INTRODUCTION

An efficient circulatory system is vital for the survival of complex multicellular organisms because without circulation, the tissues lack a supply of oxygen and nutrients, and waste substances begin to accumulate. Under such conditions, the cell function begins to diminish, which quickly leads to the death of the organism.

The heart is a hollow, cone-shaped, muscular pump, located within the thorax and resting upon the diaphragm.

SIZE AND LOCATION OF THE HEART

Heart size varies with body size with the average heart of an adult being about 6 inches long and 3 and 1/2 inches wide.

The heart is within the thoracic cavity, bordered laterally by the lungs, posteriorly by the backbone, and anteriorly by the sternum and at it's base, which is attached to several large blood vessels, lies beneath the second rib. The distal end extends down and to the left, terminating as a blunt apex at the level of the fifth intercostal space.

COVERINGS OF THE HEART

The heart and the proximal ends (attached directly to the heart) of the large blood vessels are encased by the pericardium. The pericardium consists of an outer fibrous bag, the fibrous pericardium, which surrounds a more delicate, double-layered sac, the visceral pericardium or epicardium, which covers the heart. At the base of the heart, the visceral layer turns back upon itself to become the parietal pericardium. The parietal pericardium, in turn, forms the inner lining of the fibrous pericardium, thus the two membranes form a pseudo-tri layered structure.

The fibrous pericardium is a tough, protective sac composed largely of white fibrous connective tissue. It is attached to the central portion of the diaphragm, the posterior margin of the sternum, the vertebral column, and the large blood vessels emerging from the heart. This tough membrane with it's broad attachment holds the heart firmly in place. Between the parietal and visceral layers of the pericardium is a potential space, the pericardial cavity, which contains a small amount of serous fluid, the pericardial fluid. This fluid reduces friction between the pericardial membranes as the heart moves within them.

WALL OF THE HEART

The wall of the heart is composed of three distinct layers of tissue -- an outer epicardium, a middle myocardium and an inner endocardium.

The epicardium, which is synonymous the visceral pericardium, functions as a protective layer. This membrane consists of connective tissue covered by epithelium. Its deeper portion (close to the myocardium) often contains adipose tissue, particularly along the paths of the larger blood vessels.

The myocardium is relatively thick and consists largely of the cardiac muscle tissue responsible for forcing the blood out of the heart chambers. The muscle fibers are arranged in planes, separated by connective tissues, which are richly supplied with blood capillaries, lymph capillaries, and nerve fibers, nourishing and controlling the heart.

The endocardium consists of epithelium and connective tissue, which contains many elastic and collagenous fibers. The connective tissue also contains some specialized cardiac muscle fibers, called Purkinje fibers, whose function is described in another section. This inner lining is continuous with the inner linings of the blood vessels attached to the heart.

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HEART CHAMBERS AND VALVES

The internal portion of the heart is divided into four chambers, two on the left and two on the right. The upper chambers, called atria (or atrium, singular), have relatively thin walls and receive the blood from veins (blood vessels that carry the blood toward the heart). Small earlike projections, called auricles, extend outward from the atria. The lower chambers, the ventricles, force the blood out of the heart chambers into the arteries (carrying the blood away from the heart).

The atrium and ventricle on the right side are separated from those on the left by a septum, the interatrial and the interventricular septi. The atrium on each side opens with its corresponding ventricle through an opening, which is guarded by a valve.

The opening between the right atrium and the right ventricle is controlled by a large tricuspid valve, which is composed of three parts, or cusps. This valve permits the blood to move from the right atrium into the right ventricle and prevents it from passing in the opposite direction.

Strong, fibrous strings, called chordae tendineae, are attached to the cusps on the ventricular side. These strings originate from small mounds of cardiac muscle tissue, the papillary muscles, which project inward from the walls of the ventricle. When the tricuspid valve closes, the chordae tedineae and papillary muscles prevent the cusps from swinging back into the atrium.

The right atrium receives blood from three main sources: two large veins, the superior vena cava and the inferior vena cava and a smaller vein, the coronary sinus, which drains blood into the right atrium from the wall of the heart.

The right ventricle has a thinner muscular wall than the left ventricle. The right chamber pumps the blood a fairly short distance to the lungs against a relatively low resistance to blood flow. The left ventricle, on the other hand, must force the blood to all the other parts of the body against a much greater resistance to flow. Thus it's muscular wall is considerably thicker.

An exit from the right ventricle is provided by the pulmonary trunk that divides to form the left and right pulmonary arteries, which lead to the lungs. At the base of this trunk is a pulmonary semilunar valve, which consists of three cusps. This valve allows the blood to leave the right ventricle and prevents a return flow into the ventricular chamber.

The left atrium receives the blood from the lungs through the four pulmonary veins, two from the right and two from the left lung. The blood passes from the left atrium into the left ventricle through the bicuspid or mitral valve, which prevents the blood from flowing back into the left atrium from the ventricle. As with the tricuspid valve, the cusps of the bicuspid valve are prevented from swinging back into the left atrium by the chordae tedineae and papillary muscles.

The only exit from the left ventricle is through a large artery called the aorta. At the base of the aorta, there is an aortic semilunar valve, which consists of three cusps. It opens and allows the blood to leave the left ventricle, and when it closes, it prevents the blood from backing up into the ventricle.

SKELETON OF THE HEART

At their proximal ends, the pulmonary trunk and aorta are surrounded by rings of dense fibrous connective tissue. The rings provide firm attachments for the heart valves and various muscle fibers. In addition, they prevent the outlets of the atria and ventricles from dilating during myocardial contraction. The fibrous rings together with other masses of dense fibrous tissue in the upper portion of the septum between the ventricles (interventricular septum) constitute the skeleton of the heart.