The mitral valve is a two-flap tissue valve in the heart that lies between the left atrium and the left ventricle. The mitral valve and the tricuspid valve are known collectively as the atrioventricular valves because they lie between the atria and the ventricles of the heart and control the flow of blood.
The mitral valve opens to allow the oxigen-rich blood to flow from the left atrium into the left ventricle, not letting it flow back into the atrium as the heart contracts (systole) to pump this blood to the rest of the body through the aorta.
The mitral valve has two cusp-like leaflets, the anteromedial leaflet and the posterolateral leaflet, which guard the opening. The opening is surrounded by a fibrous ring known as the mitral valve annulus. The orientation of the two leaflets resemble a bishop's miter, whence the valve receives its name.
During left ventricular diastole, after the pressure drops in the left ventricle due to relaxation of the ventricular myocardium, the mitral valve opens, and blood travels from the left atrium to the left ventricle. About 70-80% of the blood that travels across the mitral valve occurs during the early filling phase of the left ventricle. These valve leaflets are prevented from prolapsing (collapsing) into the left atrium by the action of tendons attached to the posterior surface of the valve, chordae tendineae.