atrioventricular (AV) septum

a·tri·o·ven·tric·u·lar (AV) sep·tum

(ā'trē-ō-ven-trik'yū-lăr sep'tŭm) [TA]
The small part of the membranous septum of the heart just above the septal cusp of the tricuspid valve that separates the right atrium from the left ventricle.
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