HUMAN BIOLOGY

Development anD aging 337

placenta Temporary organ

in which blood vessels of

the mother and an embryo

(later the fetus) are in con-

tact; it provides nutrients

and oxygen to an embryo

and carries away wastes.

What is the placenta?

• The placenta is an organ in which maternal and embryonic blood
  vessels are in close contact.


• The placenta provides nutrients and oxygen to the embryo
  from the mother’s bloodstream. The embryo’s bloodstream also
  discharges wastes that the mother’s bloodstream will transport
  away.

taKe-Home message

chorionic villus,

with fetal blood

vessels inside it

maternal blood

in uterine lining

placenta

amniotic

fluid

fetal blood

vessels inside

umbilical cord

umbilical

cord

pool of

maternal

blood

4 weeks 8 weeks

Ty pical position of the embryo and placenta

Artist’s depiction of the view inside the uterus, showing a fetus

connected by an umbilical cord to the pancake-shaped placenta.

The placenta consists of maternal and fetal tissue. Fetal blood flow-

ing in vessels of chorionic villi exchanges substances by diffusion

with maternal blood around the villi. The bloodstreams do not mix.

the placenta: a pipeline for oxygen,

nutrients, and other substances

n    Three weeks after fertilization, nearly a fourth of the inner

surface of the uterus has become a spongy tissue, the

developing placenta.

The placenta is the link through which nutrients and

oxygen pass from the mother to the embryo and waste

products from the embryo pass back to the mother’s blood-

stream. The placenta is a way of sustaining a developing

baby while allowing its blood vessels to develop apart from

the mother’s.

A fully developed placenta is considered an organ,

but it’s useful to think of it as a close association of the

chorion and the upper cells of the endometrium where

Figure 17.11 Animated! Blood vessels of the mother and fetus are in close contact in a full-term placenta. (© Cengage Learning)

1 7. 6

the embryo implanted (Figure 17.11).

The mother’s side of the placenta

is endo metrial tissue that contains

arterioles and venules.

As the embryo develops, very

little of its blood ever mixes with that

of its mother. Oxygen and nutrients

simply diffuse out of the mother’s

blood vessels, across the blood-filled spaces in the endome-

trium, then into the embryo’s blood vessels. Carbon dioxide

and other wastes diffuse in the opposite direction, leaving

the embryo.

Besides nutrients and oxygen, many other substances

taken in by the mother—including alcohol, caffeine, drugs,

pesticide residues, and toxins in cigarette smoke—can

cross the placenta, as can HIV.

Copyright 2016 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s).