the body systems to become functional. The rudi-
mentary heart begins beating at three weeks ges-
tational age, and by eight weeks the heart’s
formation is complete. Doppler ULTRASOUND, a non-
invasive procedure that uses sound waves to
detect motion, can detect the fetal heartbeat at 10
to 12 weeks into PREGNANCY.
PULSEand BLOOD PRESSURE—the rate and force of
blood as it flows through the arteries—are among
the basic vital signs health-care providers assess to
determine core health status and indeed life or
death. When the heart stops beating, pulse
and blood pressure cease. Cells in the BRAINand
the MYOCARDIUM, deprived of oxygen, begin to die.
Though myocardial cells are capable, to an extent,
of regenerating, brain cells are not. Only two to
three minutes without oxygen can result in per-
manent neurologic damage and even death.
The heart The heart lies behind the protective
enclosure of the rib cage, its left side beneath the
STERNUM(breastbone) and its right side extending
about to the center of the right BREAST. A tough
sac, the PERICARDIUM, encases the heart. A thin
layer of fluid between the pericardium and the
myocardium (heart MUSCLE) allows the heart to
move continuously without friction in much the
same way motor oil permits pistons to glide freely
within an engine. A crownlike network of arter-
ies, the CORONARY ARTERIES, encircles the outer sur-
face of the myocardium. This network directs
roughly 20 percent of the body’s blood supply and
70 percent of the blood’s oxygen content to the
heart with each heartbeat. A thin membrane, the
ENDOCARDIUM, lines the chambers of the heart. The
endocardium is so smooth not even platelets, the
blood’s clotting cells, can stick to it.
The heart’s four chambers collect and expel
blood through rhythmic, synchronized contrac-
tions. The two upper chambers, the atria, receive
blood into the heart. The two lower chambers, the
ventricles, pump blood from the heart. One-way
valves regulate the flow and volume of blood into
and out of each chamber. The four chambers of
the heart contract and relax in precise coordina-
tion. The walls of the heart become progressively
thicker from the atria to the ventricles, reaching
their greatest density and STRENGTHin the left ven-
tricle to support its work to contract with enough
force to pump blood to the most distant cells in
the body. A thick inner wall, the septum, sepa-
rates the heart’s chambers.
A cluster of specialized NERVEcells, the SINOA-
TRIAL (SA) NODE, generates a “pacing” electrical
impulse that starts with the right atrium and
builds momentum as it courses through the cells
and fibers of the myocardium. Other electrical
structures—the BUNDLE OFHIS, right BUNDLE BRANCH
and left bundle branch, ATRIOVENTRICULAR (AV)
NODE, and the Purkinje fibers—amplify and focus
the electrical impulses so they reach maximum
intensity when they arrive at the left ventricle.
This collective effort forms the CARDIAC CYCLE, the
sequential and coordinated contraction and relax-
ation of the atria and the ventricles.
The right atrium and right ventricle, known
collectively as the right heart, handle deoxy-
genated blood returning from the body. Blood
flows from the superior VENA CAVA(bringing blood
from the upper body) and inferior vena cava
(bringing blood from the lower body) into the
right atrium, which pumps it through the tricus-
pid valve into the right ventricle. The right ventri-
cle pumps the blood through the pulmonary valve
into the pulmonary ARTERY, which carries it to the
LUNGSfor OXYGENATION.
The left atrium and left ventricle, known collec-
tively as the left heart, handle oxygenated blood.
The PULMONARY VEINS(right and left, from the right
lung and left lung respectively) deliver oxygenated
blood from the lungs to the left atrium. The left
atrium pumps the blood through the mitral valve
into the left ventricle. The left ventricle propels
blood through the aortic valve into the AORTA, the
body’s largest blood vessel, and on its circulatory
journey.
The blood vessels Blood vessels circulate blood
throughout the body. Arteries carry blood fromthe
heart. All arteries except the pulmonary artery
carry oxygenated blood; the pulmonary artery car-
ries deoxygenated blood from the heart to the
lungs. Arteries have multilayered, muscular walls
that pulsate in coordination with the heart’s con-
tractions. The innermost layer, the intima, func-
tions in the same fashion as the endocardium to
keep the blood from sticking to the artery’s inner
walls. Veins carry blood tothe heart. All veins
except the two pulmonary veins carry deoxy-
genated blood; the pulmonary veins transport4 The Cardiovascular System