The Cardiovascular System 5oxygenated blood to the heart from the lungs.
Veins have thin, flexible walls with valves that
allow blood to flow only in one direction.
The smallest of the blood vessels, the arterioles
and the venules, have walls fractions of a millime-
ter in thickness and so narrow that only the small-
est of blood cells, oxygen-bearing erythrocytes,
can squeeze through and even they must pass sin-
gle-file. These tiny vessels mesh into the CAPILLARY
BEDS, the terminus for the blood’s journey. Blood
cells are at their busiest here, exchanging oxygen
and other nutrients for carbon dioxide and other
metabolic wastes.
Most blood vessels exist in mirror structures on
each side of the body and occur in parallel. The
femoral artery and femoral VEINrun together, for
example, one set serving each upper leg. Often
these vessels have similar names, such as the
femorals or the popliteal artery and popliteal vein.
As the coronary arteries channel oxygenated
blood to the heart, the arteries that form the CIR-
CLE OFWILLISat the base of the brain direct oxy-
gen-rich blood to the brain.
The circulation Each beat of the heart propels
80 milliliters (2.5 ounces) of blood into the AORTA,
the largest artery in the body. It takes about 20
seconds for that same volume of blood to com-
plete its journey through the body’s blood vessels
and return to the heart. Every minute, 5 to 6 liters
(0.85 to 1.5 gallons) of blood circulates through
the body—the equivalent of three 2-liter bottles of
soda. In the course of a day the volume of blood
the heart pumps is enough to fill an Olympic-size
swimming pool.
Pressure aids the heart in pushing blood through
the blood vessels. A complex interaction of hor-
mones and other chemicals regulates blood pres-
sure. Blood in the arteries is under high pressure,
helping push it to the cells that require the oxygen
and nutrients it carries. The pressure within the
arteries allows blood to defy the pull of gravity as it
courses to the body’s tissues. Because the circula-
tion is a closed system, the pressure of the arterial
flow helps send blood through the veins as well,
much as the pressure of a river’s water continues to
create current in the small streams that branch
from it. The pressure of the blood within veins is
significantly lower than the pressure within arter-
ies. Valves in the veins act as one-way gates to keep
blood flowing back to the heart. Skeletal muscles
encase the major veins, further supporting them.
With every movement these muscles massage the
veins to help move returning blood along its pas-
sage back to the heart.Health and Disorders of the
Cardiovascular System
The cardiovascular system has the capacity to
maintain healthy function in adults well into the
seventh decade and beyond, though for many
people it does not. More than 70 million Ameri-
cans are LIVING WITH CARDIOVASCULAR DISEASE; 10
million of them are disabled to an extent that they
are unable to enjoy the lifestyles they desire as a
result. CARDIOVASCULARDISEASE(CVD)—a collective
term for the many health conditions that affect
the heart and blood vessels—causes the deaths of
more than 900,000 Americans each year, making
it the leading cause of death among men and
women alike in the United States. Most CVD
among Americans is acquired, developing through
the course of life and nearly always as a conse-
quence of lifestyle factors. Genetic factors and
other health conditions, notably DIABETES, may
also contribute.FORMS OF CARDIOVASCULAR DISEASE (CVD)
ANEURYSM ARRHYTHMIA
ATHEROSCLEROSIS BUNDLE BRANCH BLOCK
CARDIOMYOPATHY CONGENITAL HEART DISEASE
CORONARY ARTERY DISEASE(CAD) DEEP VEIN THROMBOSIS(DVT)
ENDOCARDITIS HEART FAILURE
HYPERLIPIDEMIA HYPERTENSION(highBLOOD
HYPOTENSION(low blood PRESSURE)
pressure) INTERMITTENT CLAUDICATION
ISCHEMIC HEART DISEASE(IHD) KAWASAKI’S DISEASE
LONG QT SYNDROME(LQTS) MYOCARDITIS
PERICARDITIS PERIPHERAL VASCULAR DISEASE
PRIMARY PULMONARY (PVD)
HYPERTENSION RAYNAUD’S SYNDROME
RHEUMATIC HEART DISEASE SICK SINUS SYNDROME
STROKE TRANSIENT ISCHEMIC ATTACK(TIA)
VALVULAR HEART DISEASE WOLFF-PARKINSON-WHITE
SYNDROMEAbout 30,000 infants are born with CONGENITAL
HEART DISEASE—defects of the heart and blood ves-
sels—each year. Heart defects are the most com-