The bicuspid valve, also known as the mitral valve, is a structure in the left side of the heart that controls the flow of oxygenated blood. There are four chambers in the heart through which blood flows during circulation: the right and left atria, which are the upper chambers, and the right and left ventricles, which are the lower chambers. Oxygen-poor blood on the right side of the heart is kept separate from oxygen-rich blood on the left side. The bicuspid valve opens to allow oxygen-rich blood from the left atrium to enter the left ventricle. This same structure also is what prevents blood in the left ventricle from backing up into the left atrium.
A heart valve is composed of thin but tough fibrous tissue that has the capability of opening and closing to ensure that blood flows in only one direction at all times. Just as blood from the right and left sides must be kept separate, blood from the upper and lower chambers also must be prevented from mixing. A properly functioning valve acts as a gateway that opens and closes as needed with each heartbeat. Cusps or leaf-like pointed ends, called leaflets, make up the structure of heart valves. Bicuspid is the name given to the valve that has two leaflets.
The tissue that forms a wall between the atria is called the interatrial septum, and the wall dividing the ventricles is called the interventricular septum. Each heart valve is situated in a ring located between the atrium and ventricle. The base or non-pointed end of the two leaflets of the bicuspid valve are attached to the ring in which they sit, while the free or pointed edges touch each other to form a closure. These free edges are connected to another structure called the chordae tendineae, which has the appearance of flexible strings.
Chordae tendineae extend down into the ventricle and serve to prevent the leaflets or "flaps" of the valve from springing back up into the left atrium when under the pressure created by each heart beat. All of the other valves in the heart are controlled in the same manner by the chordae tendineae, but the left ventricle has to withstand the most pressure as it pumps oxygen rich blood directly to the aorta — the largest artery in the body.