upper chamber, the ATRIUM, which receives
blood from the veins, and a lower chamber, the
VENTRICLE, which receives blood from the
atrium and pumps it out into the arteries. The
openings between the chambers on each side of
the heart are separated by flaps of tissue that act
as valves to prevent backward flow of the con-
tinuously forward moving column of blood. The
one on the right has three flaps, or cusps, and is
called the TRICUSPID VALVE. The one on the
left has two flaps and is called the MITRAL, or
BICUSPID, VALVE. The outlets of the ventricles
are supplied with similar valves. On the right the
pulmonary valve is at the origin of the pulmonary
artery, and on the left the aortic valve is at the
origin of the aorta.
Physiologically, the heart acts as four inter-
related pumps. The right atrium receives deox-
ygenated blood from the body via the superior
and inferior vena cavae. It pumps this blood
through the tricuspid valve to the right ventricle.
The right ventricle pumps the blood past the
pulmonary valve through the pulmonary artery
to the lungs for oxygenation. The left atrium
receives the oxygenated blood from the lungs
through four pulmonary veins and pumps it to
the left ventricle past the mitral valve. The left
ventricle pumps the blood to all areas of the body
via the aortic valve and the aorta.
The heart muscle, the MYOCARDIUM, is
striated like the skeletal muscles of the body, but
involuntary in action, like the smooth muscles.
The walls of the atria are thin with relatively lit-
tle muscle fiber because the blood flows from the
atria to the ventricles under low pressure.
However, the walls of the ventricles, which com-
prise the bulk of the heart, are thick and muscular.
The wall of the left ventricle is considerably
thicker than that of the right, because more force
is required to pump the blood into the peripheral
systemic circulation than into the lungs located
only a short distance from the heart.
The heart acts by contraction and relaxation.
It contracts with a wringing motion, forcing blood
into the arteries. Each contraction is followed by
limited relaxation or dilation. Cardiac muscle
never completely relaxes, it always maintains a
degree of tone. Contraction of the heart is called
SYSTOLE and the period of work. Relaxation of
the heart with limited dilation is called
DIASTOLE and the period of rest. A complete
CARDIAC CYCLE is the time from onset of one
contraction, or heart beat, to the onset of the next.
The contractions of the heart are stimulated
and maintained by the SINOATRIAL NODE,
commonly called the PACEMAKER of the heart,
which is a group of hundreds of cells in the up-
per part of the right atrium that sets off electrical
impulses, causing both atria to contract
simultaneously. The normal heart rate, or number
of contractions, is about 72 beats per minute.
The BLOOD PRESSURE is the pressure the
blood exerts on the walls of the arteries. The
highest pressure is called SYSTOLIC pressure,
because it is caused when the heart is in systole,
or contraction. A certain amount of blood
pressure is maintained in the arteries even when
the heart is relaxed. This is the DIASTOLIC
pressure, because it is present during diastole, or
relaxation of the heart.
Normal blood pressure can vary considerably
with age, weight, and general condition of the in-
dividual. For young adults the systolic pressure
is between 120 and 150 mm of mercury, and the
diastolic pressure is between 70 and 90 mm of
mercury. Women have a lower blood pressure
than men. The difference between systolic and
is known as PULSE
The blood vessels of the body fall into three
1. Distributorsarteries and arterioles
3. Collectorsveins and venules
The ARTERIES are elastic tubes constructed
to withstand high pressure. They carry blood away
from the heart to all parts of the body. The
smallest branches of the arteries are called
The AORTA is the large tubelike structure
arising from the left ventricle of the heart. It
arches upward over the left lung and then down
along the spinal column through the thorax and
the abdomen, where it divides to send arteries
down both legs (fig. 3-31). The CORONARY
ARTERIES are branches of what is generally
called the ascending aorta, and they supply the
heart with blood.
There are certain branches of the aorta with
which you should be familiar, since these often
must be compressed to control hemorrhage. You
will find a discussion of pressure points in the
hemorrhage section of the chapter in this manual
entitled First Aid and Emergency Procedures.