M01.05.015 The Heart Wall

The heart wall is comprised of three distinct layers: the endocardium, myocardium, and epicardium. Each layer plays a crucial role in the heart’s function and has clinical significance. In this article, we will explore the structure and clinical relevance of these layers.

1. Endocardium

Structure

• Location: Innermost layer of the cardiac wall.
• Composition:
  • Loose connective tissue
  • Simple squamous epithelial tissue (similar to endothelium)
• Function:
  • Lines the heart’s cavities and valves
  • Regulates cardiac contractions
  • Aids in cardiac embryological development

Clinical Relevance

Endocarditis

• Definition: Inflammation of the endocardium, often affecting heart valves.
• Common cause: Infective endocarditis (bacterial infection).
• Pathophysiology: Bacteria form vegetations on the valve, causing inflammation and potential valve damage, leading to murmurs.
• Consequences: Increased susceptibility to future infections and re-infection.

2. Subendocardial Layer

Structure

• Location: Between the endocardium and myocardium.
• Composition:
  • Loose fibrous tissue
  • Contains vessels and nerves of the heart’s conduction system (e.g., Purkinje fibers)

Clinical Relevance

• Arrhythmias: Damage to this layer can disrupt the conduction system, leading to various arrhythmias.

3. Myocardium

Structure

• Location: Middle layer of the heart wall.
• Composition: Cardiac muscle (involuntary striated muscle).
• Function: Responsible for the heart’s contraction, enabling blood pumping.

Clinical Relevance

Myocarditis

• Definition: Inflammation of the myocardium, often caused by viral infections (e.g., adenovirus, coxsackie B).
• Symptoms: Chest pain, shortness of breath, tachycardia.
• Consequences: Can lead to arrhythmias, heart failure, or long-term damage to the cardiac muscle.

4. Subepicardial Layer

Structure

• Location: Between the myocardium and epicardium.
• Function: Joins the myocardium and epicardium.

5. Epicardium

Structure

• Location: Outermost layer of the heart.
• Composition:
  • Visceral layer of the pericardium
  • Connective tissue and fat
  • Simple squamous epithelial cells on the outer surface
  • Secretes lubricating fluid into the pericardial cavity

Function

• Provides a protective outer layer to the heart and secretes fluid that reduces friction during heart movement.

Key Points to Remember

• Endocardium: Lines the heart’s cavities and valves; inflammation can lead to infective endocarditis.
• Myocardium: Heart muscle responsible for contraction; inflammation can cause myocarditis and lead to arrhythmias or heart failure.
• Epicardium: Outermost layer, providing protection and secreting lubricating fluid.
• Clinical Significance: Damage to any layer can result in serious heart conditions like arrhythmias, heart failure, and valve dysfunction.

Table: Comparison of Heart Wall Layers

This summary helps students understand the anatomical layers of the heart and their clinical relevance, aiding in better comprehension for both academic and clinical practice.