Final answer:
The posterior surface of the heart, mainly composed of the left atrium, is known as the base.
Step-by-step explanation:
The posterior surface of the heart, which is mainly made up of the left atrium, is known as the base. This surface is located superiorly and attaches to the great veins such as the superior and inferior venae cavae, and the great arteries, including the aorta and pulmonary trunk. In contrast, the apex of the heart is the inferior tip located just to the left of the sternum. The sternocostal surface is the anterior surface of the heart, and the diaphragmatic surface is the inferior surface of the heart, resting on the diaphragm. The costal cartilage of the first rib articulates with the manubrium of the sternum not at the top, but lower down at its broadest part. The first costal cartilage is short and massive. It hardly permits any movement, so the two first ribs, together with the manubrium, move up and down together as one solid arch.