Chapter 19
The Cardiovascular System: Blood Vessels
745
19
Check Your Understanding
21.
What is a portal system? What is the function of the hepatic
portal system?
22.
Name the leg veins that often become varicosed.
For answers, see Appendix H.
Developmental Aspects
of Blood Vessels
Explain how blood vessels develop.
Provide examples of changes that often occur in blood
vessels as a person ages.
Te endothelial lining of blood vessels is formed by mesodermal
cells, which collect in little masses called
blood islands
through-
out the microscopic embryo. Tese blood islands form fragile
sprouting extensions that reach toward one another and toward
the forming heart to lay down the rudimentary vascular tubes.
Meanwhile, adjacent mesenchymal cells, stimulated by platelet-
derived growth factor, surround the endothelial tubes, forming
the stabilizing muscular and fibrous coats of the vessel walls.
How do blood vessels “know” where to grow? Many blood
vessels simply follow the same guidance cues that nerves follow,
which is why forming vessels oFen snuggle closely to nerves.
Whether a vessel becomes an artery or a vein depends upon the
local concentration of a differentiation factor called
vascular en-
dothelial growth factor
. As noted in Chapter 18, the heart pumps
blood through the rudimentary vascular system by the fourth
week of development.
In addition to the fetal shunts that bypass the nonfunc-
tional lungs (the
foramen ovale
and
ductus arteriosus
), other
vascular modifications are found in the fetus. A special vessel,
the
ductus venosus
, largely bypasses the liver. Also important
are the
umbilical vein
and
arteries
, large vessels that circulate
blood between the fetal circulation and the placenta where gas
and nutrient exchanges occur with the mother’s blood (see
Chapter 28). Once the fetal circulatory pattern is laid down,
few vascular changes occur until birth, when the umbilical
vessels and shunts are occluded.
Unlike congenital heart diseases, congenital vascular prob-
lems are rare, and blood vessels are remarkably trouble-free
during youth. Vessels form as needed to support body growth
and wound healing, and to rebuild vessels lost each month dur-
ing a woman’s menstrual cycle. As we age, signs of vascular dis-
ease begin to appear. In some, the venous valves weaken, and
purple, snakelike varicose veins appear. In others, more insidi-
ous signs of inefficient circulation appear: tingling fingers and
toes and cramping muscles.
Although the degenerative process of atherosclerosis begins
in youth, its consequences are rarely apparent until middle to
old age, when it may precipitate a myocardial infarction or
stroke. Until puberty, the blood vessels of boys and girls look
alike, but from puberty to about age 45, women have strikingly
less atherosclerosis than men because of the protective effects
of estrogen. By enhancing nitric oxide production, inhibiting
endothelin release, and blocking voltage-gated Ca
2
1
channels,
estrogen reduces resistance to blood flow. Estrogen also stimu-
lates the liver to produce enzymes that speed up catabolism of
LDLs and increase the production of HDLs, thus reducing the
risk of atherosclerosis (see
A Closer Look
).
Between the ages of 45 and 65, when estrogen production
wanes in women, this “gap” between the sexes closes, and males
and females above age 65 are equally at risk for cardiovascular
disease. You might expect that giving postmenopausal women
supplementary estrogen would maintain this protective effect.
Surprisingly, clinical trials have shown that this is not the case.
Blood pressure changes with age. In a newborn baby, arte-
rial pressure is about 90/55. Blood pressure rises steadily during
childhood to finally reach the adult value (120/80). AFer age 40,
the incidence of hypertension increases dramatically, as do as-
sociated illnesses such as heart attacks, strokes, vascular disease,
and renal failure.
At least some vascular disease is a product of our modern
technological culture. “Blessed” with high-protein and lipid-
rich diets, empty-calorie snacks, energy-saving devices, and
high-stress jobs, many of us are struck down prematurely. Life-
style modifications—a healthy diet, regular aerobic exercise,
and eliminating cigarette smoking—can help prevent cardio-
vascular disease. Poor diet, lack of exercise, and smoking are
probably more detrimental to your blood vessels than aging
itself could ever be!
Check Your Understanding
23.
List three differences between systemic arteries and veins
with respect to their general pathways and courses.
24.
Name three fetal shunts that are occluded shortly after birth.
Which structure does each shunt bypass?
25.
List three common age-related vascular problems.
For answers, see Appendix H.
Now that we have described the structure and function of
blood vessels, our survey of the cardiovascular system is com-
plete. Te pump, the plumbing, and the circulating fluid form
a dynamic organ system that ceaselessly services every other
organ system of the body, as summarized in
System Connec-
tions
on p. 746. However, our study of the circulatory system is
still unfinished because we have yet to examine the lymphatic
system, which acts with the cardiovascular system to ensure
continuous circulation and to provide sites from which lym-
phocytes can police the body and provide immunity. Tese are
the topics of Chapter 20.
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