The heart is a vital organ in the body that provides continuous blood circulation
through the cardiac cycle. The heart is divided into four main chambers: the two
upper chambers are called the left atrium and the right atrium, and the two
lower chambers are called the right and left ventricle. There is a thick wall of
muscle separating the right side and the left side of the heart called the septum.
Normally with each heartbeat, the right ventricle pumps the same amount of
blood into the lungs than the left ventricle pumps out into the body.
A double-walled protective sac, known as the pericardium, surrounds the heart.
The pericardium protects the heart from physical knocks and shocks, and from
infection. It anchors its surrounding structures, but has no effect over the heart
function in normal individuals. The superficial part of this sac is called the
parietal pericardium. The inner pericardium layer is called the visceral
pericardium. Together, they are known as the serous pericardium because they
contain the pericardial fluid. This pericardial fluid provides a smooth lubricated
sliding surface within which the heart can move in response to its own
contractions and to movement of the diaphragm and lungs.
The outer wall of the human heart is composed of three layers. The outer layer is
called the epicardium, where its major constituent is connective tissue and
functions as a protective layer.
The middle layer is called the myocardium, which is composed of contractile
cardiac muscles. The cells that make up cardiac muscles are called
cardiomyocytes, which contain only one nucleus. Cardiomyocytes have a high
mitochondrial density, which allows them to produce ATP quickly, making them
highly resistant to fatigue, which is important in the heart, as it is constantly
The inner layer is called the endocardium and is in contact with the blood that
the heart pumps. The endocardium also provides protection to the valves and