Left border of heart

The left border of the heart (Obtuse margin of heart) is the superolateral border located between the anterior surface and the left surface of the heart.

The left border of the heart is formed by the left ventricle, with a small superior portion contributed by the left atrial appendage.

It is termed "obtuse margin" because the angle formed between the anterior or sternocostal surface of the heart with the left lateral or left pulmonary surface of the heartis more than 90 degrees, therefore an "obtuse" angle.

In relation to the obtuse margin of the heart, the first marginal of circumflex coronary artery is called Obtuse Marginal 1 (OM1).