The heart is a muscular organ positioned on the left side of the chest. Although it's only about the size of a clenched fist, the heart is responsible for pumping blood throughout the entire circulatory system. It is a machine with parts that depend on one another to make sure that the circulation remains normal. The heart is divided into a right and left side by a muscular wall called the septum.
Valves control the flow of blood between the chambers of the heart.
The bicuspid or mitral valve separates the left atrium from the left ventricle. Valves also control the flow of blood between the heart and arteries. The aortic valve separates the left ventricle from the aorta These valves ensure that blood flows in the correct direction in order for blood to enter the ventricles from the atria and exit through the large arteries.