The mitral valve (Figure 1.02), has 2 cusps, or leaflets, attached through the chordae tendineae to papillary muscles embedded in the wall of the left ventricle. Valve closure is complex, involving eddy currents resulting from atrial contraction, papillary muscle contraction, and the sphinteric action of the valve annulus. Thus, closure is not solely or even mainly due to rising ventricular pressure!
Papillary muscle contraction also prevents valve eversion into the atrium which causes regurgitation. Valve opening involves an initial opening during rapid filling, partial closure during diastasis, re-opening during atrial systole, and a final closure immediately prior to ventricular systole. This provides the typical "M" pattern seen with echocardiography.
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