The heart is shielded by a protective structure known as the pericardium, or pericardial sac. This tough, double-walled covering contains the heart and the proximal portions of the major blood vessels. The pericardium keeps the heart securely positioned within the chest cavity (mediastinum). It also limits the heart’s movement and prevents the muscle from over-expanding when it fills with blood.
Understanding the Pericardial Sac Structure
The pericardium is composed of two primary sections: a fibrous outer layer and a serous inner layer. The outer layer, called the fibrous pericardium, is a semi-rigid sac made of dense connective tissue that provides structural support and anchors the heart within the chest. This tough, inelastic layer helps protect the heart from external forces or trauma.
The inner section is the serous pericardium, a thin membrane divided into two layers. The outer part is the parietal layer, which is firmly fused to the inner surface of the fibrous pericardium. The serous pericardium then folds back on itself to become the visceral layer, creating a continuous membrane.
Between the parietal and visceral layers is a small space called the pericardial cavity, which holds lubricating serous fluid. This fluid reduces friction between the moving surfaces, allowing the heart to beat smoothly within its protective sac. The two layers of the serous pericardium connect where the major blood vessels enter and exit the heart.
Pinpointing the Visceral Layer
The visceral pericardium is the innermost layer of the pericardial sac and is also known as the epicardium. This layer adheres directly to the surface of the heart muscle, the myocardium, essentially forming the heart’s outermost layer. It covers the exterior surface of the cardiac chambers.
The visceral layer is a thin sheet of mesothelial cells supported by connective tissue rich in elastic fibers. It is more pliable than the tough outer fibrous layer, allowing it to move freely with the contractions of the heart. It extends to cover the roots of the great vessels, such as the aorta and pulmonary trunk, before reflecting back to become the parietal layer.
This reflection around the great vessels forms two recesses or sinuses within the pericardial cavity. One is the transverse sinus, a passage located behind the aorta and pulmonary trunk but in front of the atria. The other is the oblique sinus, a cul-de-sac behind the left atrium. The point of reflection at the great vessels marks the boundary between the visceral and parietal layers.
Role of the Visceral Pericardium
The primary function of the visceral pericardium is to provide a smooth, lubricated surface for the heart as it beats within the pericardial sac. The mesothelial cells of this layer secrete the serous fluid that fills the pericardial cavity, minimizing friction. This layer ensures the heart’s activity is not compromised by rubbing against the parietal layer or the fibrous sac.
Beyond lubrication, the visceral pericardium serves as the housing layer for the heart’s own circulatory system. The coronary blood vessels, which supply oxygen and nutrients to the heart muscle, run along the surface embedded within this layer. Adipose tissue, or fat, is typically found beneath the visceral pericardium. This layer also acts as a selective mechanical barrier, offering protection against the spread of infection from surrounding organs.
Common Pericardial Conditions
When the pericardium becomes inflamed, pericarditis occurs, which is the most common disorder affecting this structure. This inflammation often results from viral infections, though it can also be triggered by autoimmune conditions, heart attacks, or kidney failure. Pericarditis can cause sharp chest pain and may lead to complications if left untreated.
Pericardial effusion involves an abnormal accumulation of fluid in the pericardial cavity. Excess fluid can put pressure on the heart, hindering its ability to fill properly. If this fluid builds up rapidly, it can lead to a medical emergency called cardiac tamponade.
Cardiac tamponade is a life-threatening condition where the non-pliable fibrous pericardium prevents the sac from expanding, causing fluid pressure to compress the heart. The compression restricts the heart chambers from fully expanding, severely limiting the amount of blood the heart can pump. Chronic inflammation can also lead to constrictive pericarditis, where the pericardium thickens and scars, becoming rigid and impairing the heart’s function over time.