False-colour scanning electron micrograph (SEM) of the endocardium, the lining of the interior of the heart. This region is next to one of the valves that divide the heart's upper & lower chambers, the atria and ventricles. Both valves are supported by projections from the walls of the ventricles called papillary muscles, part of one is visible towards the right of this image. The tricuspid valve controls blood flow between the right atrium & ventricle & the bicuspid (mitral) valve operates between the left atrium & ventricle. This exact location is not specified. Magnification: x113 at 6x7cm size.