What is the heart made up of?

Layers of Heart Wall:

There are three layers to the heart wall. From the inside out, these are:

1. Endocardium: also referred to as the intima in arteries, the endocardium is the innermost lining of the heart's chambers. It is a thin layer of flat endothelial cells that lie on top of a thin basement membrane and subendothelial connective tissue. The endocardium covers all the heart's internal structures such as the four heart valves and muscular ridges inside the atria.

2. Myocardium: consisting predominantly of cardiac muscle cells (cardiomyocytes), the myocardium is the thick middle layer of heart wall. It forms the bulk of the heart's muscle and is responsible for generating the force of heart contraction. The myocardial fibers are arranged in complex patterns, creating a meshwork that allows for efficient and coordinated contraction of the heart chambers.

3. Epicardium: also called the visceral pericardium, the epicardium is the outer layer of heart wall. It is composed of serosal tissue and consists of two layers:

- Inner layer: this is made of connective tissue and contains blood vessels, nerves, lymphatic vessels, and adipose (fat) cells.

- Outer layer: composed of mesothelial cells (simple squamous epithelial cells) that cover the surface of the heart and produce a lubricating serous fluid that reduces friction of the heart as it contracts and relaxes within the pericardial cavity.