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sinus venosus[′sī·nəs və′nō·səs]
(1) In the lancelet, which has no heart, an unpaired vessel that gathers venous blood from the portal vein and the ducts of Cuvier and passes into the abdominal aorta.
(2) In lower vertebrates—cyclostomes, fishes, and amphibians—a division of the heart; a thin-walled sac serving as a reservoir for venous blood, which, when the sinus venosus contracts, is driven into the auricle of the heart; the sinoatrial valves, located in the opening between the sinus venosus and the auricle, prevent reverse flow of blood. In the sinus venosus is found the sinus node (Remak’s ganglion), the center in which the primary impulses of heart contraction originate. In reptiles (except for Hatteria) the sinus venosus loses complete independence and enters, to one degree or another, into the structure of the right auricle. In birds and mammals the sinus venosus is formed in the embryo and subsequently (except in unkeeled birds) is completely incorporated in the wall of the right auricle, together with the sinus node; only in Monotremata are the sinoatrial valves preserved.
(3) In a number of invertebrates, the same as venous lacunae.