Editor-In-Chief: C. Michael Gibson, M.S., M.D. 
The base of the heart, directed upward, backward, and to the right, is separated from the fifth, sixth, seventh, and eighth thoracic vertebræ by the esophagus, aorta, and thoracic duct.
It is formed mainly by the left atrium, and, to a small extent, by the back part of the right atrium.
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 anterior vena cava into the lower, part of the right atrium.
This article was originally based on an entry from a public domain edition of Gray's Anatomy. As such, some of the information contained herein may be outdated. Please edit the article if this is the case, and feel free to remove this notice when it is no longer relevant.
|Anatomy of torso, cardiovascular system: heart|
|Structures||atria (interatrial septum, musculi pectinati) • ventricles (interventricular septum, trabeculae carneae, chordae tendinae, papillary muscle) • valves • cusps|
|Regions||base • apex • grooves (coronary/atrioventricular, interatrial, anterior interventricula, posterior interventricular) • surfaces (sternocostal, diaphragmatic) • borders (right, left)|
|Right heart||(vena cavae, coronary sinus) → right atrium (auricle, fossa ovalis, limbus of fossa ovalis, crista terminalis, valve of the inferior vena cava, valve of the coronary sinus) → tricuspid valve → right ventricle (conus arteriosus, moderator band/septomarginal trabecula) → pulmonary valve → (pulmonary artery and pulmonary circulation)|
|Left heart||(pulmonary veins) → left atrium (auricle) → mitral valve → left ventricle → aortic valve (aortic sinus) → (aorta and systemic circulation)|
|Layers||pericardium: fibrous pericardium • serous pericardium (pericardial cavity, epicardium/visceral layer) • pericardial sinus|
myocardium • endocardium • cardiac skeleton (fibrous trigone, fibrous rings)
|Conduction system||Cardiac pacemaker • SA node • AV node• bundle of His • Purkinje fibers|