The valve located between the left upper and lower chambers of the heart is the mitral valve. The left upper chamber is the left atrium, and the lower chamber is the left ventricle. As all heart valves, the mitral valve opens and shuts to ensure the proper flow of blood.
A normally functioning mitral valve opens with the contraction of the heart to let oxygen-rich blood travel from the left atrium to the left ventricle. It then shuts to prevent a backward flow of blood into the atrium. From the left ventricle, the blood is pushed out into the aorta and then through the body.