HUMAN BIOLOGY

Development anD aging 341

What are the basic developmental events of

the second and third trimesters?

• The second and third trimesters of gestation are when the
  organs and organ systems of a fetus enlarge and mature.


• Because the fetus exchanges gases and receives nutrients
  via its mother’s bloodstream, its circulatory system develops
  temporary vessels that bypass the lungs and liver until birth.

taKe-Home message

aorta

liver

heart

placenta urinary bladder

allantois

superior

vena cava

pulmonary

vessels

arterial duct

(ductus arteriosus)

foramen

ovale

umbilical

cord

umbilical

vein

umbilical

arteries

venous duct

(ductus venosus)

inferior

vena cava

closed

foramen

ovale

(fossa

ovalis)

hepatic

vein

hepatic portal

vein serving

the liver

pulmonary

veins

pulmonary

artery

degenerated

allantois

urinary bladder

ligament

ligament

umbilicus

(navel)

umbilical

ligaments

A B

Figure 17.15 Blood circulates in a special pattern in a fetus (arrows). A Umbilical arteries carry deoxygenated blood and wastes from
fetal tissues to the placenta. Blood in the umbilical vein picks up oxygen and nutrients from the mother’s bloodstream and returns to
the fetus. Blood mainly bypasses the lungs, moving through the foramen ovale and the arterial duct. It bypasses the liver by moving
through the venous duct. B At birth the foramen ovale closes, and the pulmonary and systemic circuits of blood flow become completely
separate. The arterial duct, venous duct, umbilical vein, and portions of the umbilical arteries become ligaments, and the allantois
degenerates. (© Cengage Learning)

won’t begin gas exchange until the newborn takes its first
breaths. Until then, its lungs receive only enough blood to
sustain their development.
Some of the blood entering the heart’s right atrium flows
into the right ventricle and on to the lungs. Most of it, how-
ever, travels through a gap in the interior heart wall (called
the foramen ovale, or “oval opening”) or into an arterial duct
(ductus arteriosus) that bypasses the lungs.
Similarly, most blood bypasses the fetal liver because
the mother’s liver performs most liver functions until birth.
Nutrient-rich blood from the placenta travels through a
venous duct (the ductus veno sus) past the liver and on to
the heart, which pumps it to body tissues. At birth, blood
pressure in the heart’s left atrium increases. This causes
a flap of tissue to close off the foramen ovale, which then

gradually seals and separates the pulmonary and sys-

temic circuits of blood flow (Figure 17.15B). The temporary

vessels that have formed in a fetus gradually close during

the first few weeks after birth.

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