In size the heart is somewhat larger than the clenched fist. It measures 12.5 cm. (5 in.) in length, 7.75 cm. (3 1/2 in.) in width, and 6.25 cm. (2 1/2 in.) in thickness. Its weight in the adult male is 250 to 300 Gm. (8 to 10 oz.), in the female it is 60 Gm. (2 oz.) less.

It lies enclosed in its pericardium in the middle mediastinum between the sternum (from the upper edge of the third costal cartilage to the sternoxiphoid articulation) in front, and the bodies of the fifth, sixth, seventh, and eighth thoracic vertebrae behind. Laterally it reaches from two centimetres to the right of the sternum nearly to the left nipple line. On each side of it are the lungs, from which it is separated by the pleurae and pericardium with the phrenic nerves between. Above are the great vessels and below it rests on the central tendon of the diaphragm.

In shape the heart resembles an acorn, the atria {auricles), forming the upper right portion and the ventricles the lower left portion. It lies with its right side resting on the diaphragm and its apex pointing forward and to the left.

For convenience one speaks of a base, an apex, a right border, a lower border, and a left border.