Human Heart - Diagram and Anatomy of the Heart - Innerbody Because the heart points to the left, about 2/3 of the heart's mass is found on the left side of the body and the other 1/3 is on the right. Anatomy of the Heart Pericardium. The heart sits within a fluid-filled cavity called the pericardial cavity. The walls and lining of the pericardial cavity are a special membrane known as the pericardium.