.. rrounded by a dense fibrous ring and is covered by the lining membrane of the heart and is protected by the mitral valve. The circular aortic opening is located in front of and to the right side of the atrial-ventricular opening from which it is separated by one of the segments of the mitral valve. The opening is protected by the semilunar valves. There are two valves located within the left ventricle; the mitral valve and the semilunar valve.

The mitral valve is attached to the circumference of the atrial-ventricular opening in the same way that the tricuspid valve is attached on the opposite side of the heart. The valve contains a few muscular fibers, is strengthened by fibrous tissue, and is formed by the lining of the heart (endocardium). It is larger, thicker, and stronger than the tricuspid, and consists of two segments of unequal size. The mitral valves are connected to many chordae tendonae.

Their attachment is the same as on the right side except they are thicker, stronger, and less numerous.The semilunar valves surround the aortic opening. They are similar in structure and mode of attachment to those of the pulmonary artery. However, they are larger, thicker, and stronger than those of the right side.

Between each valve and the cylinder of the aorta is a deep depression called the sinuses of Valsalva. The depressions are larger than those at the root of the pulmonary artery.3 Figure 1: a.Cross sectional view of the heart. b. Top view of the heart showing the four valves Histology of the Layers of the Heart: The heart and its vessels are surrounded by a conical membranous sac called the pericardium.

The pericardial sac is composed of two layers; the parietal pericardium and the visceral pericardium with the space in-between the two being called the pericardial cavity. The parietal pericardium is composed primarily of compact fibrocollagenous tissue along with elastic tissue.It is a fibrous membrane of loose irregular connective tissue that is lined internally by a mesothelium which is essentially simple squamous epithelium. The visceral pericardium forms the internal lining of the pericardium and reflects over the outer surface of the heart. This reflection forms the outer layer of the epicardium.

The visceral pericardium is also composed of compact fibrocollagenous tissue with elastic tissue but, is smooth mesothelium. The pericardial cavity is located between the parietal and visceral pericardium and contains small amounts of serous fluid. The heart tissue itself can be subdivided into three layers; (from the outside in) epicardium, myocardium, and endocardium.The epicardium is the outermost layer of the heart and consists of a loose connective tissue of fibroblasts, collagen fibers, and adipose tissue.

It contains a stroma which houses coronary arteries and veins that are surrounded by a layer of fat. These coronary branches penetrate the myocardium. The myocardium contains the main muscle mass of the heart and is composed primarily of striated muscle cells. Each of the cardiac muscle cells contain one central elongated nucleus with some central euchromatin and some peripheral heterochromatin.

The two atria have a very thin myocardial layer which increases greatly in thickness as you go from the atria to the right ventricle and into the left ventricle. The outer surface of the myocardium, next to the epicardium, is not composed of smooth muscle but is very smooth in texture. The inner surface of the myocardium is rough and is raised into trabeculations. The ventricular papillary muscles, which are for the attachment of the chordae tendinae, are extensions of the myocardium even though they are covered by endocardium. The outer layer of the myocardium is superficial bulbospiral and swirls around the ventricle in a clockwise fashion.The middle layer is circular muscles that are the ventricular constrictors.

The inner layer, which is deep bulbospiral, swirls around the ventricle in a counterclockwise fashion. The layer underneath the myocardium is known as the enodcardium. It contains a continuous smooth endothelial layer that covers all the inner surfaces of the heart, including the valves. The outer layer of the endocardium, underneath the myocardium, is irregularly arranged collagenous fibers that may contain Purkinje fibers/cells.The inner part of the endocardium contains more regularly arranged collagen and elastic fibers than the outer layer. Some myofibroblasts are present in the endocardium which is thicker in the atria than in the ventricles.

There is a subendothelial component of the endocardium underneath the endothelium. The component contains fibroblasts, scattered smooth muscle cells, elastic fibers, collagen fibers, and an amorphous ground substance that contains glycoproteins and proteoglycans. The valves of the heart are attached to the cardiac skeleton and consist of chondroid (a material resembling cartilage).The base of each valve is supported by a fibrocollagenous ring. Each valve also has a dense fibrocollagenous central plate that is covered by simple squamous epithelium. Chordae tendonae connect with the valves at the edge of each cusp as well as underneath each cusp at one end and they attach to papillary muscles in the ventricles at the other end.

Endocardial endothelium completely covers the papillary muscles, valves, and the chordae tendonae. The junctions between the cusps of each valve are known as commissures. The conducting system of the heart consists of four main components; the sinuatrial node (SA), the atrioventricular node (AV), the bundle of his, and the Purkinje fibers/cells.All the parts of this conducting system are composed of modified cardiac muscle cells. The SA node is located in the right atrium, at the point where the superior vena cava enters.

The small muscle fibers of the SA node contain a central nodal artery and desmosomes. The muscle fibers do not contain intercalated discs. The AV node is located in the medial wall, in front of the opening of the coronary sinus and above the tricuspid ring.Its small muscle fibers are more regularly arranged than those of the SA node. The AV node contains a rich nerve and blood supply.

The bundle of his has a right (single bundle) and a left (branched bundle) bundle branch located underneath the endocardium. It is histologically similar to the other components of the conducting system. The Purkinje fibers/cells can be found in clusters of about six cells which are located under the endocardium in the ventricles.The cytoplasm of Purkinje fibers appears pale under the microscope and contains many glycogen granules.

7 Physiology of the Heart: The principle function of the heart and circulatory system is to provide oxygen and nutrients and to remove metabolic waste products from tissues and organs of the body. The heart is the pump that provides the energy necessary for transporting the blood through the circulatory system in order to facilitate the exchange of oxygen, carbon dioxide, and other metabolites through the thin-walled capillaries. The contraction of the heart produces changes in pressures and flows in the heart chambers and blood vessels. The mechanical events of the cardiac cycle can be divided into four periods; late diastole, atrial systole, ventricular systole, and early diastole.6 In late diastole, the mitral and tricuspid valves are open and the pulmonary and aortic valves are closed.

Blood flows into the heart throughout diastole thus filling the atria and ventricles.The rate of filling declines as the ventricles become distended, and the cusps of the atrioventricular valves start to close. The pressure in the ventricles remains low throughout late diastole.8 In atrial systole, contraction of the atria forces additional blood into the ventricles, but approximately 70 percent of the ventricular filling occurs passively during diastole. Contraction of the atrial muscle that surrounds the openings of the superior and inferior vena cava and pulmonary veins, narrows their orifices and the inertia of the blood moving towards the heart tends to keep blood in the heart. However, there is some regurgitation of blood into the veins during atrial systole.

2&5 At the start of ventricular systole, the AV valves close. The muscles of the ventricles initially contract relatively little, but intraventricular pressure rises sharply as the muscles squeezes the blood in the ventricle. This period of isovolumetric ventricular contraction lasts about 0.05 seconds until the pressures in the ventricles exceed the pressure in the aorta and in the pulmonary artery, and the aortic and pulmonary valves (semilunar valves) open.

During this isovolumetric contraction, the AV valves bulge into the atria, causing a small but sharp rise in atrial pressure. When the semilunar valves open, the phase of ventricular ejection begins. Ejection is initially rapid, but slows down as systole progresses. The intraventricular pressure rises to a maximum and then declines somewhat before ventricular systole ends. Late in systole, the aortic pressure is actually higher than the ventricular pressure, but for a short period, momentum keeps the blood moving forward.

The AV valves are pulled down by the contractions of the ventricular muscle, and the atrial pressure drops.5 In early diastole, after the ventricular muscle if fully contracted, the already falling ventricular pressure drops even more rapidly.This is the period known as protodiastole and it lasts about 0.04 seconds. It ends when the momentum of the ejected blood is overcome and the semilunar valves close. After the valves are closed, pressure continues to drop rapidly during the period of isovolumetric relaxation.

Isovolumetric relaxation ends when the ventricular pressure falls below the atrial pressure and the AV valves open, thus allowing the ventricles to fill. Again, filling is rapid at first, then slows as the next cardiac contraction approaches.Atrial pressure continues to rise after the end of ventricular systole until the AV valves open, upon which time it drops and slowly rises again until the next atrial systole.6,2,&4 Summary: The heart is arguably the most vital organ the human body possesses.

Without the heart, none of the tissues in the body would receive the vital oxygen necessary for them to maintain survival. Heart disease is the number one killer of people in America today. Due to this disturbing fact, it is no wonder such a large percentage of the fellowships granted by the National Institutes of Health go towards heart related illnesses.