When a vein branches into set of capillary to distribute blood in some parts, such venous circulation is called portal system. Three types of portal systems are found in vertebrates:
(i) Hepatic Portal System
Hepatic portal vein carries blood that is collected by coeliacomesentric (or lienogastric) vein from alimentary canal and redistribute it in the liver. This enables liver to screen the substances absorbed in small intestine before allowing it to pass in the body.
In frog hepatic portal vein is formed by the union of coeliacomesentric vein and anterior abdominal vein.
(ii) Renal Portal System
It is absent in mammal but well developed in some fishes, all amphibians and lower reptiles.
Veins from hindlimbs and other posterior parts of body join to form renal portal vein which branches into kidney.
Here the excretory substances are filtered from blood.
Femoral veins from outer sides of hind limbs divide into outer renal portal vein and inner pelvic vein.
Sciatic vein from inner side of hind limb joins renal portal vein of its side while pelvic veins of left and right sides unite to form anterior abdominal vein running as main vein along the ventral body wall.
Blood is drained out from kidney by renal vein.
(iii) Hypothalamo-hypophysical Portal System
The vein collecting blood from anterior part and hypothalamus of brain enter and pituitary as hypophysical portal vein and branches into capillaries.
Through this system hormones of hypothalamus reach the pituitary.