Hepatic Vein Sphincters in Elasmobranchs and Their Significance in Controlling hepatic Blood flow*

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Abstract
1. Discrete muscular sphincters have been discovered on the hepatic veins of elasmobranch fishes. Their anatomy has been studied by gross dissection and by histological techniques. Their functional significance has been assessed by application of drugs to excised sphincters and by measurements of intravascular pressures in free-swimming fishes by means of chronically indwelling catheters. 2. Topical application of acetylcholine caused contraction of the sphincters and adrenaline facilitated relaxation. 3. Pressure measurements disclosed that the hepatic venous pressures in general exceed those in the central systemic veins, thus indicating partial or complete closure of the hepatic vein sphincters. The hepatic vein pressures showed spontaneous phasic changes reflecting closure and opening of the sphincters. 4. It is suggested that hepatic vein sphincters are of particular significance in elasmobranchs since negative pressures prevail in the sinus venosus and central venous sinuses, exerting suctional attraction for venous blood. A sphincteric release of hepatic blood provides a structural basis for delicate adjustments of the volume and transit time of blood in the liver and for mobilization of blood stored in the liver at times of increased demand on cardiac output. 5. The findings are discussed in relation to general mechanics of venous return in fishes and are compared with studies of hepatic vascular sphincters in other vertebrates.