Portal Systems

circulatory systems in which blood—in contrast to the ordinary systems—passes through capillaries between two veins rather than between an artery and a vein. Blood flows through afferent, or portal, veins to an organ in which these veins branch into capillaries collected in efferent veins. A portal system is found in the liver of all vertebrates and acraniates (lancelets) and in the kidneys of all vertebrates except cyclostomes and mammals. The liver’s portal system is formed by the portal vein, through which blood emerging from the intestinal canal flows into the liver. In amphibians the portal system of the liver also includes the abdominal vein, which carries some of the blood from the posterior extremities. Blood usually flows from the liver through two hepatic veins, less commonly through one (in the lancet and lamprey). The portal system of the liver provides for the deposition of nutrients (glycogen and others) and detoxifies the toxic metabolic products formed during digestion. In fish the kidney portal system is formed by the caudal vein, which branches off into two trunks—the portal veins of the kidneys, from which blood enters the posterior cardinal veins. In terrestrial vertebrates, excluding tailless amphibians (frogs and others), the portal system of the kidneys consists not only of the caudal vein but also the iliac veins of the posterior extremities; blood from the kidneys flows into the vena cava posterior. In tailless amphibians the portal system of the kidneys consists entirely of the iliac veins. It is poorly developed in birds, since much of the blood from the portal veins of the kidneys passes directly into the vena cava posterior, bypassing the capillary network. The portal system of the kidneys enables venous blood from the main organs of locomotion (the tail in fish and the hind legs in terrestrial animals) to pass through the capillary filter of the kidneys; thus, the various metabolic products are arrested in the kidneys.