Download Bio212-Mammalian Heart Dissection Instructions

Biology 212 - Mammalian Heart Dissection
Note: You are not required to hand in answers to the questions posed in this dissection
guide, but you will want to know the answers for lab practicum!
Procedures:
1. Place the heart in a dissecting pan & rinse off excess blood with tap water. Pat the
heart dry.
2. Locate the thin membrane, the pericardium, that covers the heart. (Note: some
specimens may not have intact pericardium). The pericardium (pericardial sac) is a
double-layered closed sac that surrounds the heart and anchors it. 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, inferior end) and the base (broad, superior end). Use
the diagram at the end of this handout to help orient the heart.
4. Make a slit in the pericardium near the apex and slide the heart out (your
pericardium may already be open with the heart exposed). Do not remove the
pericardium off the base of the heart yet.
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 top of the heart.
The pulmonary artery emerges from the right ventricle.
6. The vena cava and the pulmonary veins are embedded within fibrous tissue at the
base of the heart. They are thin walled and often collapsed, so are more difficult to see
than the arteries leaving the heart. The vena cava is a large diameter bluish or dark red
tube entering the right atrium. You may see both the anterior and posterior vena cava,
which will join before entering the right atrium, depending on how close to the heart
your vessels have been cut. The pulmonary veins enter the left atrium. Use your fingers
or a blunt probe in the vessels see where they lead to help you identify them (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 anterior (front) side is towards you
with the major blood vessels on the top and the apex down.
9. The heart is now in the pan in the position it would be in a body as you face the
body (if your sheep were standing up!). Locate the following chambers of the heart:
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, somewhat flimsy dark red chambers with flap-like extensions.
10. Relocate the great vessels at the base of the heart and make sure you had them
identified correctly:
Pulmonary artery - branches & carries blood to the lungs to receive oxygen. Curves out
of the right ventricle and splits, one artery going to each lung.
Aorta - Near the right atria & just behind the pulmonary artery (arteries). 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 (brachiocephalic artery).
Pulmonary veins - these vessels return oxygenated blood from the lungs to the left
atrium (they may be hard to find!). There are multiple pulmonary veins, each carrying
blood from different lobes of the lungs. Most often there are four (sometimes there are 3
or 5).
Inferior & superior vena cava - these two blood vessels are located on the right side of
the heart (your left) 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 where they enter the heart – blood flow is
not restricted.
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 whitish groove on the front of the heart.
It branches over the front & the back of the heart to supply 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 (cross section) cut through the aorta about 3 cm from where it
leaves the heart. Look down into the aorta and find the aortic semilunar valves. The
two openings for the coronary arteries lie at the base of the “pocket” of two of the
valves.
Internal Anatomy – Right Side of Heart:
13. Turn the heart over and look at its posterior (back) side. Locate the vena cava or its
entry hole in the right atrium.
Question#3: Does the vena cava have a valve at the opening to the right atrium?
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14. Insert your scissors into the vena cava and cut part way down into the right atrium.
DO NOT cut through to the ventricle. Though mostly smooth, the interior surface of the
atrium has areas of parallel ridges, the pectinate muscles, which facilitate chamber
emptying by exerting wringing compression. Find the right atrioventricular valve in the
opening between the right atrium and right ventricle. It is composed of triangular flaps
called “cusps” fastened to the inner walls of the right ventricle by strings, the chordae
tendineae, (“heart strings”) that attach to the walls of the left ventricle. 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 blow out the valves and squirt water on yourself!). You should
be able to see the valve balloon up, shutting the opening and preventing the return of
the water from ventricle to atrium.
16. Now cut longitudinally 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. You can pour some water
into the base of the pulmonary artery and watch the valve close.
17. Pour the water out of the heart and extend the cut started in the pulmonary artery
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 dissection cut should be above and
parallel to the groove of the coronary artery, but not on the artery.)
18. With your fingers, push open the heart at the cut to examine the internal structure.
19. Find the septum on the right side of the right ventricle. This thick muscular wall
separates the right & left ventricles from each other.
20. The inside of the right ventricle is slippery and shiny but has many ridges in a
disorganized arrangement. These are the the trabeculae carneae, “meaty shelves”.
They assist in ventricular emptying by exerting wringing compression.
Internal Anatomy – Left Side of Heart:
21. To examine the left side of the heart you will need to make a longitudinal cut along
the left ventricle and left atrium. Pick up the heart and rest it in your left hand, with the
right ventricle (already cut!) in your palm. You will now be looking at the left ventricle.
With scissors oriented along 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.
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).
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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.
(Some hearts may have distinct papillary muscles in their left ventricle as well) Papillary
muscles contract very early in the cardiac cycle and keep the valve flaps taut so they
won’t fall back into the atria.
Question #5: What would happen if the chordae tendineae were ruptured?
27. Take another look inside both ventricles.
Question #6: Compare and contrast the thickness of the right ventricular wall with the
thickness of the left ventricular wall. Why are they different?
28. Clean up according to the directions of your instructor.
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