Essentials of Anatomy and Physiology

The Vascular System 303

other waste products. The site of exchange between
fetus and mother is the placenta, which contains fetal
and maternal blood vessels that are very close to one
another (see Figs. 13–8 and 21–5). The blood of the
fetus does not mix with the blood of the mother; sub-
stances are exchanged by diffusion and active trans-
port mechanisms.

The fetus is connected to the placenta by the

umbilical cord, which contains two umbilical arteries

and one umbilical vein (see Fig. 13–8). The umbilical

arteriesare branches of the fetal internal iliac arteries;

they carry blood from the fetus to the placenta. In the

placenta, carbon dioxide and waste products in the

fetal blood enter maternal circulation, and oxygen

Posterior communicating

Anterior cerebral

Circle of Willis

Anterior communicating

Posterior

cerebral

Middle

cerebral

Basilar

Internal

carotid

External carotid

Vertebral

Left

common carotid

Right

common carotid

ARTERIES

Anterior

cerebral

Anterior communicating

Anterior cerebral

Middle cerebral

Internal carotid

Posterior cerebral

Posteriorcommunicating

Basilar

Vertebral

Spinal cord

Cerebellum

Medulla

Pons

Cerebrum

(temporal lobe)

(frontal lobe)Cerebrum

Figure 13–6. Circle of Willis. This anastomosis is formed by the following arteries: inter-

nal carotid, anterior communicating, posterior communicating, and basilar. The cerebral

arteries extend from the circle of Willis into the brain. The box shows these vessels in an

inferior view of the brain.

QUESTION:Why do so many vessels contribute to the circle of Willis?