Page 102 - Clinical Anatomy

ECA2 7/18/06 6:42 PM Page 87

The gastrointestinal tract 87

Fig. 67◊The composition
of the portal system.

with its neighbour above and below so that there is, in fact, a continuous
vascular arcade along the whole length of the gastrointestinal canal.

The portal system of veins
The portal venous system drains blood to the liver from the abdominal part
of the alimentary canal (excluding the anal canal), the spleen, the pancreas
and the gall-bladder and its ducts.
The distal tributaries of this system correspond to, and accompany, the
branches of the coeliac and the superior and inferior mesenteric arteries
enumerated above; only proximally (Fig. 67) does the arrangement differ.
The inferior mesenteric vein ascends above the point of origin of its artery
to enter the splenic vein behind the pancreas.
The superior mesenteric vein joins the splenic vein behind the neck of the
pancreas in the transpyloric plane to form the portal vein, which ascends
behind the first part of the duodenum into the anterior wall of the foramen
of Winslow and thence to the porta hepatis. Here the portal vein divides
into right and left branches and breaks up into capillaries running between
the lobules of the liver. These capillaries drain into the radicles of the
hepatic vein through which they empty into the inferior vena cava.

Connections between the portal and systemic
venous systems

Normally, portal venous blood traverses the liver as described above and
empties into the systemic venous circulation via the hepatic vein and infe-
rior vena cava. This pathway may be blocked by a variety of causes which
are classified into: