The pericardium is a double-walled sac enclosing the heart. The parietal pericardium (outer wall) has a tough, superficial fibrous layer of dense irregular connective tissue and a thin serous layer. The serous layer turns inward at the base of the heart to form the visceral pericardium. The pericardial sac is anchored by ligaments to the diaphragm below and the sternum anterior to it.
The pericardial cavity is between the parietal and visceral membranes holding 5-30 mL of pericardial fluid is within the cavity. Pericardial fluid lubricates the membranes that line the pericardial cavity, enabling them to slide over one another with a minimum of friction as the heart beats. The pericardium helps prevent displacement of the heart during gravitational acceleration or deceleration, acts as a barrier against infection and inflammation from the lungs, and contains pain receptors and mechano-receptors that can elicit reflex changes in the blood pressure and heart rate.
The epicardium, or visceral pericardium, consists of simple squamous epithelium overlying a thin areolar tissue layer. Some areas include a thick layer of adipose (fatty) tissue, whereas others are fat-free and translucent. This layer provides a smooth surface that allows the heart to contract and relax within the pericardium with a minimal amount of friction.
Pericarditis is inflammation of the pericardium- the membrane becomes red and swollen1. The pericardial sac may collect excess pericardial fluid causing a pericardial effusion. The fluid compresses the heart and impairs its ability to function. A pericardiocentesis may be performed to drain the excess fluid with a catheter, but if the fluid cannot be drained with a needle, then a pericardial window is performed1.
The myocardium is composed of cardiac muscle and is the thickest layer of the heart wall. Thickness is proportional to the workload on the individual chambers. Cardiac muscle spirals around the heart forming a myocardial vortex– when the ventricles contract, they exhibit a twisting motion.
The endocardium is the innermost layer that lines the valves and vessels creating a continuous, closed circulatory system. If bacteria enters the bloodstream it can infect the endocardium and clumps of bacteria (vegetation) form at the site of the infection2. IV drug abuse and dental work are common sources of infection. I once had a patient who developed endocarditis as a result of his career as an oyster shucker, so can certainly be other causes. If left untreated, endocarditis can damage the heart valves and permanently destroy the heart’s endocardium2. The heart must work harder to pump blood, eventually causing heart failure. A new or changed heart murmur is often heard from the damaged valves leaking blood.