The base of the heart is the part of the heart directed opposite to the apex. It is formed mainly by the left atrium and, to a small extent, by the back part of the right atrium.
It is directed superiorly (up), posteriorly (back) and to the right. It is separated from the 5th, 6th, 7th and 8th thoracic vertebrae by the esophagus, aorta and thoracic duct.
The base of the heart is somewhat quadrilateral in form. It is in relation above with the bifurcation of the pulmonary artery and is bounded below by the posterior part of the coronary sulcus, containing the coronary sinus.
On the right it is limited by the sulcus terminalis of the right atrium and on the left by the ligament of the left vena cava and the oblique vein of the left atrium.
The four pulmonary veins, two on either side, open into the left atrium, while the superior vena cava opens into the upper and the inferior vena cava into the lower part of the right atrium.
This article incorporates text from a public domain edition of Gray's Anatomy.