The muscular walls of the heart consist of three major "layers." The bulk of the walls is made up of a layer of cardiac muscle and is called the myocardium. The muscle is enclosed on the outside by the epicardium and on the inside by the endocardium. The heart is also covered completely by a protective sac called the pericardium.
Epicardium
The outside of the myocardium is covered with a thin layer called the epicardium.
This thin layer consists mostly of connective tissue and fat.
Myocardium
This
is the thick layer of cardiac muscle which is responsible for the contraction
and relaxation of the ventricles and atria. This layer is composed almost completely
of cardiomyocytes.
Endocardium
The
inner lining of the heart is called the endocardium. It is a smooth membrane
of endothelial cells that lines not
only the chambers of the heart, but the valves
as well. This is the same layer that covers the inside of all of the blood
vessels
of the cardiovascular
system.
Pericardium
The heart is enclosed in a bag called the pericardium. The pericardium is an extremely tough membrane that acts as protection for the heart and is not directly connected to the walls of the heart. It is composed of two layers, and between these two layers there is a small amount of fluid that serves as lubricant. This allows for the free movement (contractions) of the heart muscle within the bag. A disease called pericarditis occurs when this fibrous sac becomes inflamed.
