Ganong's Review of Medical Physiology, 23rd Edition

582 SECTION VICardiovascular Physiology

size of the conceptus increases much more, changing from a
single cell to a fetus plus a placenta that weighs 4 to 5 kg at
term in humans. Consequently, more O 2 is extracted from the
uterine blood during the latter part of pregnancy, and the O 2
saturation of uterine blood falls. Corticotrophin-releasing
hormone appears to play an important role in up-regulating
uterine blood flow, as well as in the eventual timing of birth.

PLACENTA

The placenta is the “fetal lung” (Figures 34–16 and 34–17). Its
maternal portion is in effect a large blood sinus. Into this “lake”
project the villi of the fetal portion containing the small branch-
es of the fetal umbilical arteries and vein (Figure 34–16). O 2 is
taken up by the fetal blood and CO 2 is discharged into the ma-
ternal circulation across the walls of the villi in a fashion analo-
gous to O 2 and CO 2 exchange in the lungs (see Chapter 36).
However, the cellular layers covering the villi are thicker and
less permeable than the alveolar membranes in the lungs, and
exchange is much less efficient. The placenta is also the route by
which all nutritive materials enter the fetus and by which fetal
wastes are discharged to the maternal blood.

FETAL CIRCULATION

The arrangement of the circulation in the fetus is shown dia-
grammatically in Figure 34–17. Fifty-five percent of the fetal
cardiac output goes through the placenta. The blood in the um-
bilical vein in humans is believed to be about 80% saturated
with O 2 , compared with 98% saturation in the arterial circula-
tion of the adult. The ductus venosus (Figure 34–18) diverts

some of this blood directly to the inferior vena cava, and the re-

mainder mixes with the portal blood of the fetus. The portal

and systemic venous blood of the fetus is only 26% saturated,

and the saturation of the mixed blood in the inferior vena cava

is approximately 67%. Most of the blood entering the heart

through the inferior vena cava is diverted directly to the left

atrium via the patent foramen ovale. Most of the blood from

the superior vena cava enters the right ventricle and is expelled

into the pulmonary artery. The resistance of the collapsed

lungs is high, and the pressure in the pulmonary artery is sev-

eral mm Hg higher than it is in the aorta, so that most of the

blood in the pulmonary artery passes through the ductus arte-

riosus to the aorta. In this fashion, the relatively unsaturated

blood from the right ventricle is diverted to the trunk and lower

body of the fetus, while the head of the fetus receives the better-

oxygenated blood from the left ventricle. From the aorta, some

of the blood is pumped into the umbilical arteries and back to

the placenta. The O 2 saturation of the blood in the lower aorta

and umbilical arteries of the fetus is approximately 60%.

FETAL RESPIRATION

The tissues of fetal and newborn mammals have a remarkable

but poorly understood resistance to hypoxia. However, the O 2

saturation of the maternal blood in the placenta is so low that

the fetus might suffer hypoxic damage if fetal red cells did not

FIGURE 34–15 Changes in uterine blood flow and the
amount of O 2 in uterine venous blood during pregnancy. (After
Barcroft H. Modified and redrawn with permission from Keele CA, Neil E: Samson
Wright’s Applied Physiology, 12th ed. Oxford University Press, 1971.)

Parturition

Parturition

Uterine blood flow

Fetal weight

Systemic

venous

blood

Uterine

venous

blood

Time after conception

O

saturation 2

Relative units

FIGURE 34–16 Diagram of a section through the human

placenta, showing the way the fetal villi project into the maternal

sinuses. (Reproduced with permission from Benson RC: Handbook of Obstetrics and

Gynecology, 8th ed. Originally published by Appleton & Lange. Copyright © 1983

McGraw-Hill.)

Amnion

Septum

Umbilical arteries

Umbilical

vein

Umbilical cord

Chorion

Villus

Intervillous

space

Spiral arteriole

Basal plate

Chorionic plate

Endometrium

Myometrium