10.3: Pericardium and the Heart Wall

The heart is enclosed by the pericardial sac, which protects and supports the powerful myocardium as it drives the heart’s continuous pumping action.

By the end of this section, you will be able to:

• Describe the layers and functions of the pericardium and the role of pericardial fluid.
• Identify the three layers of the heart wall and their functions.
• Explain why the left ventricle has a thicker wall than the right.

From Cover to Core: Pericardium 

The heart is enclosed in a double-layered membrane called the pericardium, or pericardial sac. This membrane also surrounds the “roots” of the major blood vessels above the heart and attaches to the diaphragm below, helping to anchor the heart securely within the thoracic cavity as it beats continuously.

The pericardium, which literally translates as “around the heart,” consists of two distinct sublayers: the sturdy outer fibrous pericardium and the inner serous pericardium. The fibrous pericardium is made of tough, dense irregular connective tissue that protects the heart and maintains its position in the thorax while also limiting the heart's motion during the heartbeat. The more delicate serous pericardium consists of two layers:

• the parietal pericardium, which is fused to the fibrous pericardium, and
• an inner visceral pericardium, or epicardium, which is fused to the heart and is also part of the heart wall.

The pericardial cavity, filled with lubricating serous fluid, lies between the epicardium and the parietal pericardium.  The serous layers of the pericardium consist of a simple squamous epithelium anchored by a layer of areolar connective tissue. The areolar connective tissue connects the parietal pericardium to the fibrous pericardium and it connects the epicardium to the myocardium.

The epithelial tissue of the serous membranes secretes the lubricating serous fluid that fills the pericardial cavity and reduces friction as the heart contracts. Adipose connective tissue can also be found in the epicardium, particularly in the areas where blood vessels run along the heart's surface. Figure  illustrates the pericardial membrane and the layers of the heart.

Figure \(\PageIndex{1}\): Pericardial Membranes and Layers of the Heart Wall.  The pericardial membrane that surrounds the heart consists of three layers and the pericardial cavity. The heart wall also consists of three layers. The pericardial membrane and the heart wall share the epicardium.  (Caption from OpenStax)  (Image credit: "Heart Wall" by Jennifer Lange is licensed under CC BY-NC-SA 4.0, modification of original by Blausen.com staff (2014).)

The pericardium is very important because it protects the heart from trauma, shock, stress, and even infections from the nearby lungs. It supports the heart and anchors it to the mediastinum — the central compartment of the thoracic cavity that lies between the lungs and contains the heart, major blood vessels, trachea, and esophagus — so it doesn’t move within the body. The pericardium lubricates the heart and prevents it from becoming too large if blood volume is overloaded (though it will not prevent chronic heart enlargement).

Despite these functions, the pericardium is still vulnerable to problems of its own. Pericarditis is the term for inflammation in the pericardium, typically due to infection. Pericarditis is often a severe disease because it can constrict and apply pressure on the heart and work against its normal function. Pericarditis comes in many types depending on which tissue layer is infected.

From Cover to Core: Heart Wall

The wall of the heart is composed of three layers of unequal thickness. From superficial to deep, these are the epicardium, the myocardium, and the endocardium (see Figure above). The outermost layer of the wall of the heart is also the innermost layer of the pericardium, the epicardium, or the visceral pericardium discussed above.

The middle and thickest layer of the heart wall is the myocardium, which is composed mainly of cardiac muscle supported by small amounts of connective tissue. The contraction of this muscular layer generates the force needed to propel blood through both the pulmonary and systemic circuits. As with skeletal muscle, the more muscle fibers that contract, the greater the force produced.

The myocardium rests on a framework of dense connective tissue known as the cardiac skeleton (described later). This structure helps anchor the muscle fibers and supports the heart’s valves. The arrangement of the cardiac muscle cells is both elegant and efficient: the fibers spiral and swirl around the chambers of the heart in complex figure-8 patterns. These loops wrap around the atria and the roots of the great vessels, while deeper layers encircle the ventricles and extend toward the apex. The outermost layers twist around both ventricles together.

This spiraling pattern of muscle ensures that when the heart contracts, it does not just squeeze but also twists, reducing the size of the chambers in a wringing motion that pushes blood efficiently to the top of the heart and into the arteries. (See the figure below for the arrangement of myocardial fibers.)

Figure \(\PageIndex{2}\): Cardiac Musculature. The swirling pattern of cardiac muscle tissue contributes significantly to the heart’s ability to pump blood effectively. (Image credit: "Heart Musculature" by OpenStax is licensed under CC BY 3.0.)

Although both ventricles pump the same volume of blood with each heartbeat, their workloads are very different. The left ventricle must push blood through the long, high-resistance systemic circuit, so its muscular wall is much thicker and more powerful. In contrast, the right ventricle only sends blood a short distance to the lungs through the low-resistance pulmonary circuit and therefore does not need to generate as much force. As a result, the heart’s left side appears more muscular when viewed in cross section. (see figure below).
 

Figure \(\PageIndex{3}\): Differences in Ventricular Muscle Thickness. The myocardium in the left ventricle is significantly thicker than that of the right ventricle.  In both relaxed and contracting states, also note the differences in the relative size of the lumens, the region inside each ventricle where the blood is contained. (Image credit: "Ventricular Muscle Thickness" by OpenStax is licensed under CC BY 3.0.)