c. Fetal Circulation. Since the fetus is located within the uterus, its lungs do
not take in air. Therefore, the pulmonary cycle does not function in the fetus.
Essentially, fetal blood flows to and from the placenta. There are certain bypasses in
the heart to avoid the pulmonary cycle. At the time of birth, the fetal circulation is
changed to the normal pattern.
Section IV. THE HEART--THE PRIMARY MOTIVE FORCE OF THE
In humans, the heart is the primary motive force for driving the blood along the
arterial vessels. The heart consists of four separate chambers. Two chambers function
as a "right heart," and two function as a "left heart." The muscular walls (myocardium)
of the chambers apply force to the blood within and force the blood to move out of the
chambers. (See Figure 10-3.)
10-24. CHAMBERS OF THE HUMAN HEART
a. Atria. Two chambers are called the atria (singular: atrium). Down the
middle, an interatrial septum separates the two atria.
(1) The muscular walls of the atria tend to be relatively thin.
(2) Attached to each atrium is an earlike appendage called an auricle. The
auricles of the atria tend to have somewhat thicker walls.
b. Ventricles. The other two chambers are the right and left ventricles.
Between the ventricles is the interventricular septum.
(1) The left ventricle tends to be cylindrical in shape. It has a relatively thick
(2) The right ventricle has a somewhat semilunar (half-moon) cross section,
since it is wrapped around one side of the left ventricle.