The human
hepatic portal system delivers about three-fourths of the blood going to the
liver. The final common pathway for transport of venous blood from the spleen, pancreas, gallbladder and the abdominal portion of the gastrointestinal tract (with the exception of the inferior part of the anal canal and sigmoid colon) is through the
hepatic portal vein. This portal vein is formed by the union of the
superior mesenteric vein and the
splenic vein posterior to the neck of the pancreas at the level of vertebral body L1. Ascending towards the liver, the portal vein passes posterior to the superior part of the duodenum and enters the right margin of the lesser omentum. It is anterior to the omental foramen and posterior to both the bile duct, which is slightly to the right, and the hepatic artery proper, which is slightly to the left. On approaching the liver, the portal vein divides into right and left branches which enter the liver parenchyma. It gives off the right and left gastric veins, the cystic vein and the para-umbilical veins as tributaries. The
hypophyseal portal system transports
hormones from the
hypothalamus to the
pituitary gland. In the
renal portal system adrenal medulla capillaries are downstream from
adrenal cortex capillaries. This portal system delivers high concentrations of adrenal cortical hormones to the adrenal medulla. In particular,
glucocorticoids induce the enzymatic conversion of
norepinephrine to
epinephrine in the adrenal medulla. By contrast, the
ganglia of
the sympathetic trunk mainly produce norepinephrine because their cells are not bathed in high concentrations of glucocorticoids. The venous blood of the
pancreatic islets is upstream from the capillary system of the exocrine pancreas via
efferent ducts. The
acini of the exocrine pancreas are therefore directly exposed to high concentrations of hormones from the endocrine pancreas, forming the pancreatic portal system. == References ==