The walls of the chambers of the heart consist of three layers:

• The endocardium
• The epicardium
• Myocardium

Page 2 Endocardium and Epicardium
The chambers themselves are lined with a simple epithelium known as the endocardium.

Likewise, the epicardium is a simple epithelium tissue. It surrounds the outside of the heart.
The epicardium is also referred to as the visceral pericardium.

Page 3 Myocardium
By far the most important of the three layers in the heart walls is the myocardium, the middle layer.

It is made up of cardiac muscle tissue.

Cardiac muscle tissue consists of fibers formed by the fusion of many individual cells (syncytium). These cardiac fibers are striated and branched.

Page 4 Myocardium of Ventricles
The myocardium is thicker in the walls of the ventricles than the atria.

This is because greater pressures are needed for the ventricles to perform their function. The wall of the left ventricle is especially thick, since it has to drive the blood throughout the body.

The inner surfaces of the ventricular walls have ridges of muscle known as the trabeculae carneae, with spaces between them.

Page 5
When the musculature within a chamber wall contracts, the lumen (cavity) decreases in diameter. This is particularly true of the left ventricle.

There is also a twisting or wringing action of the left ventricle that causes the apex of the heart to
hit against the inner surface of the chest wall--the apex beat.

Page 6
The stroke volume is the amount of blood forced out of each ventricle in one contraction.

The cardiac output is the volume of blood pumped out of the ventricles (RT into the lungs, LT into the systemic circulation) in one minute (expressed in liters per minute).

These volumes will change according to the needs of the body.