left atrioventricular orifice left atrioventricular orifice the left atrioventricular orifice (left atrioventricular opening or mitral orifice) is placed below and to the left of the aortic orifice. left atrioventricular orificedetailsidentifierslatinostium atrioventriculare sinistrumta98a12.1.04.002ta23988fma9335anatomical terminology it is a little smaller than the corresponding aperture of the opposite side. it is surrounded by a dense fibrous ring, covered by the lining membrane of the heart, and is reguarded as the bicuspid or mitral valve.