The fully divided heart

The fully divided heart

Human heart removed from a 64-year-old male

Surface anatomy of the human heart. The heart is demarcated by:
-A point 9 cm to the left of the midsternal line (apex of the heart)
-The seventh right sternocostal articulation
-The upper border of the third right costal cartilage 1 cm from the right sternal line
-The lower border of the second left costal cartilage 2.5 cm from the left lateral sternal line.^[12]

Archosaurs, (crocodilians, birds), and mammals show complete separation of the heart into two pumps for a total of four heart chambers; it is thought that the four-chambered heart of archosaurs evolved independently from that of mammals. In crocodilians, there is a small opening, the foramen of Panizza, at the base of the arterial trunks and there is some degree of mixing between the blood in each side of the heart; thus, only in birds and mammals are the two streams of blood - those to the pulmonary and systemic circulations - kept entirely separate by a physical barrier.^[6]
In the human body, the heart is usually situated in the middle of the thorax with the largest part of the heart slightly offset to the left, although sometimes it is on the right (see dextrocardia), underneath the sternum. The heart is usually felt to be on the left side because the left heart (left ventricle) is stronger (it pumps to all body parts). The left lung is smaller than the right lung because the heart occupies more of the left hemithorax. The heart is fed by the coronary circulation and is enclosed by a sac known as the pericardium; it is also surrounded by the lungs. The pericardium comprises two parts: the fibrous pericardium, made of dense fibrous connective tissue, and a double membrane structure (parietal and visceral pericardium) containing a serous fluid to reduce friction during heart contractions. The heart is located in the mediastinum, which is the central sub-division of the thoracic cavity. The mediastinum also contains other structures, such as the esophagus and trachea, and is flanked on either side by the right and left pulmonary cavities; these cavities house the lungs.^[13]