c. Heart Chambers. There are four chambers in the heart. These chambers
are essentially the same size. The upper chambers, called the atria, are seemingly
smaller than the lower chambers, the ventricles. The apparent difference in total size is
due to the thickness of the myocardial layer. The right atrium communicates with the
right ventricle; the left atrium communicates with the left ventricle. The septum
(partition), dividing the interior of the heart into right and left sides, prevents direct
communication of blood flow from right to left chambers or left to right chambers. This
is important, because the right side of the heart receives un-oxygenated blood returning
from the systemic (body) circulation. The left side of the heart receives oxygenated
blood returning from the pulmonary (lung) circulation. The special structure of the heart
keeps the blood flowing in its proper direction to and from the heart chambers.
d. Heart Valves. The four chambers of the heart are lined with endocardium.
At each opening from the chambers this lining folds on itself and extends into the
opening to form valves. These valves allow the blood to pass from a chamber but
prevent its return. The atrioventricular valves, between the upper and lower chambers,
are within the heart itself. The semilunar valves are within arteries arising from the right
and left ventricles.
(1) Atrioventricular valves. The tricuspid valve is located between the right
atrium and right ventricle. It has three flaps or cusps. The bicuspid valve or mitral valve
is located between the left atrium and left ventricle. It has two flaps or cusps.
(2) Semilunar valves. The pulmonary semilunar (half-moon shaped) valve
is located at the opening into the pulmonary artery that arises from the right ventricle.
The aortic semilunar valve is located at the opening into the aorta that arises from the
FLOW OF BLOOD THROUGH THE HEART
It is helpful to follow the flow of blood through the heart in order to understand the
relationship of the heart structures. Remember, the heart is the pump and is also the
connection between the systemic circulation and pulmonary circulation. All the blood
returning from the systemic circulation must flow through the pulmonary circulation for
exchange of carbon dioxide for oxygen. Blood from the upper part of the body enters
the heart through the superior vena cava and blood from the lower part of the body
enters through the inferior vena cava.
a. Blood from the superior vena cava and inferior vena cava enters the heart at
the right atrium. The right atrium contracts, and blood is forced through the open
tricuspid valve into the relaxed right ventricle.
b. As the right ventricle contracts, the tricuspid valve is closed, preventing back
flow into the atrium. The pulmonary semilunar valve opens as a result of the force and
movement of the blood, and the right ventricle pumps the blood into the pulmonary