Base of heart

The base of the heart (basis cordis) constitutes the dorsal aspect of the organ and is directed dorsocranially toward the vertebral column.

It is opposite the apex and is broad, relatively flat, and fixed in position compared with the more mobile ventral portions of the heart. In domestic mammals, the base lies approximately between the third and fifth intercostal spaces, depending on species, body conformation, and cardiac size.

The base of the heart represents the hilum of the organ and is formed predominantly by the atria, with the left atrium contributing the greatest portion. The atrial walls at the base are thicker and smoother than the auricles, which project cranially from the atria and are not considered part of the base proper. The left atrium occupies a dorsal and slightly caudal position, while the right atrium lies more cranially and to the right. The interatrial septum is oriented obliquely and contributes internally to the structure of the base.

The base serves as the site of attachment and entry for the great vessels. From its dorsal surface arise the ascending aorta and the pulmonary trunk, while the cranial and caudal venae cavae enter the right atrium, and the pulmonary veins enter the left atrium. These vessels are embedded in fibrous connective tissue and partially surrounded by reflections of the serous pericardium, which form the transverse and oblique pericardial sinuses. The transverse pericardial sinus lies between the arterial outflow tracts ventrally and the venous inflow dorsally, immediately adjacent to the base of the heart.

Externally, the base is related dorsally to the tracheal bifurcation, main bronchi, esophagus, and thoracic aorta. The tracheal bifurcation typically lies just cranial to the base, especially in the dog, while the esophagus courses dorsally and slightly to the left of the left atrium. This close relationship explains the clinical significance of left atrial enlargement, which may result in esophageal compression. Laterally, the base is flanked by the lungs, and its dorsal surface is in contact with mediastinal pleura.