Hepatic vein sphincters in elasmobranchs and their significance in controlling hepatic blood flow.

The liver exerts its influence in general cellular metabolism and homeostatic control within limits set by the quality and quantity of blood perfusing it. Hepatic outflow reflects hepatic portal flow which is influenced by the diverse blood flow requirements of other visceral organs engaged in metabolic and absorptive functions. In addition, the hepatic blood flow constitutes a sizeable fraction of the total venous return to the heart and is important in the maintenance of an effective overall circulation. The large size of the liver and the hepatic vasculature is as conspicuous in lower aquatic vertebrates as in higher vertebrates. This suggests an important hepatic role in vertebrate cardiovascular dynamics in general. The present paper concerns the discovery of discrete muscular sphincters of the hepatic veinsJ of elasmobranch fishes and their possible role in controlling hepatic blood flow. In a large shark, Selache maximus (weight 1500 kg.), conspicuous muscular sphincters were discovered on the hepatic veins where they empty into the sinus venosus. PI. 1, fig. 1 demonstrates these from a posterior view after ablation of the liver and the hepatic veins. In PI. 1, fig. 2 the sphincters are viewed from the opened sinus venosus. The ducts of Cuvier which connect the common cardinal sinus system to the heart showed regular valves which prevent regurgitation during contraction of the sinus venosus. The functional significance of the sphincters was investigated during studies of the general mechanics of venous return in elasmobranch fishes.