The mitral or left atrioventricular valve is situated around the left atrioventricular orifice between the left atrium and ventricle.
It acts to facilitate flow of blood between the left atrium and ventricle, but to inhibit reverse flow. It does this in several phases:
- at the beginning of diastole, left atrial pressure is greater than left ventricular pressure, the valve opens and there is passive filling of the left ventricle
- towards the middle of diastole, the increasing intraventricular pressure forces the valve leaflets to partially converge
- then, atrial systole begins with reopening of the valve and an increased rate of flow between atrium and ventricle
- at the end of atrial systole, the ventricular pressure rises dramatically as ventricular systole begins:
- the valve leaflets converge to eliminate the atrioventricular aperture
- the leaflets are prevented from ballooning into the atrium by the excitation and increase in tension of the papillary muscles
- blood is ejected from the left ventricle via the aortic valve
- when left ventricular pressure is reduced to less than left atrial pressure, the cycle begins again with passive filling of the ventricle and opening of the mitral valve
The mitral is the valve most commonly affected by pathology, typically secondary to rheumatic fever, ischaemic heart disease or congenital anomaly.