Table of Contents
- What Are the Layers of the Heart?
- Pericardium: Outermost Layer of the Heart
- Three Layers of the Heart Wall
- Lesson Summary
The heart is an amazing organ. Its main function is to pump blood throughout the body, which continues the flow of fresh oxygen to various tissues and systems. The heart is partially so efficient because of its shape. The orientation of the different compartments of the heart allows for deoxygenated blood to enter, pass to the lungs to receive oxygen, and exit back to the body. As the heart pumps, the alignment of the valves and compartments of the heart prevent the oxygen-rich blood and oxygen-depleted blood from mixing and flowing backward.
The heart's strong contractions are the driving force of the circulatory system. With each contraction, blood is forced out of one area and into the next. These contractions are the result of strong muscle tissues that compose the walls of the heart. The heart's walls are divided into four layers. Together, these layers both compose the muscle tissue that makes up the heart and provides essential functions to assist in muscular contractions. The layers of the heart are as follows:
In this lesson, investigate the layers of the heart wall to better understand how they function and contribute to the circulatory system. Continue reading to answer the following questions:
As the heart beats, it expands to fill with blood and constricts to expel blood out. Because the heart changes in size as it functions, its movement would likely cause friction with other organ systems and tissues within the chest cavity. Excess friction would wear down tissue and eventually decrease the health and efficacy of the exposed tissues. To curb this potential issue, the heart is contained within a thin sac referred to as the pericardium. The pericardium contains the parietal pericardium and epicardium.
The pericardium is a layer of connective tissue filled with lubricating fluids. These fluids (called serous fluids) reduce the friction within the pericardial sac and protect the heart from potential pathogens. The production of these fluids is controlled by the epicardium and parietal pericardium. In addition to protection and lubrication, the pericardium is important in maintaining the position of the heart within the body cavity. It also prevents the heart from stretching too much by restricting the amount it can expand.
While the pericardium does not compose the actual heart, the heart wall is composed of three additional yet separate layers. These layers work together to form the major structure of the heart and allow it to function after endless hours of pumping. The three layers of the heart, as discussed above, are the epicardium, myocardium, and endocardium.
The outside surface of the heart is referred to as the epicardium. "Epi" refers to outside, and "cardium" refers to the heart. Thus, the epicardium is the outside of the heart. The epicardium is sometimes referred to as the visceral pericardium. This is because it is composed of connective tissue that, in part, connects to the pericardium and forms its second layer.
The epicardium has an important function within the cardiovascular system. While the heart's main job may be to circulate blood, it does not benefit directly from the blood it pumps. Instead, it must receive freshly oxygenated blood via a system of vessels, like all other organs in the body. This fresh blood is delivered through a system of blood vessels that reside within the epicardium. Thus, the epicardium is largely responsible for the delivery of fresh oxygen and nutrients to the cardiac tissue of the heart.
The myocardium is the middle layer of the heart and composes most of the heart's tissue. The myocardium is composed of muscular tissue and generates the contractions that control the pumping of the heart. This muscle is known as cardiac muscle and differs from the other types of muscles in the body. Unlike skeletal muscle, cardiac muscle tissue is involuntary and contains intercalated discs. These discs increase the rate at which electrical stimuli travel through the heart, generating more uniform contractions. In addition, cardiac muscle does not fatigue and can thus contract repetitively without becoming tired or sore. The thickest section of the myocardium is found in the right ventricle of the heart, which requires the strongest contractile forces.
The myocardium is also the site of Purkinje fibers. These fibers are a network of nerves that reside at the base of the heart. When these nerves fire, they stimulate the muscular tissue of the heart to contract. This contraction starts at the base of the heart and travels cranially, causing the lower parts of the heart to contract before the upper parts. In this way, the contractions occur in a pattern that is conducive for the emptying and filling of the heart's chambers.
Cardiomyocytes are another important structure found within the myocardium. These specialized cells control the rate at which the heart contracts and relaxes. Thus, cardiomyocytes are often referred to as the pacemakers of the heart.
The innermost layer of the heart is referred to as the endocardium. "Endo" means "inner," thus endocardium refers to the inner portion of the heart. The endocardium is a smooth layer of tissue that lines the inside chambers of the heart. It works to separate blood within the heart from the cardiac muscle composing the myocardium and allows for a slick surface for blood to pass over without getting stuck. The endocardium also contains blood vessels like the epicardium, thus it assists in delivering nutrients and oxygen to tissues of the heart. The valves of the heart are also composed of the endocardium and function to prevent the backflow of fluid through the heart as it pumps.
The endocardium also plays an important role in the conduction of electrical signals. When the fibers within the myocardium fire, their signal must be carried through the tissue of the heart to innervate different areas of the muscular myocardium. The small nerves that send these impulses reside within the endocardium.
The endocardium is composed of three layers:
The endothelium is composed of special endothelial cells which assist in the passage of material from the inside of the heart to the myocardium. The elastic layer is composed of another involuntary muscle known as smooth muscle. This is the same type of muscle found within the intestines and provides involuntary contractions. These contractions constrict blood vessels within the endocardium and force blood to move. The subendocardial layer is the layer closest to the myocardium. It acts as a layer of connective tissue to adhere the endocardium to the myocardium.
The heart is composed of three layers. The pericardium is the outside layer that does not compose part of the heart. It houses fluid that reduces friction to the heart and protects it as it pumps. The epicardium is the outermost layer of the heart and is sometimes referred to as the visceral pericardium. This is because it composes the inside layer of the pericardium and houses important blood vessels which supply the heart's tissues with oxygen and nutrients.
The myocardium is the middle layer of the heart that is composed of contractile cardiac muscle. It is innervated by the Purkinje fibers, which stimulate it to contract. These contractions are controlled by cardiomyocytes, which act as pacemakers within the heart. The innermost layer of the heart is the endocardium, which is smooth and composes the valves and inner walls of the heart. The innermost surface of the endocardium is the endothelium. The middle layer contains smooth muscle and vessels and is called the elastic tissue layer. The subendocardial is the layer that connects the endocardium to the myocardium. Together, the pericardium and layers of the heart wall support the structure and position of the heart.
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The three layers of the heart wall are the epicardium, myocardium, and endocardium. There is a fourth layer that envelopes the heart called the pericardium.
The epicardium is the outer layer that contains blood vessels. The myocardium is the middle layer that is composed of contractile tissues. The endocardium is the innermost layer and composes the valves, inner lining of the chambers, and contains vessels and nerves.
The pericardium is a layer of tissue that surrounds the heart and contains fluid to reduce friction. While the epicardium is technically an extension of the pericardium, it is often referred to as a separate layer because it composes part of the heart's wall.
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