M01.05.015 The Heart Wall

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Learning Objective

Understand the structure and function of the three layers of the heart wall—endocardium, myocardium, and epicardium—and their clinical correlations.

Overview

The heart wall is composed of three distinct layers:

Endocardium (innermost)
Myocardium (middle, muscular layer)
Epicardium (outermost)

Each layer has a unique structure and function, and pathology affecting any of them can lead to significant cardiovascular disease.

Endocardium

The endocardium is the inner lining of the heart wall, covering the internal chambers and valves.

It is composed of simple squamous epithelium and loose connective tissue, resembling the endothelium of blood vessels. Beyond providing a smooth lining for efficient blood flow, it contributes to cardiac development and helps regulate myocardial contractions.

Subendocardial Layer

The subendocardial layer lies between the endocardium and myocardium, housing Purkinje fibres and other components of the cardiac conduction system. Damage to this layer (e.g., from ischemia) can lead to cardiac arrhythmias.

Clinical Relevance

Endocarditis is inflammation of the endocardium, often affecting valves, usually caused by bacterial infection. This can form vegetations, cause valvular damage, murmurs, and increase the risk of embolic events or heart failure.

Activity

Myocardium

The myocardium forms the bulk of the heart wall and consists of involuntary striated cardiac muscle. It is the contractile layer responsible for pumping blood.

Subepicardial Layer

Between myocardium and epicardium lies the subepicardial layer, containing coronary vessels, nerves, and connective tissue.

Clinical Relevance

Myocarditis is inflammation of the myocardium, often triggered by viruses like Coxsackie B. Symptoms include chest pain, palpitations, and shortness of breath. It can impair contraction, causing arrhythmias, cardiomyopathy, and heart failure.

Epicardium

The epicardium is the outermost layer, equivalent to the visceral layer of the serous pericardium. Composed of loose connective tissue, fat, and lined by simple squamous epithelium (mesothelium).

The epicardium functions to: