mitral valve

Meaning: The mitral valve (bicuspid or left atrioventricular valve) sits between the left atrium and left ventricle and keeps blood moving one way. It opens when left atrial pressure exceeds left ventricular pressure, letting oxygenated blood fill the ventricle, and closes when the ventricle contracts and its pressure rises. Chordae tendineae attached to papillary muscles stop the flaps from everting into the atrium. Valves move passively with pressure differences; they are not opened by their own muscle.

On the MCAT: Expect questions that trace blood flow: pulmonary veins → left atrium → mitral valve → left ventricle → aortic valve → aorta. On a pressure-versus-time graph, the mitral valve closes where ventricular pressure first rises above atrial pressure, at the start of systole. Its closure, together with the tricuspid valve, produces the first heart sound (S1). A leaky valve lets blood flow back into the left atrium; a narrowed valve raises left atrial pressure, which can back up into the pulmonary circulation.

Common confusion: Students mix the mitral valve up with the tricuspid valve. The mitral valve is on the left and has two cusps; the tricuspid valve is on the right and has three. Both are atrioventricular valves that close at S1, while the aortic and pulmonary semilunar valves close at S2.