Human Physiology, 14th edition (2016)

Reproduction 743

cavities within the decidua basalis that are located between the
chorionic villi ( fig. 20.48 ). Because of the structure of the pla-
centa, only two cell layers separate molecules in the maternal
blood from those in the fetal blood. In this way, maternal and
fetal blood are brought close together but never mix within the
placenta.
The placenta serves as a site for the exchange of gases
and other molecules between the maternal and fetal blood.
Oxygen diffuses from mother to fetus, and carbon dioxide dif-
fuses in the opposite direction. Nutrient molecules and waste
products likewise pass between maternal and fetal blood; the
placenta is, after all, the only link between the fetus and the
outside world.
But the placenta is not merely a passive conduit for
exchange between maternal and fetal blood. It has a very high
metabolic rate, utilizing about a third of all the oxygen and
glucose supplied by the maternal blood. The rate of protein
synthesis is, in fact, higher in the placenta than in the liver.
Like the liver, the placenta produces a great variety of enzymes
capable of converting hormones and exogenous drugs into less
active molecules. In this way, potentially dangerous molecules
in the maternal blood are often prevented from harming the
fetus.

Endocrine Functions

of the Placenta

The placenta secretes both steroid hormones and protein hor-

mones. The protein hormones include chorionic gonadotropin

( hCG ); growth hormone-variant ( hGH-V ), also known as

placental growth hormone ( PGH ); and chorionic somato-

mammotropin ( hCS ), also called placental lactogen ( hPL ).

These have actions similar to those of some anterior pituitary

hormones ( table  20.7 ). Chorionic gonadotropin has LH-like

effects (previously described); hGH-V works like growth hor-

mone and supplants maternal GH, which stops being secreted

at about 21–25 weeks of pregnancy. Through the stimulation of

protein synthesis, gluconeogenesis, and lipolysis, it helps to pro-

vide nutrients for the fetus. The growth-hormone-like effects of

hCS supplement these actions. Though hCS also has structural

similarities to prolactin, its possible effects on lactation have not

been demonstrated.

Pituitary-like Hormones from the Placenta

The importance of chorionic gonadotropin in maintaining the

mother’s corpus luteum for the first 5½ weeks of pregnancy

has been previously discussed. There is also some evidence

that hCG may in some way help to prevent immunological

rejection of the implanting embryo.

Chorionic somatomammotropin acts together with growth

hormone from the mother’s pituitary to produce a diabetic-like

effect in the pregnant woman. These two hormones promote

(1) lipolysis and increased plasma fatty acid concentration;

Placenta Uterine wall Amniotic fluid

Figure 20.47

Amniocentesis. In this

procedure, amniotic fluid

containing suspended

cells is withdrawn for

examination. Various

genetic diseases can be

detected prenatally by this

means.

Figure 20.48 The circulation of blood within the

placenta. Maternal blood is delivered to and drained from the

spaces between the chorionic villi. Fetal blood is brought to

blood vessels within the villi by branches of the umbilical artery

and is drained by branches of the umbilical vein.

Intervillous pool of

maternal blood

Maternal vein

Mother Fetus

Uterine

endometrium

Chorion

frondosum

Umbilical artery

Umbilical vein

Umbilical cord

Maternal artery

Maternal vein

Maternal artery