The Pericardial Sac Consists of Two Layers
The pericardial sac is composed of two layers (B) as clearly defined in the 2015 ESC Guidelines for the diagnosis and management of pericardial diseases 1.
Anatomy of the Pericardial Sac
The pericardium is a double-walled sac containing the heart and the roots of the great vessels. Its structure consists of:
- Serous visceral layer (also known as epicardium when it comes into contact with the myocardium)
- Fibrous parietal layer
These two layers enclose the pericardial cavity, which contains pericardial fluid (20-60 mL of plasma ultrafiltrate) 2. The pericardial fluid provides lubrication during cardiac movement, reducing friction between the heart and surrounding structures 2, 3.
Detailed Structure
The pericardial layers have distinct characteristics:
- Visceral layer (epicardium): Forms a mesothelial monolayer that directly covers the heart surface 2
- Parietal layer: Consists of an outer fibrous layer made of organized collagen and elastin fibers 4
The parietal pericardium itself has a complex structure with three sublayers 5:
- Serosa (mesothelial cells and submesothelial space)
- Fibrosa (containing collagen fibrils and elastic fibers)
- Epipericardial connective tissue
Functional Significance
The pericardium serves several important functions:
- Mechanical protection for the heart and great vessels
- Lubrication to reduce friction during cardiac movement
- Fixation of the heart to the mediastinum
- Protection against infection
- Limitation of acute cardiac dilation
Clinical Relevance
Understanding the two-layered structure of the pericardium is essential for diagnosing and managing pericardial diseases such as:
- Pericarditis (inflammation of the pericardial layers)
- Pericardial effusion (abnormal accumulation of fluid between the layers)
- Constrictive pericarditis (scarring and loss of elasticity of the pericardium)
- Cardiac tamponade (compression of the heart due to fluid accumulation)
Advanced imaging techniques like echocardiography, CT, and MRI can visualize these layers and help diagnose pericardial pathologies 1, 6.