The veins that collect blood from different parts of the body and enter another organ and re-form the capillary network are called portal veins. It consists of hepatic portal and renal portal. Portal veins are:
(i) Caudal vein: The caudal vein enters the trunk from the tail and divides into two branches to form the renal portal vein. The renal portal vein brings blood from the digestive system to the liver.
(ii) Segmental vein: It collects blood from different parts of the body.
(iii) Transverse anastomosis veins: Right and left cardinal sinuses join with some transverse veins to form transverse anastomosis veins.
(iv) Sub clavian vein: It supplies blood directly from the thoracic fins to the sinus venosus.
(v) Hepatic veins: Two hepatic veins arise from the liver and bring blood to the sinus venosus.