follow, unlike the arterial system. However, the larger
veins are commonly located parallel to the course
taken by their counterpart in the arterial system. For
instance, the renal vein parallels the renal artery, the
common iliac vein parallels the common iliac artery,
and so forth.
THREE PRINCIPAL VENOUS SYSTEMS.
The three principal venous systems in the body are the
pulmonary, portal, and systemic.
The pulmonary system is composed of four
vessels, two from each lung, which empty into the left
atrium. These are the only veins in the body that carry
freshly oxygenated blood.
The portal system consists of the veins that
drain venous blood from the abdominal part of the
digestive tractthe spleen, pancreas, and gallbladder,
but not the lower rectumand deliver it to the liver.
There, it is distributed by a set of venous capillaries. The
blood in the portal system conveys absorbed substances
from the intestinal tract to the liver for storage,
Figure 1-36.Principal vessels of the venous system.