Page 10, Cardiac Cycle, Dr. D. Penney
The AV Valves:
The mitral valve, 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. Closure is not solely or even mainly due to rising ventricular pressure! Papillary muscle contraction also prevents valve eversion into the atrium which would cause 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 is the typical "M" pattern seen with echocardiography.
Although the right and left hearts are driven by a common pacemaker, the timing of their mechanical events do not occur simultaneously. Right atrial contraction precedes left atrial because it is the site of the sinoatrial node, the primary pacemaker. Left ventricular contraction precedes right ventricular because the left bundle branch is bigger and thus depolarization of the left ventricle begins first. The earlier opening and later closing of the pulmonic than the aortic valve hinges on the fact that afterload is far lower on the former than on the latter.
Next Page
Previous Page
Back To Index