The wall of the heart is composed of three layers of unequal thickness: the epicardium, myocardium, and endocardium. These layers surround the chambers of the heart and contribute to its structure and function. The myocardium forms the bulk of the heart wall, while the epicardium and endocardium provide external and internal coverings.

Between the parietal and visceral layers is the pericardial cavity, a narrow space filled with serous fluid. This lubricating fluid reduces friction between the surfaces of the heart and the pericardium during cardiac contractions.

The epicardium is composed of a layer of simple squamous epithelium known as the mesothelium, supported by loose connective tissue. This tissue contains blood vessels, nerves, and fat and contributes to the secretion of the serous fluid within the pericardial cavity.

Myocardium

The myocardium is the middle and thickest layer of the heart wall and consists primarily of cardiac muscle tissue. It forms more than seventy percent of the ventricular wall and is responsible for the pumping action of the heart. 

The myocardium is built upon a framework of connective tissue containing blood vessels and nerve fibers that support and regulate cardiac function.

Cardiac muscle fibers are striated and arranged in bundles and layers that follow a highly complex pattern. Rather than running in straight lines, the fibers spiral and curve around the chambers of the heart. 

The muscle fibers form figure-eight arrangements around the atria and the bases of the great vessels, while deeper fibers wrap around the ventricles and converge toward the apex. This intricate arrangement allows the heart to contract efficiently and generate the force necessary to propel blood through the pulmonary and systemic circulations.

The myocardium of the ventricles is much thicker than that of the atria because the ventricles perform the major work of pumping blood. 

The left ventricle possesses the thickest muscular wall because it must generate high pressures to propel blood throughout the systemic circulation. In contrast, the right ventricle pumps blood only to the lungs and therefore requires less force.

The right ventricle has a thinner myocardium and the distinction between its muscular layers is less pronounced. Its middle fibers also run transversely, while the deeper fibers project into the ventricular cavity and contribute to internal structures.

The muscular tissue of the interventricular septum arises from both ventricles, although the majority originates from the left ventricle. The uppermost portion of the septum remains thin and fibrous and is known as the membranous septum.

Fibrous Skeleton of the Heart

Many cardiac muscle fibers originate from dense fibrous rings that surround the atrioventricular openings. These rings separate the musculature of the atria from that of the ventricles and provide attachment sites for the heart valves. Together, these structures form the fibrous skeleton of the heart.

The right fibrous ring completely surrounds the right atrioventricular opening, whereas the left ring is incomplete because the root of the aorta interrupts its anterior portion. The left ring is connected to two dense connective tissue masses known as the right and left fibrous trigones.

The fibrous skeleton provides structural support, maintains the shape of the valve openings, and electrically isolates the atria from the ventricles.

Endocardium

The endocardium is the innermost layer of the heart wall. It lines the chambers of the heart and covers the surfaces of the valves. The endocardium is continuous with the inner lining, or intima, of the blood vessels.

This layer consists primarily of simple squamous epithelium called endothelium, supported by a thin layer of connective tissue rich in elastic fibers. The endocardium is thicker in the atria than in the ventricles. In the ventricular walls, it is so thin that the reddish muscular tissue beneath it may be seen through the membrane.

Within the auricular appendages, the endocardium is relatively thin, whereas in the venous portions of the atria it becomes thicker and more opaque.

Although once regarded as merely a lining membrane, the endocardium is now known to play an active physiological role. Endothelial cells contribute to the regulation of myocardial contraction, influence the growth and maintenance of cardiac muscle cells, and secrete substances called endothelins. These molecules help regulate vascular tone, ionic balance, and the contractile properties of the myocardium.

Through its close association with the myocardium and the circulatory system, the endocardium contributes to the efficient function of the heart as a pump.