C H A P T E R 4 2
Introduction to the cardiovascular system
649
CIRCULATION
The purpose of the heart’s continual pumping action
is to keep blood flowing to and from all of the body’s
tissues and cells. Blood delivers oxygen and much-
needed nutrients to the cells for producing energy, and it
carries away carbon dioxide and other waste products of
metabolism. The steady circulation of blood is essential
for the proper functioning of all of the body’s organs,
including the heart.
The circulation of the blood follows two courses:
•
Heart–lung or pulmonary circulation
: The right side
of the heart sends blood to the lungs, where carbon
dioxide and some waste products are removed from
the blood and oxygen is picked up by the red blood
cells.
•
Systemic circulation
: The left side of the heart sends
oxygenated blood out to all of the cells in the body.
In addition, the heart muscle, like any other muscle,
requires adequate oxygen and nutrients to function.
This is accomplished via coronary circulation.
The blood moves from areas of high pressure to
areas of lower pressure. The system is a “closed” system;
that is, it has no openings or holes that would allow
blood to leak out. The closed nature of the system is
what keeps the pressure differences in the proper rela
tionship so that blood always flows in the direction in
which it is intended to flow (Figure 42.6).
Pulmonary circulation
The right atrium is a very low pressure area in the car
diovascular system. All of the deoxygenated blood from
the body flows into the right atrium from the inferior
and superior venae cavae (see Figure 42.1) and from
the great cardiac vein, which returns deoxygenated
blood from the heart muscle. As the blood flows into
the atrium, the pressure increases. When the pressure
becomes greater than the pressure in the right ventricle,
most of the blood flows into the right ventricle; this is
called the rapid-filling phase. At this point in the cardiac
cycle, the atrium is stimulated to contract and pushes
the remaining blood into the right ventricle. The ventri
cle is then stimulated to contract; it generates pressure
that opens the pulmonic valve (see Figure 42.1) and
sends blood into the pulmonary artery, which takes
the blood into the lungs, a very low pressure area. The
Veins—
distensible,
thin walls
Pulmonary vessels—
distensible,
thin walls
Arteries—
elastic,
thick walls
PULMONARY CIRCULATION
SYSTEMIC CIRCULATION
FIGURE 42.6
Blood flow through the
systemic and pulmonary vasculature
circuits.
