when the left atrium contracts, the blood pressure in the left atrium becomes higher than that in the left ventricle. this causes the bicuspid valve to open and blood enters the left ventricle.the bicuspid valve or mitral valve separates the left atrium from the left ventricle. this is similar in structure and function to the tricuspid valve except that it has two flaps instead of three. when the left ventricle contracts, blood leaves through a large artery, the aortic arch.