Internal Anatomy Of The Heart

A cross section cut through the heart reveals three layers (Fig. 7): (1) a superficial visceral pericardium or epicardium (epi = "upon" + "heart"); (2) a middle myocardium (myo = "muscle" + "heart"); and (3) a deep lining called the "endocardium" (endo = "within," derived from the endoderm layer of the embryonic trilamina). The endocardium is a sheet of epithelium called endothelium that rests on a thin layer of connective tissue basement membrane. It lines the heart chambers and makes up the valves of the heart.

The myocardium is the tissue of the heart wall and the layer that actually contracts. The myocardium consists of cardiac muscles in a spiral arrangement of myocardium that squeeze blood through the heart in the proper directions (inferiorly through the atria and superiorly through the ventricles). Unlike all other types of muscle cells, cardiac muscle cells: (1) branch, (2) join at complex junctions called intercalated discs so that

Intercalated Discs
Fig. 5. Cardiac tamponade. Under normal circumstances, only serous fluid exists between the visceral and parietal layers of the pericardium. A condition called cardiac tamponade occurs when there is an accumulation of fluid in the pericardial space that leads to compression of the heart.

they form cellular networks, and (3) each contain single, centrally located nuclei. A cardiac muscle cell is not called a fiber. The term cardiac muscle fiber, when used, refers to a long row of joined cardiac muscle cells.

Like skeletal muscle, cardiac muscle cells are triggered to contract by the flow of Ca2+ ions into the cell. Cardiac muscle cells are joined by complex junctions called intercalated discs.

The discs contain adherans to hold the cells together, and there are gap junctions to allow ions to pass easily between the cells. The free movement of ions between cells allows for the direct transmission of an electrical impulse through an entire network of cardiac muscle cells. This impulse in turn signals all the muscle cells to contract at the same time. For more details on the electrical properties of the heart, refer to Chapter 9.

Venous Sinus

Fig. 6. Pericardial sinuses. A blind-ended sac called the oblique pericardial sinus is formed from the venous reflections of the inferior vena cava and pulmonary veins. Another sac, the transverse pericardial sinus, is formed between the arterial reflections above and the venous reflections of the superior vena cava and pulmonary veins below.

Fig. 6. Pericardial sinuses. A blind-ended sac called the oblique pericardial sinus is formed from the venous reflections of the inferior vena cava and pulmonary veins. Another sac, the transverse pericardial sinus, is formed between the arterial reflections above and the venous reflections of the superior vena cava and pulmonary veins below.

Pericardium And Heart Wall

Fig. 7. Internal anatomy of the heart. The walls of the heart contain three layers: the superficial epicardium; the middle myocardium, which is composed of cardiac muscle; and the inner endocardium. Note that cardiac muscle cells contain intercalated disks that enable the cells to communicate and allow direct transmission of electrical impulses from one cell to another. (Fig. 21.3, p. 553 from Human Anatomy, 4th Ed. by Frederic H. Martini, Michael J. Timmons, and Robert B. Tallitsch. © 2003 by Frederic H. Martini, Inc. and Michael J. Timmons.)

Fig. 7. Internal anatomy of the heart. The walls of the heart contain three layers: the superficial epicardium; the middle myocardium, which is composed of cardiac muscle; and the inner endocardium. Note that cardiac muscle cells contain intercalated disks that enable the cells to communicate and allow direct transmission of electrical impulses from one cell to another. (Fig. 21.3, p. 553 from Human Anatomy, 4th Ed. by Frederic H. Martini, Michael J. Timmons, and Robert B. Tallitsch. © 2003 by Frederic H. Martini, Inc. and Michael J. Timmons.)

4.1. Cardiopulmonary Circulation

To understand the internal anatomy of the heart, its function must be understood. The heart has two primary functions: (1) to collect oxygen-poor blood and pump it to the lungs for release of carbon dioxide in exchange for oxygen, and (2) to collect oxygen-rich blood from the lungs and pump it to all tissues in the body to provide oxygen in exchange for carbon dioxide.

The four chambers in the heart can be segregated into the left and the right side, each containing an atrium and a ventricle. The right side is responsible for collecting oxygen-poor blood and pumping it to the lungs. The left side is responsible for collecting oxygen-rich blood from the lungs and pumping it to all tissues in the body. Within each side, the atrium is a site for the collection of blood before pumping it to the ventricle. The ventricle is much stronger, and it is a site for the pumping of blood out and away from the heart (Fig. 8).

The right ventricle is the site for the collection of all oxygen-poor blood. The large superior and inferior venae cavae, among other veins, carry oxygen-poor blood from the upper and lower parts of the body to the right atrium. The right ventricle pumps the blood out of the heart and through the pulmonary trunk. The term trunk is a term that indicates an artery that bifurcates. The pulmonary trunk bifurcates into the left and right pulmonary arteries that enter the lungs. It is important to note that the term artery is always used for a vessel that carries blood away from the heart. This is irrespective of the oxygen content of the blood that flows through the vessel.

Once oxygenated, the oxygen-rich blood returns to the heart from the right and left lung through the right and left pulmonary veins, respectively (vein, a vessel carrying blood toward the heart). Each pulmonary vein bifurcates before reaching the heart. Thus, there are four pulmonary veins that enter the left atrium. Oxygen-rich blood is pumped out the heart by the left ventricle and into the aortic artery. The right side of the heart, the pulmonary artery, and pulmonary veins are part of the pulmonary circuit. This is because of their role in collecting blood from the tissues of the body and pumping it to the lungs (pulmo = "lungs"). The left side of the heart, the aortic artery, and the venae cavae are part of the systemic circuit. This is because of their role in collecting blood from the lungs and pumping it to all of the tissues of the body

Observing the heart from a superior viewpoint, the pulmonary trunk assumes the left, most anterior location projecting upward from the base of the heart. The aorta assumes a central location, and the superior vena cava is in the right, most posterior location. The general pattern of blood flow through the heart is shown in Fig. 9. Note that the function of atria is generally to collect; the function of ventricles is to pump. The right side is involved in pulmonary circulation, and the left side is involved in the systemic circulation. There is a unidirectional flow of blood through the heart; this is accomplished by valves.

4.2. The Right Atrium

The interior of the right atrium has three anatomically distinct regions, each a remnant of embryological development (Fig. 10): (1) The posterior portion of the right atrium has a smooth wall and is referred to as the sinus venarum (embryo-logically derived from the right horn of the sinus venosus);

Interior Right Atrium With Label

Fig. 8. Cardiopulmonary circulation. The four chambers in the heart can be segregated into the left and the right sides, each containing an atrium and a ventricle. The right side is responsible for collecting oxygen-poor blood and pumping it to the lungs. The left side is responsible for collecting oxygen-rich blood from the lungs and pumping it to the body. An artery is a vessel that carries blood away from the heart; a vein is a vessel that carries blood toward the heart. The pulmonary trunk and arteries carry blood to the lungs. Exchange of carbon dioxide for oxygen occurs in the lung through the smallest of vessels, the capillaries. Oxygenated blood is returned to the heart through the pulmonary veins and collected in the left atrium.

Fig. 8. Cardiopulmonary circulation. The four chambers in the heart can be segregated into the left and the right sides, each containing an atrium and a ventricle. The right side is responsible for collecting oxygen-poor blood and pumping it to the lungs. The left side is responsible for collecting oxygen-rich blood from the lungs and pumping it to the body. An artery is a vessel that carries blood away from the heart; a vein is a vessel that carries blood toward the heart. The pulmonary trunk and arteries carry blood to the lungs. Exchange of carbon dioxide for oxygen occurs in the lung through the smallest of vessels, the capillaries. Oxygenated blood is returned to the heart through the pulmonary veins and collected in the left atrium.

9. head and upper limbs

9. head and upper limbs

Thebesian Veins

Fig. 9. Cardiac circulation. Blood collected in the right atrium is pumped into the right ventricle. On contraction of the right ventricle, blood passes through the pulmonary trunk and arteries to the lungs. The left atrium pumps the blood into the left ventricle. Contraction of the left ventricle sends the blood through the aortic artery to all tissues in the body. The release of oxygen in exchange for carbon dioxide occurs through capillaries in the tissues. Return of oxygen-poor blood is through the superior and inferior venae cavae, which empty into the right atrium. Note that a unidirectional flow of blood through the heart is accomplished by valves. Reprinted from Principals of Human Anatomy, by G.J. Tortora, © 1999 Biological Sciences Textbooks, Inc. This material is used by permission of John Wiley & Sons, Inc.

9. trunk and lower limbs

Fig. 9. Cardiac circulation. Blood collected in the right atrium is pumped into the right ventricle. On contraction of the right ventricle, blood passes through the pulmonary trunk and arteries to the lungs. The left atrium pumps the blood into the left ventricle. Contraction of the left ventricle sends the blood through the aortic artery to all tissues in the body. The release of oxygen in exchange for carbon dioxide occurs through capillaries in the tissues. Return of oxygen-poor blood is through the superior and inferior venae cavae, which empty into the right atrium. Note that a unidirectional flow of blood through the heart is accomplished by valves. Reprinted from Principals of Human Anatomy, by G.J. Tortora, © 1999 Biological Sciences Textbooks, Inc. This material is used by permission of John Wiley & Sons, Inc.

Embryological Heart Tube

Fig. 10. Embryonic origin of the internal anatomy of the heart. The embryonic heart at day 22 is a linear heart tube. At this time, there are four divisions, and each contains structures that will remain associated with the division throughout development. During development, the linear tube folds to form two superior chambers (atria) and two inferior chambers (ventricles). SA, sinoatrial.

Fig. 10. Embryonic origin of the internal anatomy of the heart. The embryonic heart at day 22 is a linear heart tube. At this time, there are four divisions, and each contains structures that will remain associated with the division throughout development. During development, the linear tube folds to form two superior chambers (atria) and two inferior chambers (ventricles). SA, sinoatrial.

(2) the wall of the anterior portion of the right atrium is lined by horizontal, parallel ridges of muscle bundles that resemble the teeth of a comb, hence the name pectinate muscle (pectin = "a comb," embryologically derived from the primitive right atrium); and (3) the atrial septum (primarily derived from the embryonic septum primum and septum secundum). For more details on the embryology of the heart, refer to Chapter 2.

The purpose of Fig. 10 is to demonstrate that the smooth posterior wall of the right atrium holds most of the named structures of the right atrium. It receives both the superior and inferior venae cavae and the coronary sinus. It also contains the fossa ovalis, the sinoatrial node, and the atrioventricular node.

The inferior border of the right atrium contains the opening or ostium of the inferior vena cava and the os or ostium of the coronary sinus (Fig. 11). The coronary sinus is located on the posterior (inferior) side of the heart and receives almost all of the deoxygenated blood from the vasculature of the heart. The os of the coronary sinus opens into the right atrium anteriorly and inferiorly to the orifice of the inferior vena cava. A valve of the inferior vena cava (eustachian valve, a fetal remnant; Bartolommeo E. Eustachio, Italian Anatomist, 1520-1574) guards the orifice of the inferior vena cava. The valve of the coronary sinus (Thebesian valve; Adam C. Thebesius, German physician, 1686 to 1732) covers the opening of the coronary sinus to prevent backflow. Both of these valves vary in size and presence. For more details on the valves of the heart, refer to Chapter 27. These two venous valves insert into a prominent ridge, the sinus septum (eustachian ridge), which runs mediallateral across the inferior border of the atrium and separates the os of the coronary sinus and inferior vena cava.

On the medial side of the right atrium, the interatrial septum (atrial septum) has interatrial and atrioventricular parts. The

Atrial Septal Anatomy

Fig. 11. Internal anatomy of the right atrium. The interior of the right atrium has three anatomically distinct regions: (1) the posterior portion, which has a smooth wall; (2) the wall of the anterior portion, which is lined by horizontal, parallel ridges of pectinate muscle; and (3) the atrial septum. IVC, inferior vena cava; SA, sinoatrial; SVC, superior vena cava.

Fig. 11. Internal anatomy of the right atrium. The interior of the right atrium has three anatomically distinct regions: (1) the posterior portion, which has a smooth wall; (2) the wall of the anterior portion, which is lined by horizontal, parallel ridges of pectinate muscle; and (3) the atrial septum. IVC, inferior vena cava; SA, sinoatrial; SVC, superior vena cava.

Anotomical Land Marks Ivc

Fig. 12. Koch's triangle: three landmarks used to triangulate the location of the atrioventricular node (Koch's node) of the conduction system, including (1) coronary sinus, (2) atrioventricular opening, and (3) tendon of Todaro. Adapted from F. Anselme, B. Hook, K. Monahan, et al. (1966) Heterogeneity of retrograde fast-pathway conduction pattern in patients with atrioventricular nodal reentry tachycarda. Circulation 93, pp. 960-968.

Fig. 12. Koch's triangle: three landmarks used to triangulate the location of the atrioventricular node (Koch's node) of the conduction system, including (1) coronary sinus, (2) atrioventricular opening, and (3) tendon of Todaro. Adapted from F. Anselme, B. Hook, K. Monahan, et al. (1966) Heterogeneity of retrograde fast-pathway conduction pattern in patients with atrioventricular nodal reentry tachycarda. Circulation 93, pp. 960-968.

fossa ovalis (a fetal remnant) is found in the interatrial part of the atrial septum. It appears as a central depression surrounded by a muscular ridge or limbus. The fossa ovalis is positioned anterior and superior to the ostia of both the inferior vena cava and the coronary sinus. A tendinous structure, the tendon of Todaro (Francesco Todaro, Italian anatomist, 1839-1918), connects the valve of the inferior vena cava to the central fibrous body (the right fibrous trigone ["triangle"]) as a fibrous extension of the membranous portion of the interventricular septum. It courses obliquely within the eustachian ridge and separates the fossa ovalis above from the coronary sinus below. This tendon is a useful landmark in approximating the location of the atrioventricular node (conduction system).

To approximate the location of the atrioventricular node, found in the floor of the right atrium and the atrial septum, it is necessary to form a triangle (triangle of Koch; Walter Koch, German Surgeon, unknown-1880) using lines that cross (1) the os of the coronary sinus posteriorly, (2) the right atrioventricular opening anteriorly, and (3) the tendon of Todaro superiorly (Fig. 12).

Sinus Node

Fig. 13. The location of the sinoatrial node. Human cadaver heart demonstrating that the position of the sinoatrial node (pacemaker of the conduction system) in the smooth muscle portion of the right atrium is indicated by three lines: the sulcus terminalis, the lateral border of the superior vena cava, and the superior border of the right auricle. Note the muscle fiber bundles in the wall of the pectinate portion of the right atrium. SVC, superior vena cava.

Fig. 13. The location of the sinoatrial node. Human cadaver heart demonstrating that the position of the sinoatrial node (pacemaker of the conduction system) in the smooth muscle portion of the right atrium is indicated by three lines: the sulcus terminalis, the lateral border of the superior vena cava, and the superior border of the right auricle. Note the muscle fiber bundles in the wall of the pectinate portion of the right atrium. SVC, superior vena cava.

In the lateral wall and the septum of the smooth portion of the right ventricle are numerous small openings in the endocardial surface. These openings are the ostia of the smallest cardiac (Thebesian) veins. These veins function to drain deoxygenated blood from the myocardium to empty into the right atrium, which is the collecting site for all deoxygenated blood.

In the anterior-superior portion of the right atrium, the smooth wall of the interior becomes pectinate. The smooth and pectinate regions are separated by a ridge, the crista terminalis (crista = "crest" + "terminal"). The ridge represents the end of the smooth wall and the beginning of the pectinate wall. It begins at the junction of the right auricle with the atrium and passes inferiorly over the "roof" of the atrium. The crista runs inferiorly and parallel to the openings of the superior and inferior vena cavae. Recall that the crista terminalis separates the sinus venosus and the primitive atrium in the embryo and remains to separate the smooth and the pectinate portions of the right atrium after development.

The crista terminalis on the internal side results in a groove on the external side, the sulcus terminalis. This is a useful landmark in approximating the location of the sinoatrial node (pacemaker of the conduction system). The intersection of three following lines indicates the position of the sinoatrial node: (1) the sulcus terminalis, (2) the lateral border of the superior vena cava, and (3) the superior border of the right auricle (Fig. 13).

On the "floor" of the right atrium is the atrioventricular portion of the atrial septum, which has muscular and membranous components. At the anterior and inferior aspect of the atrial septum, the tricuspid valve annulus (annulus = "ring") is attached to the membranous septum. As a result, a portion of the membranous septum lies superior to the annulus and therefore functions as a membranous atrial, and membranous ventricular, septum.

4.3. The Right Ventricle

The right ventricle receives blood from the right atrium and pumps it to the lungs through the pulmonary trunk and arteries. Most of the anterior surface of the heart is formed by the right ventricle (Fig. 14). Abundant, coarse trabeculae carneae ("beams of meat") characterize the walls of the right ventricle. Trabeculae carneae are analogous to pectinate muscle of the right atrium (as bundles of myocardium) and are found in both the right and left ventricles. The outflow tract, conus arteriosus ("arterial cone") or infundibulum ("funnel"), carries blood out

Internal Right Ventricle Structures
Fig. 14. Internal anatomy of the right ventricle. Coarse trabeculae carneae characterize the walls of the right ventricle. The conus arteriosus makes up most of the outflow tract. The right atrioventricular or tricuspid valve is made up of three sets of cusps, cordae tendineae and papillary muscles.

of the ventricle in an anterior-superior direction and is relatively smooth walled. A component of the conus arteriosus forms part of the interventricular septum. This small septum, the infundibular (conal) septum, separates the left and right ventricular outflow tracts and is located just inferior to both semilunar valves. Four distinct muscle bundles, collectively known as the semicircular arch, separate the outflow tract from the rest of the right atrium.

4.3.1. Tricuspid Valve

Blood is pumped from the right atrium through the atrioven-tricular orifice into the right ventricle. When the right ventricle contracts, blood is prevented from flowing back into the atrium by the right atrioventricular valve or tricuspid ("three cusps") valve. The valve consists of the annulus, three valvular leaflets, three papillary muscles, and three sets of chordae tendineae (Figs. 14 and 15). The atrioventricular orifice is reinforced by the annulus fibrosus of the cardiac skeleton (dense connective tissue). Medially, the annulus is attached to the membranous ventricular septum.

The tricuspid valve has three leaflets: anterior (superior), posterior (inferior), and septal. The anterior leaflet is the larg est and extends from the medial border of the ventricular septum to the anterior free wall. This, in effect, forms a partial separation between the inflow and outflow tracts of the right ventricle. The posterior leaflet extends from the lateral free wall to the posterior portion of the ventricular septum. The septal leaflet tends to be somewhat oval in shape and extends from the annulus of the orifice to the medial side of the inter-ventricular septum (on the inflow side), often including the membranous part of the septum.

Papillary ("nipple") muscles contract and "tug" down on chordae tendineae ("tendinous cords") attached to the leaflets to secure them in place in preparation for the contraction of the ventricle. This is done to prevent the prolapse of the leaflets into the atrium. This is somewhat analogous to the tightening of the sails on a yacht in preparation for a big wind. Note that the total surface area of the cusps of the atrioventricular valve is approximately twice that of the respective orifice, so that considerable overlap of the leaflets occurs when the valves are in the closed position. The leaflets remain relatively close together even during ventricular filling. The partial approximation of the valve surfaces is caused by eddy currents that prevail behind the leaflets and by tension exerted by the

Trabeculae Carneae

Fig. 15. Valves of the heart. During ventricular systole, atrioventricular valves close to prevent the regurgitation of blood from the ventricles into the atria. The right atrioventricular valve is the tricuspid valve; the left is the bicuspid valve. During ventricular diastole, the atrioventricular valves open as the ventricles relax, and the semilunar valves close. The semilunar valves prevent the backflow of blood from the great vessels into the resting ventricles. The valve of the pulmonary trunk is the pulmonary semilunar valve, and the aortic artery has the aortic semilunar valve. To the right of each figure are photographs of human cadaveric hearts.

Fig. 15. Valves of the heart. During ventricular systole, atrioventricular valves close to prevent the regurgitation of blood from the ventricles into the atria. The right atrioventricular valve is the tricuspid valve; the left is the bicuspid valve. During ventricular diastole, the atrioventricular valves open as the ventricles relax, and the semilunar valves close. The semilunar valves prevent the backflow of blood from the great vessels into the resting ventricles. The valve of the pulmonary trunk is the pulmonary semilunar valve, and the aortic artery has the aortic semilunar valve. To the right of each figure are photographs of human cadaveric hearts.

chordae tendineae and papillary muscle. As the filling of the ventricle reduces, the valve leaflets float toward each other, but the valve does not close. The valve is closed by ventricular contractions, and the valve leaflets, which bulge toward the atrium but do not prolapse, stay pressed together throughout ventricular contraction (Fig. 15). The junction between two leaflets is called a commissure and is named by the two adjoining leaflets (anteroseptal, anteroposterior, and posteroseptal). Each commissure contains a relatively smooth arc of valvular tissue delineated by the insertion of the chordae tendineae.

There are three papillary muscles, just as there are three leaflets or cusps. The anterior papillary muscle is located in the apex of the right ventricle. This is the largest of the papillary muscles in the right ventricle, and it may have one or two heads. When this papillary muscle contracts, it pulls on chordae tendineae attached to the margins of the anterior and posterior leaflets. The posterior papillary muscle is small and located in the posterior lateral free wall. When this papillary contracts, it pulls on chordae tendineae attached to the posterior and septal leaflets. The septal papillary muscle (papillary of the conus) arises from the muscular interventricular septum near the outflow tract (conus arteriosus). This papillary muscle more often consists of a collection of small muscles in close proximity and has attachments to the anterior and septal valve leaflets. In addition, chordae tendineae in this region may extend simply from the myocardium and attach to the valve leaflets directly without a papillary muscle (Fig. 14). The most affected is the septal leaflet, which has restricted mobility because of extensive chordae tendineae attachment directly to the myocardium.

Near the anterior free wall of the right ventricle is a muscle bundle of variable size and the moderator band (occasionally absent). This muscle bundle extends from the interventricular septum to the anterior papillary muscle and contains a component of the right bundle branch of the conduction system. It seems logical that the anterior papillary muscle, with its remote location away from the septum, would need special conduction fibers for it to contract with the other papillary muscles and convey control of the valve leaflets equal to the other valve leaflets. The moderator band is a continuation of another muscle bundle, the septal band (septal trabeculae) called septomar-ginal trabecula, and is a component of the semicircular arch (delineation of the outflow tract).

4.3.2. Pulmonary Semilunar Valve

During ventricular systole, blood is pumped from the right ventricle into the pulmonary trunk and arteries toward the lungs. When the right ventricle relaxes, in diastole, blood is prevented from flowing back into the ventricle by the pulmonary semilunar valve (Figs. 14 and 15). The semilunar valve is composed of three symmetric, semilunar-shaped cusps. Each cusp looks like a cup composed of a thin membrane. Each cusp acts like an upside-down parachute facing into the pulmonary trunk, opening as it fills with blood. This filled space or recess of each cusp is called the sinus of Valsalva (Antonio M. Valsalva, 16661723). On complete filling, the three cusps contact each other and block the retrograde flow of blood. Each of the three cusps is attached to an annulus such that the cusp opens into the lumen, forming a U shape. The annulus is anchored to both the right ventricular infundibulum and the pulmonary trunk. The cusps are named according to their orientation in the body: anterior, left (septal), and right.

The cusps collapse against the arterial wall as the right ventricle contracts, sending blood flowing past them. When the ventricle rests (diastole), the cusps meet in the luminal center. There is a small thickening on the center of the free edge of each cusp, at the point where the cusps meet. This nodule (of Arantius or Morgagni; Giulio C. [Aranzi] Arantius, Italian anatomist and physician, 1530-1589; Giovanni B. Morgagni, Italian anatomist and pathologist, 1682-771) ensures central valve closure. Radiating from this nodule around the free edge of the cusp is a ridge, the linea alba ("line" + "white").

4.4. The Left Atrium

The left atrium (Fig. 16) receives oxygenated blood from the lungs via the left and right pulmonary veins. The pulmonary veins enter the heart as two pairs of veins inserting posteriorly and laterally into the left atrium. In addition, the smallest (Thebesian) veins drain deoxygenated blood from the atrial myocardium directly into the atrium.

The left atrium is found midline, posterior to the right atrium and superior to the left ventricle. Anteriorly, a left atrial appendage (auricle) extends over the atrioventricular (coronary) sul-cus. The walls of the atrial appendage are pectinate, and the walls of the left atrium are smooth; this reflects their embryo-logical origin. The atrial appendage is derived from the primitive atrium (a strong pumping structure), and the atrium is derived from the fetal pulmonary vein as a connection with the embryonic pulmonary venous plexus. The venous structures are absorbed into the left atrium, resulting in the posteriolateral connections of the right and left pulmonary veins. The atrial septum of the left atrium is derived from the embryonic septum primum, resulting in the adult structure called the valve of the foramen ovale (a sealed valve flap).

4.5. The Left Ventricle

The left ventricle receives blood from the left atrium and pumps it through the aortic artery to all of the tissues of the body (Fig. 16). Most of the left lateral surface of the heart is formed by the left ventricle, also forming part of the inferior and posterior surface. As with the right ventricle, abundant trabeculae carneae characterize the walls of the left. However, in contrast to the right ventricle, the muscular ridges tend to be relatively fine. Also in contrast to the right ventricle, the myocardium in the wall of the left ventricle is much thicker. The interventricu-lar septum appears from within the left ventricle to bulge into the right ventricle; this creates a barrel-shaped left ventricle.

4.5.1. Bicuspid (Mitral) Valve

Blood is pumped from the left atrium through the left atrio-ventricular orifice into the left ventricle. When the left ventricle contracts, blood is prevented from flowing back into the atrium by the left atrioventricular valve or bicuspid ("two cusps") valve (Figs. 15 and 16). The valve consists of the annulus, two leaflets, two papillary muscles, and two sets of chordae tendineae.

The atrioventricular orifice is partly reinforced by the annu-lus fibrosus of the cardiac skeleton. The annulus fibrosus supports the posterior and lateral two-thirds of the annulus. The remaining medial third is supported by attachment to the left atrium and fibrous support to the aortic semilunar valve.

The bicuspid valve has two leaflets: anterior (medial or aortic) and posterior (inferior or mural, "wall"). The two apposing leaflets of the valve resemble a bishop's hat or mitre. Thus, the bicuspid valve is often referred to as the mitral valve (Fig. 17).

The anterior leaflet is typically a trapezoidal shape. The distance from its attachment on the annulus to its free edge is longer than the length of attachment across the annulus. In contrast, the posterior leaflet is found to be relatively narrow, with a very long attachment distance across the annulus. The distance from annulus to free edge in the anterior cusp is twice as long as in the posterior cusp. The posterior cusp is so long and narrow that the free edge is often subdivided into the anterior, central, and posterior crescent shapes.

Papillary muscles, in conjunction with chordae tendineae, attach to the leaflets to secure them in place. This is done in preparation for the contraction of the ventricle to prevent the prolapse of the leaflets up into the atrium. As with the other atrioventricular valve (tricuspid), the total surface area of the two cusps of the valve is significantly greater than the area described by the orifice. There is considerable overlap of the leaflets when the valves are in the closed position (Fig. 15).

As with the tricuspid valve, the leaflets remain relatively close together, even when the atrium is contracting and the ventricle is filling. The partial approximation of the valve surfaces is caused by eddy currents that prevail behind the leaflets and by tension exerted by the chordae tendineae and papillary muscle. In the open position, the leaflets and commissures are in an oblique plane of orientation roughly parallel to the ventricular septum. The valve is closed by ventricular contractions.

Left Atrium Internal Anatomy

Fig. 16. Internal anatomy of the left atrium and ventricle. The left atrium receives oxygenated blood from the lungs via the left and right pulmonary veins. The pulmonary veins enter the heart as two pairs of veins inserting posteriorly and laterally. Anteriorly, the pectinate left auricle extends over the smooth-walled atrium. Most of the left lateral surface of the heart is formed by the left ventricle. Trabeculae carneae characterize the walls, and the myocardium is much thicker than the left ventricle. The interventricular septum bulges into the right ventricle, creating a barrel-shaped left ventricle.

Fig. 16. Internal anatomy of the left atrium and ventricle. The left atrium receives oxygenated blood from the lungs via the left and right pulmonary veins. The pulmonary veins enter the heart as two pairs of veins inserting posteriorly and laterally. Anteriorly, the pectinate left auricle extends over the smooth-walled atrium. Most of the left lateral surface of the heart is formed by the left ventricle. Trabeculae carneae characterize the walls, and the myocardium is much thicker than the left ventricle. The interventricular septum bulges into the right ventricle, creating a barrel-shaped left ventricle.

The valve leaflets, which bulge toward the atrium, stay pressed together throughout the contraction and do not prolapse. The junctions of the two leaflets are called the "anterolateral" and the "posteromedial" commissures. The line of apposition of the leaflets during valvular closure is indicated by a fibrous ridge.

There are two distinct papillary muscles of the left ventricle that extend from the ventricular free wall toward and perpendicular to the atrioventricular orifice. The anterior papillary muscle is typically slightly larger than the posterior, and each papillary muscle consists of a major trunk that often has mul

Mitre

Mitral Valve Pope Hat
Fig. 17. The mitral valve. The mitral (left atrioventricular or bicuspid) valve is so named because of its resemblance to a cardinal's hat, known as a mitre. Left: Photo of the Pope that appears on the Vatican Web site.

tiple heads from which extend the chordae tendineae. The chordae tendineae of each papillary muscle extend to the two valvular commissures and to the multiple crescent shapes of the posterior cusp. Thus, each papillary muscle pulls on chordae from both leaflets. In addition, the posterior leaflet has occasional chordae that extend simply from the ventricular myocardium without a papillary muscle (similar to the septal papillary muscle of the right ventricle).

4.5.2. Aortic Semilunar Valve

During ventricular systole, blood is pumped from the left ventricle into the aortic artery to all of the tissues of the body. When the left ventricle relaxes in diastole, blood is prevented from flowing back into the ventricle by the aortic semilunar valve (Figs. 15 and 16). Like the pulmonary semilunar valve, the aortic valve is composed of three symmetric, semilunar-shaped cusps; each cusp acts like an upside-down parachute facing into the aortic artery, opening as it fills with blood. The filled space or recess of each cusp is called the sinus of Valsalva. On complete filling, the three cusps contact each other and block the flow of blood. Each of the three cusps is attached to an annulus ("ring") such that the cusp opens into the lumen, forming a U-shape. The cusps are firmly anchored to the fibrous skeleton within the root of the aorta. A circular ridge on the innermost aspect of the aortic wall, at the upper margin of each sinus, is the sinotubular ridge, the junction of the sinuses and the aorta.

At the sinotubular ridge, the wall of the aorta is thin, bulges slightly, and is the narrowest portion of the aortic artery. The cusps are named according to their orientation in the body: left and right (both facing the pulmonary valve) and posterior. Within the sinuses of Valsava, there are openings or ostia (ostium = "door or mouth") into the blood supply of the heart called coronary arteries. These ostia are positioned below the sinotubular junction near the center of the sinuses. Only the two sinuses facing the pulmonary valve (left and right) have ostia that open into the left and right coronary arteries, respectively. Coronary arteries carry oxygenated blood to the myocardium of the heart. During ventricular diastole, the aortic valve snaps shut as pressure in the aorta increases. Under such pressure, the walls of the great artery distend, the sinuses fill, and blood is sent under great pressure through the coronary ostia into the coronary arteries. The posterior (noncoronary) sinus is in a position that abuts the fibrous skeleton and the annuli of both atrioventricular valves (Fig. 15).

When the left ventricle contracts, the cusps collapse against the arterial wall as blood flows past them. When the ventricle rests (diastole), the cusps meet in the luminal center. As with the pulmonary valve, there is a small thickening on the center of the free edge of each cusp, at the point where the cusps meet. This nodule (of Arantius or Morgagni) ensures central valve closure. Radiating from this nodule around the free edge of the cusp is a ridge, the linea alba. This valve is exposed to

Cardiac Skeleton
Fig. 18. The cardiac skeleton. A dense connective tissue that functions to attach the atrial and ventricular myocardium, support and reinforce the openings of the four valves of the heart, and electrically separate the ventricles from the atria. Courtesy of Jean Magney, University of Minnesota.

a greater degree of hemodynamic stress than the pulmonary valve. The aortic cusps can thicken, and the linea alba can become more pronounced. For this and other reasons, the aortic pulmonary valve is the most likely valve to be surgically repaired or replaced (see Chapter 27).

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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Responses

  • Niklas
    What are the layers of the heart?
    4 years ago
  • Serena
    How does a mitral valve look like the popes had?
    2 years ago
  • Sarah Kuefer
    What does the conus form heart embryology?
    2 years ago
  • reuben
    What is annulus fibrosus in heart?
    1 year ago
  • joona
    What is the structure of the tricuspid valve?
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