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.
In this article, we will look at the anatomy of the layers of the heart wall and their clinical correlations.
Pro Feature - 3D Model
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, closely resembling the endothelium of blood vessels.
Beyond providing a smooth lining for efficient blood flow, the endocardium contributes to cardiac development and plays a role in regulating myocardial contractions.
Subendocardial Layer
The subendocardial layer is a thin layer of loose connective tissue that lies between the endocardium and the myocardium.
It serves as a crucial interface, housing the Purkinje fibres and other components of the heart’s conducting system.
Because it contains the conduction pathways, damage to this layer (such as from ischaemia) can lead to various cardiac arrhythmias.
Fig 1
The layers of the heart wall.
Clinical Relevance
Endocarditis
Endocarditis refers to inflammation of the endocardium, most commonly affecting the heart valves.
It is typically caused by bacterial infection, leading to the formation of vegetations (masses of platelets, fibrin, microorganisms, and inflammatory cells) on the valve surfaces.
This can result in valvular damage, the development of murmurs, and an increased risk of recurrent infections. If left untreated, complications may include heart failure or embolic events.
Myocardium
The myocardium forms the bulk of the heart wall and consists of involuntary striated cardiac muscle.
It is the contractile layer responsible for generating force to pump blood throughout the body.
Subepicardial Layer
The subepicardial layer lies between the myocardium and the epicardium. It contains coronary vessels, nerves, and connective tissue.
Clinical Relevance
Myocarditis
Myocarditis is inflammation of the myocardium, often triggered by viral infections (e.g. coxsackie B virus). Common symptoms include chest pain, palpitations, and shortness of breath.
This inflammation can directly damage the myocardial tissue, impairing its ability to contract effectively. As a result, myocarditis can lead to arrhythmias, cardiomyopathy, and ultimately heart failure due to reduced cardiac output.
Epicardium
The epicardium is the outermost layer of the heart wall and is equivalent to the visceral layer of the serous pericardium.
It is composed of loose connective tissue and fat, and is lined externally by simple squamous epithelial cells (mesothelium).
The epicardium has several functions:
- Serves as a protective outer covering
- Secretes pericardial fluid, which lubricates the heart within the pericardial cavity
- Reduces friction during cardiac contractions