The venous system in frogs includes veins that bring blood from various parts of the body to the heart. There are two pre caval veins and a post caval vein that unite and open into the sinus venosus. The post caval vein collects blood from the hind limbs, kidneys, gonads, and alimentary canal and empties into the sinus venosus. Two portal vein systems exist - the renal portal system transports blood from the hind limbs to the kidneys, and the hepatic portal system transports blood from the alimentary canal to the liver. Blood composition in frogs is similar to other vertebrates, containing plasma, red blood cells, white blood cells, and platelets.