Download Heart dissection

Mammalian Heart Dissection
Procedures:
1. Place the heart in a dissecting pan & rinse off the excess blood with tap water. Pat the heart
dry.
2. Examine the heart and locate the thin membrane or pericardium that still covers the heart.
(Note: some specimens may not have intact pericardium). The pericardium or pericardial sac is a
double-layered closed sac that surrounds the heart and anchors it. The pericardium consists of
two tissues layers - the visceral pericardium that covers the surface of the heart & the parietal
pericardium covering the inner surface of the parietal sac. These two tissue layers are continuous
with each other where the vessels enter or leave the heart. The slender gap between the parietal
& visceral surfaces is the pericardial cavity & is filled with fluid to reduce friction between the
layers as the heart pumps.
3. Determine the apex (pointed, ventral end) and the base (broad, dorsal end). Only the left
ventricle extends all the way to the apex.
4. If your heart is enclosed in pericardium, make a slit near the apex and slide the heart out. Do
not remove the pericardium off the base of the heart yet to avoid cutting through the great
vessels.
5. At the base of the heart the pericardium blends with the coverings of the great vessels. The
aorta and pulmonary artery are the two largest vessels. Both have thick rubbery white or gray
walls. The aorta emerges from the center of the base of the heart. The pulmonary artery
emerges from the right ventricle.
6. The vena cava and the pulmonary veins are also embedded within the fibrous tissue, but
they are thinner walled and often collapsed, so are more difficult to see. The vena cava is a thinwalled, but large diameter bluish or dark red tube. You may see both the anterior and posterior
vena cava, which will join and enter the right atrium. The pulmonary veins enter the left atrium.
Use your fingers or a blunt probe to test openings and see where they lead to help you identify
the great vessels (be gentle so you do not break any valves). After identifying the great vessels,
carefully remove the remainder of the pericardium from the base of the heart.
7. Gently squeeze the heart and see if you can determine the left side from the right side. The
left ventricle (which extends all the way to the apex) is thicker.
Question #1: Which ventricle pumps blood to the body?
8. Place the heart in the dissecting pan so that the ventral (front) side is towards you with the
major blood vessels on the top and the apex down. The left ventricle should be on your right
side. The front of the heart can also be recognized by a groove that extends from the right side
of the broad end of the heart diagonally to a point above and to your left of the apex.
9. The heart is now in the pan in the position it would be in a body as you face the body. Locate
the following chambers of the heart from this surface:
Left atria - upper chamber to your right
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Left ventricle - lower chamber to your right
Right atria - upper chamber to your left
Right ventricle - lower chamber to your left
The atriums are wrinkled, darker red chambers with flap-like extensions.
10. While the heart is still in this position in the dissecting pan, relocate the great vessels at the
base of the heart and make sure you had them identified correctly:
Pulmonary artery - this blood vessel branches & carries blood to the lungs to receive oxygen &
can be found curving out of the right ventricle.
Aorta - major vessel located near the right atria & just behind the pulmonary arteries to the
lungs. Locate the curved part of this vessel known as the aortic arch. Branching from the aortic
arch is a large artery that supplies blood to the upper body.
Pulmonary veins - these vessels return oxygenated blood from the right & left lungs to the left
atrium.
Inferior & superior vena cava - these two blood vessels are located on your left of the heart
and connect to the right atrium. Deoxygenated blood from the body enters the heart through
these vessels. Use your probe to feel down into the right atrium. These vessels do not contain
valves to control blood flow.
Question #2: How does the diameter of the vena cava compare to the diameter of the aorta?
Which vessel has thicker walls? Can you make an inference about blood volume, pressure or
velocity from your observations?
11. Locate the coronary artery which lies in the groove on the front of the heart & it branches
over the front & the back side of the heart to supply fresh blood with oxygen & nutrients to the
heart muscle itself. You may need to scrape away some fat to see it clearly.
IMPORTANT: At this point, check with your instructor to demonstrate that you have correctly
identified the major vessels, R & L atria and R & L ventricles BEFORE you begin cutting.
12. Make a transverse cut through the aorta about 3 cm from where it leaves the heart. Look
down into the aorta and find the aortic semilunar valves. There are two openings for coronary
arteries at the base of the “pocket” of two of the valves. Insert a blunt probe into each opening
and observe where it appears on the outside of the heart.
Internal Anatomy – Right Side of Heart:
13. Turn the heart over and look at its posterior side. Locate the vena cava or its entry hole in the
right atrium.
Question: Does the vena cava have a valve at the opening to the right atrium?
14. Insert your scissors into the vena cava and cut part way into the right atrium. DO NOT cut
all the way through to the ventricle. Find the right atrioventricular valve placed in the opening
between the right atrium and right ventricle. It is composed of triangular flaps called “cusps”
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fastened to the inner walls of the right ventricle by strings, chordae tendineae, (“heart stings”)
that attach it to papillary muscles of the heart. There are three flaps so this valve is also called the
tricuspid valve.
15. Fill the right atrium with water and allow it to trickle down into the right ventricle. Now
GENTLY squeeze the right ventricle from the bottom up (BE CAREFUL - if you squeeze too
hard you will squirt water and blood on yourself). You should be able to see the valve balloon
up into the opening, shutting the opening and preventing the return of the water to the atrium.
16. Now cut through the wall of the anterior side of the pulmonary artery until you can see the
pulmonary semilunar valve which controls blood flow away from the right ventricle at the
entrance to the pulmonary artery. Pour some water into the base of the pulmonary artery and
watch the valve close.
Question #3: Compare and contrast the closure of the semilunar valves with the closure of the
atrioventricular valves.
17. Pour the water out of the heart and extend the cut started in the pulmonary artery and
continue cutting down into the wall of the right ventricle to its apex. Be careful to cut just deep
enough to go through the wall of the heart chamber. (Your cutting line should be above &
parallel to the groove of the coronary artery.)
18. With your fingers, push open the heart at the cut to examine the internal structure. If there is
dried blood inside the chambers, rinse out the heart.
19. Find the septum on the right side of the right ventricle. This thick muscular wall separates the
right & left pumping ventricles from each other.
20. The inside of the right ventricle is slippery and shiny but has many ridges in a crossing and
disorganized arrangement. These are the pectinate muscles, which facilitate chamber emptying
by exerting a wringing compression.
21. The inside of the ventricle also has crossed ridges of muscle bands. These are called the
trabeculae carneae, “meaty shelves”. They assist in ventricular emptying.
Internal Anatomy – Left Side of Heart:
22. To examine the left side of the heart you will need to make a cut lengthwise along the left
ventricle and left atrium. Once again place the heart on the dissecting tray and position it with
the ventral side up (left ventricle on your right), with the base at the top and the apex pointed
down. You will want to cut along the outer border of the left side of the heart by following the
procedure below.
23. Pick up the heart and rest it in your left hand, with the right ventricle in your palm. You will
now be looking at the left ventricle. With a scissor oriented with the length of the heart, cut into
the left ventricle wall. Remember this wall is thick so the cut must be deep. Once you are
through the ventricular wall, extend the cut down to the apex, and then up to the base of the
heart, through the atrium.
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24. Examine the left atrium. Find the openings of the pulmonary veins from the lungs.
Question #4: Is there a valve between the pulmonary veins and the left atrium?
25. Observe the left atrioventricular valve placed in the opening between the left atrium and
left ventricle. There are two flaps so this valve is also called the bicuspid valve or the mitral
valve.
26. Notice that the chordae tendineae are attached to mounds of cardiac muscle protruding from
the inner surface of the ventricle. These are the papillary muscles. Papillary muscles contract
very early in the cardiac cycle and keep the valve flaps taut so they cannot prolapse into the atria.
Question #5: What would happen if the chordae tendineae were ruptured?
27. Compare the inside of the left ventricle with the right ventricle.
Question #6: Compare and contrast the thickness of the right ventricular wall with the thickness
of the left ventricular wall. Also compare the shape of the right ventricular chamber with the
shape of the left ventricular chamber.
28. Clean up according to the directions of your instructor.
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