and maternal blood supplies do not mix). Accord-
ingly, the fetal LUNGSdo not function. Blood flows
to and from the fetus through the umbilical arter-
ies and veins (UMBILICAL CORD).
In the adult heart the right ventricle pumps
blood through the PULMONARY ARTERIESto the LUNGS
for OXYGENATION. The blood returns to the heart via
the PULMONARY VEINS. Because the fetal lungs are
nonfunctional, the fetal circulatory system
bypasses the lungs. An opening (shunt) between
the atria, the foramen ovale, allows blood to flow
from the right atrium to the left atrium, which
pumps it to the left ventricle. A small amount of
blood goes from the right atrium to the right ven-
tricle, which pumps it into the pulmonary ARTERY.
A shunt between the aorta and the pulmonary
artery, the ductus arteriosus, directs the blood into
the aorta where it mixes with the blood the left
ventricle pumps into the AORTA. With the first
breath following birth the lungs inflate and the
changes in pressure initiate a series of biochemical
actions that cause these shunts to close, establish-
ing blood circulation through the lungs. Within a
few days of birth the ductus arteriosus becomes
the ligamentum arteriosum, a strip of connective
tissue that stabilizes the aorta and the pulmonary
artery. The umbilical veins retreat to form the
round ligament supporting the LIVER and the
umbilical arteries to form ligaments that support
the abdominal muscles.
Cardiovascular Changes at MenopauseEstrogen, the HORMONEresponsible for female FER-
TILITY, is essential for lipid metabolism. The high
estrogen levels that mark fertility seem to exert a
protective action on a woman’s cardiovascular sys-
tem, lowering the likelihood for HYPERLIPIDEMIA
and related health conditions such as ATHEROSCLE-
ROSISand CORONARY ARTERY DISEASE(CAD). During
the 35 to 40 years of her fertility, a woman’s risk
for cardiovascular disease is a third to half that of
a man of comparable age and health status. At
MENOPAUSEestrogen levels drop significantly and a
woman’s risk for cardiovascular disease takes a
significant jump. Some studies suggest that during
the first five years following menopause, a
woman’s risk for HEART ATTACKis greater than that
of a man who is of comparable age and health
status.
Hormone replacement therapy (HRT) to restore
estrogen levels after menopause became a stan-
dard medical approach in the 1950s. In the 1990s
numerous studies revealed significant increases in
the risks for BREAST CANCER and uterine CANCER
associated with HRT as well as failed to find sup-
portive evidence that HRT improved cardiovascu-
lar health in women after menopause, and health
experts withdrew recommendations for its routine
use. Current recommendations suggest women,
like men, make nutritious eating choices, get daily
physical exercise, maintain healthy weight, and
not smoke as the key preventive measures to
lower their risk for cardiovascular disease in
midlife and beyond.Lifestyle Choices to Maintain
Cardiovascular Health
Current research strongly supports the role of
lifestyle choices in maintaining cardiovascular
health, even to the extent that many researchers
believe appropriate choices beginning in early
childhood could prevent as much as 90 percent of
acquired cardiovascular disease. Healthy adults
who are in their 70s and 80s who do not have any
form of cardiovascular disease or other chronic
health conditions do not have significant changes
in cardiovascular function. Weight management,
not smoking, nutritious food choices, and daily
physical exercise are the cornerstones of lifestyle
measures to preserve cardiovascular health. Many
researchers believe the healthy cardiovascular sys-
tem has the capacity to function efficiently well
into the eighth decade of life and beyond.
See also CARDIOVASCULAR DISEASE PREVENTION;
CONGENITAL HEART DISEASE; ESTROGENS; LIFESTYLE AND
CARDIOVASCULAR HEALTH; LIGAMENT; MUSCLE; PREG-
NANCY; SMOKING CESSATION; WEIGHT LOSS AND WEIGHT
MANAGEMENT.aneurysm A weakened and often distended
(stretched) area in the wall of an ARTERY. Though
an aneurysm may develop in any artery, the most
common location is the descending or abdominal
AORTA. An aneurysm is potentially life-threatening.
The continual pressure of the BLOOD flowing
through the artery pressures the weakened area,
which can cause the layers of the artery’s wall to
further split and separate, called a dissecting10 The Cardiovascular System