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Sheep Heart Dissection
Sheep have a four-chambered heart, just like humans. By studying the anatomy of a sheep's heart, you can learn
about how your own heart pumps blood through your body and keeps you alive! Use this as a dissection guide
complete enough for a high school lab, or just look at the labeled images to get an idea of what the heart looks
like. If you do the dissection yourself, you will need a preserved sheep heart.
Observation: External Anatomy
Most heart diagrams show the left atrium and ventricle on the right side of the diagram. Imagine the heart in the
body of a person facing you. The left side of their heart is on their left, but since you are facing them, it is on
your right.
1. Identify the right and left sides of the heart. Look closely and on one side
you will see a diagonal line of blood vessels that divide the heart. The half that
includes all of the apex (pointed end) of the heart is the left side. Confirm this
by squeezing each half of the heart. The left half will feel much firmer and
more muscular than the right side. (The left side of the heart is stronger
because it has to pump blood to the whole body. The right side only pumps
blood to the lungs.)
2. Turn the heart so that the right side is on your right, as if it were in your
body. Examine the flaps of darker tissue on the top of the heart. These ear-like
flaps are called auricles. Find the large opening at the top of the heart next to
the right auricle. This is the opening to the superior vena cava, which brings
blood from the top half of the body to the right atrium (the atria are the top
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chambers in the heart). Stick a probe down this vessel. You should feel it open
into the right atrium. A little down and to the left of the superior vena cava there is another blood vessel
opening. Insert your probe into this; it should also lead into the right atrium. This is the inferior vena cava,
which brings blood from the lower tissues. You can also see another blood vessel next to the left auricle. This is
a pulmonary vein that brings blood from the lungs into the left atrium.
3. Sticking straight up from the center of the heart is the largest blood vessel
you will see. This is the aorta, which takes oxygenated blood from the left
ventricle to the rest of the body (the ventricles are the lower chambers of the
heart). The aorta branches into more than one artery right after it leaves the
heart, so it may have more than one opening on your heart specimen. Look
carefully at the openings and you should be able to see that they are
connected to each other.
4. Behind and to the left of the aorta there is another large vessel. This is the
pulmonary artery which takes blood from the right ventricle to the lungs.
Dissection: Internal Anatomy
1. Insert your dissecting scissors or scalpel
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into the superior vena cava and make an incision down through the wall of
the right atrium and ventricle, as shown by the dotted line in the external
heart picture. Pull the two sides apart and look for three flaps of membrane.
These membranes form the tricuspid valve between the right atrium and the
right ventricle. The membranes are connected to flaps of muscle called the
papillary muscles by tendons called the chordae tendinae or "heartstrings."
This valve allows blood to enter the ventricle from the atrium, but prevents
backflow from the ventricle into the atrium.
2. Insert your probe into the pulmonary artery and see it come through to the
right ventricle. Make an incision down through this artery and look inside it
for three small membranous pockets. These form the pulmonary semilunar
valve which prevents blood from flowing back into the right ventricle.
3. Insert your dissecting scissors or scalpel into the left auricle at the base
of the aorta and make an incision down through the wall of the left atrium
and ventricle, as shown by the dotted line in the external heart picture.
Locate the mitral valve (or bicuspid valve) between the left atrium and
ventricle. This will have two flaps of membrane connected to papillary
muscles by tendons.
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4. Insert a probe into the aorta and observe where it connects to the left
ventricle. Make an incision up through the aorta and examine the inside
carefully for three small membranous pockets. These form the aortic
semilunar valve which prevents blood from flowing back into the left
ventricle.
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Blood Flow
Now consider all the parts you've found and how the blood flows through them. Draw a diagram of the heart
and use red and blue arrows to show the flow of blood:
→ deoxygenated blood
→ oxygenated blood
Blood from the tissues → superior and inferior vena cava → right atrium → tricuspid valve → right ventricle
→ pulmonary semilunar valve → pulmonary artery → lungs → pulmonary veins → left atrium → bicuspid
(mitral) valve → left ventricle → aortic semilunar valve → aorta → body tissue.
Illustration of the Physical Form of the Heart
Diagram (1): Physical Appearance and
Major Components of the Heart
The heart is a muscular cone-shaped organ
about the size of a clenched fist of the same
person.
It is located in the upper body (chest area)
between the lungs, and with its pointed end
(called the apex) downwards, forwards, and
pointing towards the left.
The main purpose of the heart is to pump blood around the body.
The basic structure of the heart (illustrated above) may be described as follows:
The Heart is divided into separate right and left sections by the interventricular septum, or "septum" when the
context is clearly that of the heart. Each of these (right and left) sections is also divided into upper and lower
compartments known as atria and ventricles, respectively. The four main chambres of the heart are therefore
the:
Right Atrium (Labelled "RA" in the diagrams on this page);
Right Ventricle (Labelled "RV" in the diagrams on this page);
Left Atrium (Labelled "LA" in the diagrams on this page);
Left Ventricle (Labelled "LV" in the diagrams on this page).
Deoxygenated blood (from the body) is pumped through the right atrium and the right ventricle (to the lungs),
while oxygenated blood (from the lungs) is pumped through the left atrium and the left ventricle (to the body).
Deoxygenated blood enters the right atrium from the Superior vena cava and the Inferior vena
cava.
Deoxygenated blood leaves the right ventricle by Pulmonary artery, which takes blood to the
lungs via the right and left brances of the pulmonary artery.
Oxygenated blood enters the left atrium from the Pulmonary veins. These may be labelled as
"right pulmonary veins" and "left pulmonary veins".
Oxygenated blood leaves the left ventricle by Ascending aorta, which takes blood to the body via
its system of arteries, arterioles, and capillaries. Major arteries leading from the heart (via the
ascending aorta) include the brachiocephalic artery, the left common carotid artery, and the left
subclavian artery (illustrated above). These are just a few of the main arteries of the body.
It is essential that blood flows in the correct direction through the heart so the structure of the heart includes a
series of valves.
The Tricuspid valve separates the right atrium from the right ventricle.
The Pulmonic / Pulmonary valve separates the right ventricle from the pulmonary artery.
The Mitral (also known as the Bicuspid) valve separates the left atrium from the left ventricle.
The Aortic valve separates the right ventricle from the ascending aorta.