C H A P T E R 4 2
Introduction to the cardiovascular system
643
T
he cardiovascular system is responsible for delivering
oxygen and nutrients to all of the cells of the body and
for removing waste products for excretion. The cardio
vascular system consists of a pump—the heart—and an
interconnected series of vessels that continually move
blood throughout the body.
STRUCTURE AND FUNCTION OF
THE HEART
The heart is a hollow, muscular organ that is divided
into four chambers. The heart may actually be viewed as
two joined hearts: a right heart and a left heart, each of
which is divided into two parts, an upper part called the
atrium
(literally “porch” or entryway) and a lower part
called the
ventricle
.
Attached to each atrium is an appendage called the
auricle
, which collects blood that is then pumped into
the ventricles by atrial contraction. The right auricle
is quite large; the left auricle is very small. The ventri
cles pump blood out of the heart to the lungs or the
body. Between the atria and ventricles are two cardiac
valves—thin tissues that are anchored to an annulus,
or fibrous ring, which also gives the hollow organ some
structure and helps to keep the organ open and divided
into distinct chambers.
A partition called a septum separates the right half
of the heart from the left half. The right half receives
deoxygenated blood from everywhere in the body
through the
veins
(vessels that carry blood towards the
heart) and directs that blood into the lungs. The left half
receives the now-oxygenated blood from the lungs and
directs it into the aorta. The aorta delivers blood into
the systemic circulation by way of
arteries
(vessels that
carry blood away from the heart) (Figure 42.1). The
aorta delivers blood into the systemic circulation by way
of arteries. The circulatory system is composed of about
96,500 kilometres of interconnecting blood vessels
that carry the needed oxygen and nutrients to the cells
and carry away the metabolic waste products from the
tissues.
Cardiac cycle
The heart, a muscle that contracts thousands of millions
of times in a lifetime, possesses structural and func
tional properties that are different from those of other
muscles. The fibres of the cardiac muscle, or
myocard
ium
, form two intertwining networks called the atrial
and ventricular
syncytia
. These interlacing structures
enable the atria and then the ventricles to contract syn
chronously when excited by the same stimulus.
Cardiac:
Cardiac cycle
Simultaneous contraction is a necessary property for
a muscle that acts as a pump. A hollow pumping mecha
nism must also pause long enough in the pumping cycle
to allow the chambers to fill with fluid. The heart muscle
relaxes long enough to ensure adequate filling; the more
completely it fills, the stronger is the subsequent con
traction. This occurs because the muscle fibres of the
heart, stretched by the increased volume of blood that
has returned to them, spring back to normal size. This is
similar to the stretching of a rubber band, which returns
to its normal size after it is stretched—the further it is
stretched, the stronger is the spring back to normal. This
property is defined through
Starling’s law of the heart
.
During
diastole
—the period of cardiac muscle relax
ation—blood returns to the heart from the systemic and
pulmonary veins, flowing into the right and left atria,
respectively. When the pressure generated by the blood
troponin:
chemical in heart muscle that prevents the reaction between actin and myosin, leading to muscle relaxation; it is inactivated
by calcium during muscle stimulation to allow actin and myosin to react, causing muscle contraction
veins:
vessels that return blood to the heart; distensible tubes
ventricle:
bottom chamber of the heart, which contracts to pump blood out of the heart
Aorta
Pulmonary
arteries
Pulmonic
valve
Right
atrium
Tricuspid
valve
Inferior
vena cava
Superior
vena cava
Pulmonary
veins
Mitral
valve
Left
atrium
Left
ventricle
Right
ventricle
FIGURE 42.1
Blood flow into and out of the heart. Deoxygenated blood
enters the right atrium from the great cardiac vein and the superior
and inferior venae cavae and flows through the tricuspid valve into
the right ventricle, which contracts and sends the blood through
the pulmonic valve into the pulmonary artery and to the lungs.
Oxygenated blood from the lungs enters the left atrium through the
pulmonary veins and passes through the mitral valve into the left
ventricle, which contracts and ejects the blood through the aortic
valve into the aorta and out to the systemic circulation.
