Lymph Nodes Circulatory System
myocardium of the ventricles are called the trabeculae
(trah-BEK-yoo-lee) carneae (KAR-neh-ee). The muscle
tissue of the atria and ventricles is separated by connec-tive
tissue that also forms the valves. This connective tis-sue
divides the myocardium into two separate muscle masses.
Externally, a groove called the coronary sulcus (KOR-
ah-nair-ee SULL-kus) separates the atria from the
ventricles (see Figure 14-1). Two other sulci (plural) are
seen externally. The anterior interventricular sulcus
and the posterior interventricular sulcus separate the
right and left ventricles from one another. The sulci contain
a vary-ing amount of fat and coronary blood vessels
(vessels that supply the heart tissue with blood).
The Great Vessels of the Heart
The right atrium receives blood from all parts of the body
except the lungs. It receives this blood through three veins.
The superior vena cava, also known as the anterior
vena cava, brings blood from the upper parts of the body,
the head, neck, and arms. The inferior vena cava, also
known as the posterior vena cava, brings blood from
the lower parts of the body, the legs, and abdo-men. The
coronary sinus drains the blood from most of the vessels
that supply the walls of the heart with blood. This blood in
the right atrium is then squeezed into the right ventricle.
The right ventricle pumps the blood into the next major
vessel, the pulmonary trunk, which splits into the right
pulmonary artery and the left pulmonary artery.
These arteries each carry the blood to a lung. In the lungs,
the blood releases the carbon dioxide it has been carrying
and picks up oxygen. The oxygenated blood returns to the
heart via four pulmonary veins that empty into the left
atrium. The blood is then squeezed into the left ventricle.
The left ventricle pumps the blood into the next great
vessel, the ascending aorta. From here the aortic blood
goes to the coronary arteries (which supply the walls of
the heart with oxygenated blood), the arch of the aorta
(which sends arteries to upper parts of the body), and the
descending thoracic aorta, which becomes the
abdominal aorta. These arteries transport oxygenated
blood to all parts of the body.
The size of the chambers and the thickness of the
chamber walls vary, due to the amount of blood received
and the distance this blood must be pumped. The right
atrium, which collects blood coming from all parts of the
body except the lungs, is slightly larger than the left atrium,
which receives only the blood coming from the lungs. The
thickness of the chamber walls also varies.
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Ventricles have thick walls, whereas the atria are thin
walled. They are assisted with pumping blood by the re-
duced pressure caused by the expanding ventricles as they
receive the blood. The thickness of the two ventricle walls
varies also. The left ventricle has walls thicker than the
right ventricle (see Figure 14-4) since it must pump the
oxygenated blood at high pressure through thou-sands of
miles of blood vessels in the head, trunk, and extremities.The Valves of the Heart
The valves of the heart are designed in such a way as to
prevent blood from flowing back into the pumping
chamber. There are two atrioventricular valves between the
atria and their ventricles. The valve between the right
atrium and the right ventricle is called the -tricuspid (try-
KUSS-pid) valve because it consists of three flaps or
cusps (see Figure 14-4B). These flaps are made of -fibrous
connective tissue that grows out of the walls of the heart
and is covered with endocardium. The pointed ends of the
cusps project down into the ventricle. Cords called
chordae (KOR-dee) tendineae (TIN-din-ee) con-nect
the pointed ends of the flaps or cusps to small conical
projections called the papillary (PAP-ih-layr-ee)
muscles located on the inner surface of the ventricle (see
-Figure 14 - 4A and B).
The atrioventricular valve between the left atrium and
the left ventricle is known as the bicuspid (bye-KUSS-
pid) or mitral (MYE-tral) valve. As the name in-dicates,
it has two cusps or flaps, whose pointed ends project down
into the ventricle with the same structures as the tricuspid
valve. It is the only valve in the heart with only two cusps;
all others have three cusps. The two arteries that leave the
heart (the ascending aorta and the pulmonary trunk) also
have valves that prevent blood from flowing back into the
pumping chamber. These are called the semilunar valves.
The pulmonary semilunar valve is found in the
opening where the pul-monary trunk exits the right
ventricle. The aortic semi-lunar valve is found in the
opening where the ascending aorta leaves the left ventricle.
Both of the valves are made of three semilunar cusps that
allow blood to flow only in one direction.Blood Flow Through The Heart
As we discuss blood flow through the heart, it is easier to
study its path by beginning at one point, going in a one-way
direction, and ending at the same point we began. We will
do this, but remember, the heart actually pumps