right border of heart right border of heart the right border of the heart (right margin of heart) is a long border on the surface of the heart, and is formed by the right atrium. the atrial portion is rounded and almost vertical; it is situated behind the third, fourth, and fifth right costal cartilages about 1.25 cm. from the margin of the sternum. the ventricular portion, thin and sharp, is named the acute margin; it is nearly horizontal, and extends from the sternal end of the sixth right coastal cartilage to the apex of the heart. right border of heartsternocostal surface of heart. (right margin visible but not labeled.)detailsidentifierslatinmargo dexter cordisanatomical terminology