The Human Heart

The Human Heart Biology – Histology The Human Heart. Abstract: Dorland’s Illustrated Medical Dictionary defines the heart as “the viscus of cardiac muscle that maintains the circulation of the blood”. It is divided into four cavities; two atria and two ventricles. The left atrium receives oxygenated blood from the lungs. From there the blood passes to the left ventricle, which forces it via the aorta, through the arteries to supply the tissues of the body.

The right atrium receives the blood after it has passed through the tissues and has given up much of its oxygen. The blood then passes through the right ventricle into the lungs where it gets oxygenated. There are four major valves in the heart; the left atrioventricular valve (also known as the mitral or bicuspid valve), the right atrioventricular valve (tricuspid), aortic valve, and the pulmonary valve. The heart tissue itself is nourished by the blood in the coronary arteries.2 Position of the Heart Within the Body: The heart is placed obliquely in the chest. The two atria are directed upwards and backwards to the right and are at the level of the fifth through the eight dorsal vertebrae.

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The apex of the heart points downwards and forwards to the left and corresponds to the interspace between the fifth and sixth ribs, two inches below the left nipple. Its atrial border corresponds to a line drawn across the sternum on a level with the upper border of the third costal cartilage. Its lower border (apex) corresponds to a line drawn across the lower end of the same bone, near the xiphoid process. Its upper surface is rounded and convex, directed upwards and forwards, and formed mainly by the right ventricle and part of the left ventricle. The posterior surface of the heart is flattened and rests upon the diaphragm muscle.

Of its two borders, the right is the longest and thinnest, the left is shorter but thicker and round. Size: In an adult, the heart measures about five inches in length, three and a half inches in the broadest part of its transverse diameter, and two and a half inches in its antero-posterior. The average weight in the male varies from ten to twelve ounces. In the female, the average weight is eight to ten ounces. The heart will continue to grow in size up to an advanced period of life. This growth is more obvious in men than in women.3 Circulation of Blood in an Adult: The heart is subdivided by a longitudinal muscular septum into two lateral halves which are named right and left according to their position.

A transverse muscle divides each half into two cavities. The upper cavity on each side is called the atria/auricle, and the lower side is called the ventricle. The right atrium and ventricle form the venous side of the heart. Dark venous blood is pumped into the right atrium from the entire body by the superior (SVC) and inferior vena cava (SVC), and the coronary sinus. From the right atrium, the blood passes into the right ventricle and from the right ventricle, through the pulmonary artery into the lungs.3 Once the blood becomes oxygenated/arterialized by its passage through the lungs, it is returned to the left side of the heart by the pulmonary veins which open into the left atrium.

From the left atrium, the blood passes into the left ventricle where it is distributed by the aorta and its subdivisions through the entire body. Morphology of Each Heart Chamber: The right atrium is a little longer than the left. Its walls are also somewhat thinner than the left. The right atrium is capable of containing about two ounces of fluid. It consists of two parts, a principle cavity/sinus, and an appendix auriculae.

The sinus is a large quadrilateral-shaped cavity located between the IVC and the SVC. Its walls are extremely thin and are connected on the lower surface with the right ventricle and internally with the left atrium. The rest of the right atrium is free and unattached. The appendix auricle is a small conical muscular pouch. It projects from the sinus forwards and to the left side, where it overlaps the root of the pulmonary artery.6 There are four main openings into the right atrium; the SVC, IVC, coronary sinus, and the atriculo-ventricular opening.

The larger IVC returns blood from the lower half of the body and opens into the lowest part of the right atrium, near the septum. The smaller SVC returns blood from the upper half of the body and opens into the upper and front part of the right atrium. The coronary sinus opens into the right atrium between the IVC and auriculo-ventricular opening. It returns blood from the cardiac muscle of the heart and is protected by a semicircular fold of the lining membrane of the atrium, called the coronary valve. The auriculo-ventricular opening is the large oval aperture of communication between the right atrium and ventricle.

There are two main valves located within the right atrium; the Eustachian valve and the coronary valve.3 The Eustachian valve is located between the anterior margin of the IVC and the auricule-ventricular orifice. It is semilunar in form. The coronary valve is a semicircular fold of the lining membrane of the right atrium, protecting the orifice of the coronary sinus. The right ventricle is triangular-shaped and extends from the right atrium to near the apex. Its anterior surface is rounded and convex and forms the larger part of the front of the heart.

Its posterior surface is flattened, rests on the diaphragm muscle, and forms only a small part of this surface. Its inner wall is formed by the partition between the two ventricles, the septum, and bulges into the cavity of the right ventricle. Superiorly, the ventricle forms a conical structure called the infundibulum from which the pulmonary artery arises. The walls of the right ventricle are thinner than those of the left ventricle. The thickest part of the wall is at the base and it gradually becomes thinner towards the apex.

The cavity can contain up to two ounces of fluid. There are two openings in the right ventricle; the auriculo-ventricular opening and the opening of the pulmonary artery. The auriculo-ventricular opening is the large oval opening between the right atrium and the right ventricle. The opening is about an inch in diameter. It is surrounded by a fibrous ring, covered by the lining membrane of the heart (endocardium), and is larger than the opening between the left atrium and the left ventricle.

It is protected by the tricuspid valve. The opening of the pulmonary artery is round and is situated at the top of the conus arteriosus, close to the septum. It is on the left side and is in front of the auriculo-ventricular opening. It is protected by the semilunar valves.3 There are two main valves associated with the right ventricle; the tricuspid valve and the semilunar valves. The tricuspid valve consists of three segments of a triangular shape, formed by the lining membrane of the heart (endocardium).

They are strengthened by a layer of fibrous tissue and muscular fibers.1 These segments are connected by their bases to the auriculo-ventricular orifice, and by their sides with one another, so as to form a continuous membrane which is attached around the margin of the auriculo-ventricular opening. Their free margin and ventricular surfaces are attached to many delicate tendinous cords called chordae tendinae. The central part of each valve segment is thick and strong while the lateral margins are thin and indented. The chordae tendinae are connected with the adjacent margins of the main segment of the valves. The semilunar valves guard the opening of the pulmonary artery.

They consist of three semicircular folds formed by the endothelial lining of the heart and are strengthened by fibrous tissue. They are attached by their convex margins to the wall of the artery at its junction with the ventricle. The straight borders of the valve are unattached and are directed upwards in the course of the vessel, against the sides of which they are pressed during the passage of blood along its canal. The free margin of each valve is somewhat thicker than the rest of the valve and is strengthened by a bundle of tendinous fibers. During the passage of blood along the pulmonary artery, these valves are pressed against the sides of its cylinder.

During ventricular diastole (rest), when the current of blood along the pulmonary artery is checked and partly thrown back by its elastic walls, these valves become immediately expanded and close the entrance of the tube. 3 The left atrium is smaller but thicker than the right atrium. It consists of two parts; a principle cavity/sinus and an appendix auriculae. The sinus is cuboidal in form and is covered in the front by the pulmonary artery and the aorta. Internally, it is separated from the right atrium by the septum auricularum. Behind the sinus on each side, it receives the pulmonary veins.

The appendix auriculae in the left atrium is narrower and more curved than the same structure in the right atrium. Its margins are more deeply indented, presenting a kind of foliated appearance. Its direction is forwards towards the right side, overlapping the root of the pulmonary artery. There are two main openings in the left atrium; the openings of the four pulmonary veins and the atrial-ventricular opening. Two of the four pulmonary veins open into the right side of the atrium and two open into the left side.

The two veins on the left exit into the atrium through a common opening. None of the pulmonary veins have valves. The atrial-ventricular opening is the large oval opening of blood flow between the atrium and the ventricle. It is smaller than the same opening between the right atrium and ventricle.3 The left ventricle is longer and more conical shaped than the right ventricle. It forms a small part of the left side of the anterior surface of the heart and a large portion of the posterior surface.

It also forms the apex of the heart because it extends beyond the right ventricle. Its walls are nearly twice as thick as those of the right ventricle. They are thickest in the broadest part of the ventricle, becoming gradually thinner towards the base and also towards the apex, which is the thinnest part of the left ventricle. There are two main openings in the left ventricle; the atrial-ventricular opening and the aortic opening. The atrial-ventricular opening is located behind and to the left side of the aortic opening. The opening is a little smaller than the same opening between the right atrium and ventricle. Its position corresponds to the center of the sternum. It is su …