mitral valve

The mitral valve is a one-way door between the left atrium and the left ventricle. Like a flap that swings open to let blood pour forward and then snaps shut to stop it flowing backward, it keeps oxygen-rich blood moving in the right direction.

It has two leaflets (hence its older name, the bicuspid valve), said to resemble a bishop's mitre — the pointed hat that gives the valve its name. The leaflets open during ventricular filling and close tightly when the ventricle contracts, preventing blood from leaking back into the atrium.

The leaflets are tethered by thin cords (the chordae tendineae) to papillary muscles inside the ventricle, which act like guy-ropes holding a tent flap so the valve does not blow inside out under pressure. When this apparatus fails, blood leaks backward (mitral regurgitation); when the valve narrows, the opening is restricted (mitral stenosis).