The Heart Wall

he heart is an extraordinary organ. It is about the size of a clenched fist, weighs about 10.5 ounces and is shaped like a cone. Along with the circulatory system, the heart works to supply blood and oxygen to all parts of the body. The heart is located in the chest cavity just posterior to the breastbone, between the lungs, and superior to the diaphragm. It is surrounded by a fluid filled sac called the pericardium, which serves to protect this vital organ.

The heart wall is composed of connective tissue, endothelium, and cardiac muscle. It is the cardiac muscle that enables the heart to contract and allows for the synchronization of the heart beat. The heart wall is divided into three layers: epicardium, myocardium, and endocardium.

• Epicardium - outer protective layer of the heart.
• Myocardium - muscular middle layer wall of the heart.
• Endocardium - inner layer of the heart.

Epicardium

 Heart Interior Anatomy. Stocktrek Images/Getty Images

Epicardium (epi-cardium) is the outer layer of the heart wall. It is also known as visceral pericardium as it forms the inner layer of the pericardium. The epicardium is composed primarily of loose connective tissue, including elastic fibers and adipose tissue. The epicardium functions to protect the inner heart layers and also assists in the production of pericardial fluid. This fluid fills the pericardial cavity and helps to reduce friction between pericardial membranes. Also found in this heart layer are the coronary blood vessels, which supply the heart wall with blood. The inner layer of the epicardium is in direct contact with the myocardium.

Myocardium

 This is a colored scanning electron micrograph (SEM) of healthy heart (cardiac) muscle fibrils (blue). The muscle fibrils, or myofibrils, are crossed by transverse tubules (running vertically). These tubules mark the division of the myofibrils into contractile units known as sarcomeres. Cardiac muscle is under subconscious control and continuously contracts to pumps blood around the body without tiring. Steve Gschmeissner/Science Photo Library/Getty Images

Myocardium (myo-cardium) is the middle layer of the heart wall. It is composed of cardiac muscle fibers, which enable heart contractions. The myocardium is the thickest layer of the heart wall, with its thickness varying in different parts of the heart. The myocardium of the left ventricle is the thickest as this ventricle is responsible for generating the power needed to pump oxygenated blood from the heart to the rest of the body. Cardiac muscle contractions are under the control of the peripheral nervous system, which directs involuntary functions including heart rate.

Cardiac conduction is made possible by specialized myocardial muscle fibers. These fiber bundles, consisting of the atrioventricular bundle and Purkinje fibers, carry electrical impulses down the center of the heart to the ventricles. These impulses trigger the muscle fibers in the ventricles to contract.

Endocardium

 This is a false-color scanning electron micrograph (SEM) showing an aggregation of red blood cells on the endocardium, the lining of the heart. P. MOTTA/University 'LA SAPIENZA', Rome/Getty Images

Endocardium (endo-cardium) is the thin inner layer of the heart wall. This layer lines the inner heart chambers, covers heart valves, and is continuous with the endotheliumof large blood vessels. The endocardium of heart atria consists of smooth muscle, as well as elastic fibers. An infection of the endocardium can lead to a condition known as endocarditis. Endocarditis is typically the result of an infection of the heart valves or endocardium by certain bacteria, fungi, or other microbes. Endocarditis is a serious condition that can be fatal.