Interior of the heart, computer artwork. The heart is divided into four chambers; two upper chambers (atria) and two lower chambers (ventricles). Blood flows from the atria to the ventricles. Valves between the atria and ventricles prevent the backflow of blood. The tricuspid valve (bottom left) lies between the right atrium and ventricle, and the bicuspid, or mitral, valve (centre right) lies between the left atrium and ventricle. The valves are prevented from inverting by chordae tendinae (thin lines), which are attached to papillary muscles.