897
CHAPTER TWENTY-THREE
Reproductive System
FIGURE 23C
Fetal alcohol syndrome. Some
children whose mothers drank alcohol during
pregnancy have characteristic f
at ±aces. Women
who drink excessively while pregnant have a 30%
to 45% chance o± having a child a²
ected to some
degree by prenatal exposure to alcohol. Two mixed
drinks per day seems to be the level above which
damage is likely to occur.
In an adult heart, the blood from the right atrium enters
the right ventricle and is pumped through the pulmonary trunk
and pulmonary arteries to the lungs. In the fetus, however,
the lungs are nonfunctional, and the blood largely bypasses
them. As blood from the inferior vena cava enters the fetal
right atrium, much of it is shunted directly into the left atrium
through an opening in the atrial septum. This opening is called
the
foramen ovale
(fo-ra
men o-val
e), and the blood passes
through it because the blood pressure in the right atrium is
somewhat greater than that in the left atrium. Furthermore, a
small valvelike structure (septum primum) located on the left
side of the atrial septum overlies the foramen ovale and helps
prevent blood from moving in the reverse direction.
The rest of the fetal blood entering the right atrium,
including a large proportion of the deoxygenated blood
entering from the superior vena cava, passes into the right
ventricle and out through the pulmonary trunk. Only a small
volume of blood enters the pulmonary circuit because the
lungs are collapsed and their blood vessels have a high resis-
tance to blood flow. However, enough blood reaches the
lung tissues to sustain them.
Most of the blood in the pulmonary trunk bypasses the
lungs by entering a fetal vessel called the
ductus arterio-
sus
(duk
tus ar-te
re-o
sus), which connects the pulmonary
oxygen-carrying hemoglobin in the fetal blood is about 50%
greater than in the maternal blood. Also, fetal hemoglobin
has a greater attraction for oxygen than does adult hemo-
globin. Thus, at the oxygen partial pressure of the placen-
tal capillaries, fetal hemoglobin can carry 20% to 30% more
oxygen than adult hemoglobin. Different genes encode the
protein subunits of hemoglobin in embryos, fetuses, and
individuals after birth.
In the fetal cardiovascular system, the
umbilical vein
transports blood rich in oxygen and nutrients from the pla-
centa to the fetal body. This vein enters the body through
the umbilical ring and travels along the anterior abdominal
wall to the liver. About half the blood it carries passes into
the liver, and the rest enters a vessel called the
ductus veno-
sus
(duk
tus ven-o
sus), which bypasses the liver.
The ductus venosus extends a short distance and joins
the inferior vena cava. There, oxygenated blood from the
placenta mixes with deoxygenated blood from the lower
parts of the fetal body. This mixture continues through the
inferior vena cava to the right atrium.
RECONNECT
To Chapter 15, Path of Blood Through the Heart,
pages 560–561.
placentas lack important growth ±actors. The
result o± these assaults is poor growth be±ore
and a±ter birth. Cigarette smoking during
pregnancy raises the risk o± spontaneous
abortion, stillbirth, prematurity, and low birth
weight.
Nutrients and Malnutrition
Certain nutrients in large amounts, particularly
vitamins, act in the body as drugs. The acne
medication
isotretinoin
(Accutane) is a deriva-
tive o± vitamin A that causes spontaneous
abortions and de±ects o± the heart, nervous
system, and ±ace in the ±etus. A vitamin A-based
drug used to treat psoriasis, as well as excesses
o± vitamin A itsel±, also cause birth de±ects. This
is because some ±orms o± vitamin A are stored
in body ±at ±or up to three years a±ter ingestion.
Malnutrition during pregnancy causes
intrauterine growth retardation (IUGR), which
may have delayed health e²
ects. Fetal physi-
ology adapts to starvation to best use avail-
able nutrients. Insulin resistance changes
to compensate ±or lack o± muscle tissue.
Cardiovascular changes shunt blood to vital
organs. Starvation also raises stress hormone lev-
els, sti²
ens arteries, and too ±ew kidney tubules
±orm. These changes set the stage ±or type 2 dia-
betes mellitus, hypertension, stroke, and coro-
nary artery disease years later. Paradoxically, the
in±ant is scrawny, but the older child tends to be
obese, and di³
culty losing weight may persist.
Occupational Hazards
Some teratogens are encountered in the work-
place. Increased rates o± spontaneous abor-
tion and birth de±ects have been noted among
women who work with textile dyes, lead, certain
photographic chemicals, semiconductor mate-
rials, mercury, and cadmium. We do not know
much about the role o± the male in environmen-
tally caused birth de±ects. Men whose jobs expose
them to sustained heat, such as smelter workers,
glass manu±acturers, and bakers, may produce
sperm that can ±ertilize an egg and possibly lead
to spontaneous abortion or a birth de±ect. A virus
or a toxic chemical carried in semen may also
cause a birth de±ect.
Small head
circumference
Low nasal
bridge
Eye folds
Short nose
Small midface
Thin upper lip
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