The seven major veins in the body bring blood back toward the
heart. From the capillaries blood enters small veins, called
venules, that merge into larger and larger veins, until they
finally join the body's largest vein, the vena cava, returning
the oxygen-poor blood to the right atrium of the heart. The vena
cava actually has two branches: the lower branch brings blood
from the lower part of the body, while the upper branch carries
blood from the upper part of the body and the brain. On its
journey from the heart to the tissues, blood is forced through
the arteries at high pressure. But on the return journey through
the veins and back to the heart, the blood flows at low
pressure. It's kept moving by the muscles in the arms and legs
that compress the walls of the veins, and by valves in the veins
that prevent the blood from flowing backward. Because each type
of blood vessel performs a different job under very different
pressure, the structures of the arteries, veins, and capillaries
are quite different. Ve