The base of the heart, directed superiorly, posteriorly, and to the right, is separated from the 5th, 6th, 7th, and 8th thoracic vertebrae

Thoracic vertebrae

In human anatomy, twelve thoracic vertebrae compose the middle segment of the vertebral column, between the cervical vertebrae and the lumbar vertebrae. They are intermediate in size between those of the cervical and lumbar regions; they increase in size as one proceeds down the spine, the upper...

 by the esophagus

Esophagus

The esophagus is an organ in vertebrates which consists of a muscular tube through which food passes from the pharynx to the stomach. During swallowing, food passes from the mouth through the pharynx into the esophagus and travels via peristalsis to the stomach...

, aorta

Aorta

The aorta is the largest artery in the body, originating from the left ventricle of the heart and extending down to the abdomen, where it branches off into two smaller arteries...

, and thoracic duct

Thoracic duct

In human anatomy, the thoracic duct of the lymphatic system is the largest lymphatic vessel in the body. It is also known as the left lymphatic duct, alimentary duct, chyliferous duct, and Van Hoorne's canal....

.

It is formed mainly by the left atrium

Left atrium

The left atrium is one of the four chambers in the human heart. It receives oxygenated blood from the pulmonary veins, and pumps it into the left ventricle, via the mitral valve.-Foramen ovale:...

, and, to a small extent, by the back part of the right atrium

Right atrium

The right atrium is one of four chambers in the hearts of mammals and archosaurs...

.

Somewhat quadrilateral in form, it is in relation above with the bifurcation of the pulmonary artery

Pulmonary artery

The pulmonary arteries carry deoxygenated blood from the heart to the lungs. They are the only arteries that carry deoxygenated blood....

, and is bounded below by the posterior part of the coronary sulcus

Coronary sulcus

The atria of the heart are separated from the ventricles by the coronary sulcus ; this contains the trunks of the nutrient vessels of the heart, and is deficient in front, where it is crossed by the root of the pulmonary artery.-External links:*...

, containing the coronary sinus

Coronary sinus

The coronary sinus is a collection of veins joined together to form a large vessel that collects blood from the myocardium of the heart. It is present in all mammals, including humans...

.

On the right it is limited by the sulcus terminalis

Terminal sulcus (heart)

The separation of the right atrial pectinate muscle from the sinus venarum is indicated externally by a groove, the terminal sulcus , which extends from the front of the superior vena cava to the front of the inferior vena cava, and represents the line of union of the sinus venosus of the embryo...

 of the right atrium

Right atrium

The right atrium is one of four chambers in the hearts of mammals and archosaurs...

, and on the left by the ligament of the left vena cava and the oblique vein of the left atrium

Left atrium

The left atrium is one of the four chambers in the human heart. It receives oxygenated blood from the pulmonary veins, and pumps it into the left ventricle, via the mitral valve.-Foramen ovale:...

.

The four pulmonary veins, two on either side, open into the left atrium

Left atrium

The left atrium is one of the four chambers in the human heart. It receives oxygenated blood from the pulmonary veins, and pumps it into the left ventricle, via the mitral valve.-Foramen ovale:...

, while the superior vena cava

Superior vena cava

The superior vena cava is truly superior, a large diameter, yet short, vein that carries deoxygenated blood from the upper half of the body to the heart's right atrium...

 opens into the upper, and the inferior vena cava into the lower, part of the right atrium

Right atrium

The right atrium is one of four chambers in the hearts of mammals and archosaurs...

.