How does the bicuspid valve function?

May be normal: The aortic valve is comprised of three leaflets, or cusps, that swing open and closed. Approximately 1% of the population is born with only 2 cups. These swing open and shut in a similar fashion. Depending on the precise architecture of the 2 cusps, there may be increased turbulence of blood flow through the valve, and over time more wear and tear leading to thickening/obstruction, or leaks.

Mitral valve: Books call it the bicuspid valve. Doctors call it the mitral valve. It opens to allow filling of the left ventricle during diastole. It closes when the lv contracts (during systole) to prevent oxygenated blood from returning to the left atrium (and lungs via the pulmonary veins), promoting emptying of the lv through the aortic valve & aorta.