the left atrium. Finally cut through the pericardium behind the great vessels and remove the heart.
 
Blood Supply of the heart
Identify the right coronary and left coronary arteries (Netter, 204). Note that they arise from the aorta distal to two of the cusps of the aortic valve, the coronary cusps. Clean the coronary arteries. Identify the following branches of the right coronary: right marginal, posterior interventricular, and the artery to the sinoatrial node. Identify the following branches of the left coronary: anterior interventricular (LAD) and the circumflex branch.
 
On the posterior surface of the heart identify the coronary sinus (Netter, 204). The small cardiac vein, the middle cardiac vein and the great cardiac vein all drain into the coronary sinus. Anterior cardiac veins drain directly into the right atrium from the margin of the right ventricle.
 
Interior of the heart
Using the attached diagrams, make incisions to open the four chambers of the heart.
1. Right Atrium
Open the right atrium by making a vertical incision to the leftof the atrioventricular groove, from the auricle to the inferior margin. Make transverse cuts at the upper and lower ends of the vertical incision. Extend the transverse incisions to the right margin of the atrium. Clean out remnants of fixed blood from the chamber and identify the following (Netter, 208):
 
a.) crista terminalis demarcating the rough area from the smooth atrial area
b.) musculi pectinati - muscular ridges
c.) the smooth portion, representing the embryonic sinus venosus, contains the openings of
     the superior vena cava, inferior vena cava and the coronary sinus.
d.) fossa ovalis, limited superiorly by a sharp curved ridge (limbus fossa ovalis) and inferiorly by
     the valve of the inferior vena cava.
e.) the atrioventricular orifice
f.)  the location of the sinuatrial node. The SA node is a small collection of specialized conducting
     tissue located in the wall or the right atrium at the upper end of the crista terminalis on the right
     of the opening of the superior vena cava. If you can't identify the tissue at least determine its
     location.
g.) the location of the atrio-ventricular node (AV node) at the antero-inferior part of the interatrial
     septum immediately above the opening of the coronary sinus. Again, you may not be able to
     identify the tissue, but should be able to determine the location.
 
2. Right Ventricle
Make an incision in the ventricular wall on the right side of the anterior interventricular groove, extending from the root of the pulmonary trunk to the inferior broder. Make another incision on the left side of the atrioventricular groove extending from the root of the pulmonary trunk to the inferior border. Connect the upper ends of these incisions by a transverse incision. Reflect the anterior wall of the right ventricle downwards to expose the contents of the right ventricle. If necessary, remove the flap by cutting through the lower ends of it.
 
Identify the following features of the right ventricle (Netter 208):
a. note the greater thickness of the ventricle wall compared to the atrial wall
b. muscular ridges called trabeculae carneae (note the prominent moderator band, the
    septomarginal trabecula)
c. papillary muscles
d. the smooth upper portion known as the infundibulum
e. the atrioventricular valve (tricuspid valve).
f. chordae tendinae
g. the pulmonary valves
 
3. Left Ventricle
Open the cavity of the left ventricle by two incisions, the first parallel to the interventricular groove, and the second along the inferior margin of the heart.
 
Identify the following features (Netter 209):
a. the greater thickness of the left ventricular wall than that of the right
b. the trabeculae carneae
c. the anterior and posterior papillary muscles
d. the mitral valve. Note the two cusps, anterior and posterior
e. the chordae tendinae
f. the aortic valve, coronary vs non-coronary cusps
g. Note that the postero-superior part of the interventricular septum is membranous.
 
4. Left Atrium
Open the left atrium by a vertical incision midway between the right and left sets of pulmonary veins. Identify the following features (Netter 209):
a. the smooth interior of the left atrium
b. the openings of the four pulmonary veins
c. the atrioventricular orifice
 
Review questons:
1. What is cardiac tamponade?
2. What is the oblique sinus?
3. What is the transverse sinus?
4. What are the locations of the cell bodies of efferent (motor) and afferent (sensory) fibers of the
    phrenic nerve?
5. What nerves supply the heart?
6. What is the septomarginal (moderator) band?
7. What is the 'pacemaker' of the heart?
8. Trace an impulse from the pacemaker of the heart to the apex of the left ventricle, naming the
    components of the conduction system through which it passes
9. Why does a heart attack sometimes produce pain along the medial side of the arm?
 
Answers:
1. Compression of the heart as a result of accumulation of fluid (eg. bleeding) in the pericardial
    sac. This rapidly becomes life threatening as it prevents normal function of the heart.
 
2. The oblique sinus is part of the pericardial sac posterior to the heart. It is a blind pouch formed
    by the reflection of the parietal pericardium to the visceral pericardium around the pulmonary
    veins and the venae cavae

3. The transverse sinus is part of the pericardial sac located between the atria and the great
    vessels (aorta and pulmonary trunk).
 
4. Cell bodies of efferent fibers to the diaphragm are located in the 3rd, 4th and 5th segments of
    the cervical spinal cord. Afferent fibers (from the diaphragm and mediastinum) have their cell
    bodies in the dorsal root ganglia of the 3rd, 4th and 5th spinal nerves
 
5. cardiac branches of the vagus and of the sympathetic trunk.
 
6. A prominent band of muscular tissue in the right ventricle. It carries a branch of the AV bundle,
    part of the conducting system of the heart.
 
7. The sinuatrial node
 
8. SA node, muscle of atrium, AV node, AV bundle (bundle of His), left bundle branch, Purkinje
    fibers of left ventricle to apex.
 
9. This is 'referred pain'. Afferent fibers from the heart enter the spinal cord at the same level as
    those from the medial side of the arm. However, as far as I know the phenomenon is not
    completely understood.