A double-leaflet mitral valve guards the left atrioventricular orifice. The mitral valve has two cusps, anterior and posterior. The mitral valve is located posterior to the sternum at the level of the 4^th costal cartilage. Each of its cusps receives tendinous cords from more than one papillary muscle. These muscles and their cords support the mitral valve, allowing the cusps to resist the pressure developed during contractions (pumping) of the left ventricle. The cords become taut just before and during systole, preventing the cusps from being forced into the left atrium. As it traverses the left ventricle, the bloodstream undergoes two right angle turns, which together result in a 180° change in direction. This reversal of flow takes place around the anterior cusp of the mitral valve.