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Doors of the Heart

Working with the heart's tireless muscles are four heart valves, equally durable, equally essential. Opening and closing with every heartbeat, these valves are one-way doors that control the flow of blood through the heart. Their perfect functioning keeps blood coursing through the body in a fast, endless stream. If damaged or deformed, the valves can disrupt the heart's perfect labor and create a potentially lethal back-up of blood in the veins and arteries, even within heart itself.All four of the valves consist of strong, thin cusps of tissue anchored to the tough rings of the heart's skeleton. The cusps are made of single sheets of fibrous tissue blanketed by folds of endocardial cells. At the base of each valve cusp, the fibrous layer merges with the ring to form a continuous, flexible hinge.

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Chambers of the Heart

The first of the heart's four chambers, the right atrium, receives purplish blood, short of oxygen and laden with carbon dioxide. This used blood arrives through the body's two major veins, the superior and inferior venae cavae, and from the many minute blood vessels that drain blood from the walls of the chamber itself. The atrial walls form a small, smooth dome of muscle with a pouch of folded tissue, the right auricle, perched near the top.Along the inner surface of the auricle, bundles of muscle called musculi pectinati line up in parallel ridges like the teeth of a comb.The right atrium holds about three-and-a-half tablespoons of blood. Its walls are less than an eighth of an inch thick. Two layers of muscle form the right atrial wall. A superficial layer spans both atria, and an inner layer, composed of many small bundles, arches over the atrial cavity at right angles to the superficial layer. From the right atrium, the dark, venous blood flows through the tricuspid valve to the right ventricle. A larger, stronger chamber than the atrium, the right ventricle holds slightly more than a quarter cup of blood. Its walls, which are a quarter of an inch thick, are composed of three spiraling layers of muscle: the superficial and deep sinospiral layers and the superficial bulbo spiral layer. All three bands of muscle anchor firmly onto the skeleton of the heart.Toward the top of the chamber, near the pulmonary artery, the inner surface of the ventricle is smooth. Throughout the rest of the chamber, small bundles of muscle rise from the walls of the ventricle, stretch for fractions of an inch across the chamber, then merge with other bundles. Living threads, they weave an intricate web of tissue across the ventricular wall. Papillary muscles climb up the wall and branch into the short, strong fibers of the chordae tendineae, which anchor the cusps of the tricuspid valve. When the right ventricle contracts, blood rushes into the pulmonary artery and to the lungs.Four pulmonary veins empty into the main cavity of the left atrium, also a smooth chamber, except for the musculi pectinati, inside the left auricle. Although constructed of two overlapping layers of muscle in the same way as the wall of the right atrium, the left atrial wall is slightly thicker and more powerful than its counterpart.From the left atrium, blood flows through the mitral valve to the left ventricle. This chamber holds the same volume of blood as the right ventricle, but its walls are three times thicker, making it by far the most powerful chamber in the heart. Its papillary muscles and the cusps of the mitral valve are thicker and stronger than their counterparts on the right side of the heart to withstand higher blood pressure.Along with the three spiral layers shared by both ventricles, a fourth layer of muscle, the deep bulbo spiral, winds from the aortic and mitral valve rings around the left ventricle and back. This extra layer gives the left ventricle greater strength than any other chamber of the heart. It is strength much needed, because with the contraction of the left ventricle, red blood pushes open the aorta, the first step on its long circuit through the blood vessels of the body.

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About the Heart

You probably think you know what the heart looks like. But you may be wrong. The heart does not look very much like the shapes people draw on Valentine's Day. And it certainly isn't flat, like a paper valentine. A real, live heart is shaped something like an ice-cream cone, with a pointed bottom and a rounded top, like two scoops of ice cream. It is hollow and can fill up with blood. An adult's heart is about the size of a large orange. It weights a little less than a pound.The heart is in the middle of the chest. It fits snugly between the two lungs. But the heart is tipped over, so that there is a little more of it on the left side than on the right. The pointed tip at the bottom of the heart touches the front wall of the chest. Every time the heart beats, it goes 'thump' against the chest wall. You can feel the thumps if you press there with your hand. You can hear them with your ear.The heart is a pump. Its walls are made of thick muscle. They can squeeze (contract) to send blood rushing out. The blood does not spill all over the place when it leaves the heart. It flows smoothly in tubes called blood vessels. First the blood flows into tubes called arteries. The arteries that leave the heart are thick tubes. The biggest one, called the aorta, is an inch wide. But the arteries soon branch again and again, to form many smaller tubes. These blood vessels carry blood to all parts of the body. The farther from the heart, the more blood vessels there are. The tiniest blood vessels, called capillaries, are so small you would need a microscope to see them. Capillaries join to form larger blood vessels. These tubes carry blood back toward the heart. The bigger ones are called veins. The closer to the heart, the fewer the veins there are, and the larger they are. The largest veins empty into the heart.

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The Crimson Stream

The path of blood forms a double loop. One route, called systemic circulation, wends its way through all the muscles, organs and tissues of the body, then back to the heart. A second, shorter circuit, the pulmonary circulation, travels only through the lungs. Pulmonary circulation changes blood from a purplish, breathless fluid into a bright crimson stream rich in oxygen.The pulmonary artery sprouts from the right ventricle and branches into smaller left and right arteries which lead to each lung. Inside the lungs, the arteries divide into smaller and smaller vessels until finally they spread out into a bed of capillaries. Hundreds of miles of these microscopic vessels thread their way through the lungs and wrap around pockets of lung tissue called alveoli. The lungs contain about 750 million of these small air sacs, their combined surface area totaling more than 750 square feet, about the size of a racquetball court.Blood in the capillaries is separated from the air in the alveoli by two thin membranes, each only one cell thick, and by a thin film of fluid. As blood flows through the capillaries, the gases on either side of this infinitesimal divide strain to reach equilibrium. Since the pressure of oxygen in the alveolar air is higher than its pressure in the blood, molecules of oxygen diffuse across the membranes into the blood. The greater pressure of carbon dioxide in the blood forces the gas to flow from the capillaries into the alveoli.Blood makes a complete circuit from the right side of the heart through the lungs, back through the left side of the heart and out the aorta every two-and-a-half seconds when a person is at rest. During exercise, the blood can travel this short loop in about one second. A remarkable blood protein, hemoglobin, is largely responsible for the speed with which this vital exchange occurs. Rich in iron, hemoglobin can unload carbon dioxide and absorb oxygen sixty times faster than blood plasma, the fluid portion of blood. Each molecule of hemoglobin carries four molecules of oxygen to the tissues of the body. It is hemoglobin, when combined with oxygen, that gives blood its bright red color.Leaving the capillaries, the red blood journeys through progressively larger blood vessels until it flows into one of the four pulmonary veins that empty into the left atrium. From the left atrium, blood flows through the left ventricle and out the aorta to the body. When oxygen-rich blood reaches the tissues of the body, a process much like the transfer of gases in the lungs, but in reverse, occurs. Because the pressure of oxygen in the blood is higher than its pressure in the oxygen-starved cells, molecules of the gas diffuse across the capillary and cell membranes into the cell. Differing pressures move carbon dioxide in the opposite direction. As the blood is drained of oxygen, the molecules of hemoglobin lose their brilliant red stain, leaving the blood a dull purple, the color it wears on its long journey back through the heart to the lungs.

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The Circulation of Blood

The circulatory system is actually made up of two circles. The left ventricle of the heart sends blood out through the big artery called the aorta. From there it flows into the branching arteries that carry it to various parts of the body. The veins that carry blood back from the body parts empty the blood into the right atrium of the heart. The blood then flows down into the right ventricle, which pumps it into a different circle of blood vessels. This circle goes from the right ventricle into the lungs and back to the heart, into the left atrium. From there the blood flows into the left ventricle. Then, in the next heartbeat, this blood is pumped out into the body again by the left ventricle. The two circles of blood vessels are separate, but the heart is in the middle of each of them.Why does the heart pump blood in two separate circles? The answer involves two important gases, oxygen (02) and carbon dioxide (C02). All the cells of the body need energy to do their work. They get energy by combining sugars or other food materials with oxygen. This chemical reaction is something like burning. The chemical reaction in a burning fire gives off heat and light. The chemical reaction inside body cells gives off heat and other forms of energy. This energy provides the power we need to talk and move and think.

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The Heartbeat

The heart never rests. Your heart started beating about eight months before you were born. It will beat over 2 1/2 billion times in your lifetime. In all those years it will never take a day off -- not even a minute off. Your body needs the flow of fresh blood that the heart keeps pumping.What is a heartbeat? It has two parts. First the heart muscle contracts. The contraction starts with the atria, which push blood into the ventricles. Then the walls of the ventricles squeeze together and force the blood out into the arteries. The contraction is the first part of the heartbeat. After that, the heart muscle relaxes. The heart gets larger again, and blood can flow in from the veins.Why doesn't blood flow back in from the arteries, the same way it went out? It can't, because the heart has some little trapdoors to stop it. These trapdoors are called valves. There is a valve at the bottom of the aorta, and another on in the large artery -- called the pulmonary artery -- that leads to the lungs. Trapdoor valves also guard the openings from the atria into the ventricles. So there is always one-way traffic in the heart. The valves keep blood from flowing backward.If you listen to a person's chest with a stethoscope, you can hear the sounds of the heartbeat clearly. Each beat has two sounds, like 'lub-dub'. The 'lub' is a long, booming sound. The 'dub' is a short, snapping sound. The 'lub' is the sound of the ventricles contracting and the atria closing. When the ventricles start to relax, the valves to the arteries snap shut. That is the 'dub' sound.

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Introduction

The heart is a muscular pump with two duties to perform: (1) it must pump venous blood to the lungs, so that the red blood cells may exchange their cargoes of carbon dioxide for new cargoes of oxygen; and (2) it must pump this oxygenated blood, received from the lungs, to all parts of the body. In consequence, the heart is a double pump whose two parts work in unison. The right side receives the venous blood and pumps it to the lungs; the left side received the oxygenated blood from the lungs and pumps it to the body at large. The circulation to and from the lungs is called the lesser or pulmonary circulation; the circulation to and from the body at large is called the greater or systemic circulation.Each side of the heart consists of: (1) a "receiving" chamber, the atrium (Latin = antechamber), which pumps its blood through an orifice guarded by a valve into (2) a "discharging" chamber, the ventricle (Latin = little belly), which pumps its blood through an orifice also guarded by a valve into an artery.The walls of the heart consist of three layers. The outermost layer is a thin membrane known as epicardium. The middle layer, known as the myocardium, consists of special cardiac muscle and is responsible for the heart's ability to pump. The innermost layer, known as endocardium, lines the myocardium and is a very delicate membrane that is in touch with the blood vessels. Besides these three layers, there exists a fibrous framework for the heart (often referred to as its "skeleton") in the form of tough rings which, in essence, surround the orifices of the heart; they give attachment to the heart muscle and to the delicate valves that guard the orifices.

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Introduction

The heart is a muscular pump with two duties to perform: (1) it must pump venous blood to the lungs, so that the red blood cells may exchange their cargoes of carbon dioxide for new cargoes of oxygen; and (2) it must pump this oxygenated blood, received from the lungs, to all parts of the body. In consequence, the heart is a double pump whose two parts work in unison. The right side receives the venous blood and pumps it to the lungs; the left side received the oxygenated blood from the lungs and pumps it to the body at large. The circulation to and from the lungs is called the lesser or pulmonary circulation; the circulation to and from the body at large is called the greater or systemic circulation.Each side of the heart consists of: (1) a "receiving" chamber, the atrium (Latin = antechamber), which pumps its blood through an orifice guarded by a valve into (2) a "discharging" chamber, the ventricle (Latin = little belly), which pumps its blood through an orifice also guarded by a valve into an artery.The walls of the heart consist of three layers. The outermost layer is a thin membrane known as epicardium. The middle layer, known as the myocardium, consists of special cardiac muscle and is responsible for the heart's ability to pump. The innermost layer, known as endocardium, lines the myocardium and is a very delicate membrane that is in touch with the blood vessels. Besides these three layers, there exists a fibrous framework for the heart (often referred to as its "skeleton") in the form of tough rings which, in essence, surround the orifices of the heart; they give attachment to the heart muscle and to the delicate valves that guard the orifices.

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Pericardium

The heart is enclosed in a double walled serous sac, known as the pericardium (Greek = around the heart). The two layers of pericardium, parietal and visceral, are separated from one another only by a minimal amount of serous fluid sufficient to keep their smooth contiguous surfaces moist and glistening.The pericardial sac is literally a bag whose walls are a continuous sheet of fibrous tissue. This, fibrous sac - the fibrous pericardium - adheres to the upper surface of the central tendon of the diaphragm and loosely encloses the heart. Its inner surface is lined by the parietal serous pericardium. By the protection the pericardium affords it, the heart moves and beats in a frictionless environment. The visceral pericardium is closely applied to the myocardium where it is also known as epicardium; it resembles the serous coat of a digestive organ.

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Right Atrium

The right atrium receives the venous blood returning from all parts of the body except the lungs. If the chamber is opened from the front, a large vein, the superior vena cava, will be seen entering it vertically from above; a still larger one, the inferior vena cava, will be seen entering it vertically from below. The superior vena cava returns blood from everything above the diaphragm (lungs excepted), the inferior vena cava from everything below the diaphragm. The two veins are in line with one another, and their right borders are in line and are continuous with the right border of the heart. The posterior wall of the chamber between the veins is smooth and gives the appearance of being a continuation of the veins, which indeed, developmentally, it is.The superior vena cava has no valve, but the inferior vena cava has, on its left, a narrow crescentic fold of endothelium, inappropriately called its valve. This fold sweeps upwards to outline an oval depression in the interatrial wall of the chamber known as the fossa ovalis, whose significance is appreciated only when the circulation before birth is understood.In the fetus, the inferior vena cava returned blood not only from the lower half of the body but also from the placenta and, since this placental blood contained oxygen and food products necessary to the life of the fetus, it had to be transferred at once to the left side of the heart for delivery to all parts of the body. It could not be pumped to the lungs, since before birth they were not functioning as respiratory organs. The fossa ovalis indicates the site of the fetal foramen ovale, and through this foramen the blood from the inferior vena cava passed directly to the left atrium. It closes at birth and then the blood no longer passes through the fetal foramen ovale. After birth blood is pumped to the lungs. If a foramen ovale remains open, it allows unoxygenated blood to mingle with oxygenated blood and one of several types of "blue baby" results.

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Right Ventricle

The right ventricle receives blood from the right atrium through the tricuspid orifice which is toward the back and right of the ventricle. The blood, pumped by the ventricle into the pulmonary trunk (which lies above and in front), has to take a U-shaped course between entrance and exit. The chamber itself is crescentic in cross section since, to the left, the muscular interventricular septum bulges into the right ventricle and reduces the size of the chamber.The walls of the chamber are rough because of the existence of many coarse bundles of muscle fibers, which are in the form of ridges and bridges. There also project from the walls nipple-like papillary muscles to whose apices are attached delicate tendinous cords; these resemble the cords of a parachute and are attached at their other ends to the margins of delicate and transparent flaps known as cusps and comparable to the silk of a parachute. The cusps are attached by their bases to the fibrous ring that surrounds the tricuspid orifice. When the ventricle contracts, the cusps come together and occlude the opening; the contraction of the papillary muscles tenses the cords and prevents the cusps from suffering the same fate as that suffered by an umbrella blown inside out on a windy day. Such a catastrophe occurs only very rarely.Below the opening of the pulmonary trunk, the ventricular wall is smooth. The entrance to the pulmonary trunk is guarded by the valve of the pulmonary trunk (pulmonary valve), which consists of three semilunar valvules or cusps whose bases are fixed to the fibrous ring at the base of the artery. These valvules are the walls of little pockets between them and the vessel wall. As the blood rushes out between the valvules, it compresses them against the wall of the vessel so that they offer no obstruction to the stream. When the ventricle relaxes, blood, attempting to re-enter the ventricle, fills the pockets behind the valvules and forces them together until they completely occlude the exit.

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Left Atrium

The left atrium receives oxygenated blood from the lungs via the pulmonary veins. These veins, therefore, are unlike veins in general in that they contain oxygenated blood. As a rule, four pulmonary veins enter the left atrium, two from each lung. The left atrium, which presents few features of interest, is essentially a chamber resulting from the confluence of pulmonary veins. To the left lies a little auricle which "peeks" around the left border of the heart it is the only part whose interior is not smooth. Since the left atrium (almost alone) forms the posterior surface (or base) of the heart, it opens forward and into the back of the left ventricle. The opening and its double flapped valve are known as the left atrioventricular or mitral orifice and valve. The name mitral was given to this valve because the two cusps together suggested the shape of a bishop's miter.

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Left Ventricle

The left ventricle is thick walled and is circular in cross section. Again, the blood must take a U-shaped course between the entrance behind and the exit above. It is also equipped with muscular ridges and bridges; it possesses papillary muscles, tendinous cords, and cusps. The cusps are two in number (hence the obsolete name "bi-cuspid") and their bases are attached to the fibrous ring surrounding the mitral orifice - the entrance to the chamber. The aorta is the great artery leaving the left ventricle above, and it lies in close apposition with the right side of the pulmonary trunk and somewhat behind it. It is guarded at its entrance by the aortic valve, exactly similar to the valve of the pulmonary trunk.

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The Interior of the Heart

The Interatrial Septum The interior of the atrial portion of the heart is divided by the interatrial septum into right and left chambers. This septum is a composite structure, being derived from two independent septa of the embryonic atrium, neither one of which was formed as a complete partition in itself. The openings in the two embryonic septa do not normally coincide in position, so that when fusion of the septa is completed, usually during the first year of postnatal life, the impervious partition characteristic of the adult heart is formed.Traces of the two originally independent parts of the interatrial septum are, however, clearly recognizable in the adult. The crescentic margin of the old valvula foraminis ovalis can be seen more or less firmly adherent to the left side of the septum. The area cephalic to this margin represents the location of ostium II of interatrial septum primum of embryologic descriptions. The main muscular part of the interatrial septum is derived from a septum (interatrial septum secundum) that forms somewhat later, immediately to the right of the septum primum. Septum secundum retains throughout fetal life an oval opening called the foramen ovale, the margin of which is seen on the right side of the adult interatrial septum as the limbus fossae ovalis. After the valve of the foramen ovale (septum primum) has fused to the left atrial side of the septum secundum, the foramen ovale becomes a more or less oval depression on the right side of the interatrial septum, called the fossa ovalis.In some 20 to 25 per cent of adult hearts the fusion of the valve of the foramen ovale with the septum secondum is not complete. By following the direction of the inferior vena cava, a slender probe may be slipped under the limbus fossae ovalis, between the valve of the foramen ovale and the muscular portion of the septum, into the left atrium. Such openings are vestiges of an important fetal blood route which is abandoned postnatally, after the lungs have become completely functional. Failure of complete fusion between the two parts of the embryonic interatrial septum with resulting probe-patency does not appear to be a handicap to an otherwise normal heart, and is of sufficient frequency to be regarded as a variant of the normal. Probe-patency should be sharply distinguished from a true valvular defect such as exists when the valve of the foramen ovale is incompetent to guard the foramen ovale.

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Right Atrium

The inferior vena cava passes through the diaphragm and enters the caudal side of the right atrium. The inferior caval orifice is partially guarded along its ventral aspect by an incompetent valve flap of variable fullness, the valve of the inferior vena cava (eustachian valve). Dorso-caudally on the wall of the right atrium, between the atrioventricular orifice and the fossa ovalis, is located the opening of the coronary sinus, guarded by the valve of the coronary sinus (thebesian valve).Leading from the right atrium ventrally, slightly caudally, and to the left is the right atrioventricular orifice which is guarded by the tricuspid valve. Extending between the right sides of the superior and inferior caval orifices there is a prominent muscular ridge, the crista terminalis, which underlies the sulcus terminalis. As the crista terminalis extends caudally it becomes less distinct. Its general course is continued by the valve of the inferior vena cava.Cephalically the crista terminalis passes to the right of the orifice of the superior vena cava, and continues as a muscular ridge which forms the sinistral margin of the opening into the right auricular appendage. This appendage projects cephalically from the right atrium and lies in contact externally with the ascending aorta. The interior of the right auricular appendage is trabeculated by muscular bands, the pectinate muscles. These appear to arise from the most cephalic part of the crista terminalis, and radiate out over the inner surface of the auricular appendage, forming the shell-like pattern which has given them their name.The portion of the right atrium bounded laterally by the crista terminalis, and medially by the interatrial septum is smooth-walled, and is called the sinus venarum. It is the adult derivative of the enlarged right horn of the sinus venosus of the embryo. The lower part of the crista terminalis marks the original line of attachment of the upper part of the right sinus valve; the part of the crista lying cephalic to the superior vena cava, on the cephalic wall of the atrium, is derived from the extension of the venous valves, especially the right one, which was known in the embryo as the septum spurium.

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Right Ventricle

In contrast with the wall of the left ventricle, the trabeculated part of the right ventricular wall makes up approximately two-thirds of its thickness and only its outer third is solid. The cephalic part of the right ventricle leading into the pulmonary trunk is called the pulmonary cone, and is delimited from the rest of the right ventricular cavity by a muscular ridge, the supraventricular crest (crista supraventricularis). The main portion of the right ventricular chamber is crescentic in cross section, since the interventricular septum is concave on its left side and convex on its right.

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Left Atrium

The left atrium is situated to the left of, and somewhat dorsal to, the right atrium. It lies dorsal to the root of the aorta, and its auricular appendage protrudes to the left of the pulmonary trunk. Opening into the dorsal wall of the left atrium are the right and left superior and inferior pulmonary veins. The orifices of these four veins are not guarded by valves. The left atrioventricular ostium, guarded by the mitral valve, lies on the ventral side of the atrium, facing slightly caudally and to the left. The inner face of the left atrium is relatively smooth, but the inner surface of the left auricular appendage is distinguished by well-marked pectinate muscles.

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Left Ventricle

In the adult the left ventricle has the form of a narrow cone, tapering to form the apex of the heart. The left ventricle forms the gently curved left cephalic border of the heart (Margo obtusus), about half of the diaphragmatic surface, and a small part of the sternocostal surface.The greater part of the inner surface of its wall is thrown into myocardial ridges of variable size. These ridges (trabeculae carneae) may either stand out in relief, or be undercut so that they form muscular bands completely covered by endocardium. In general the myocardium of the left ventricle consists of an outer zone of relatively solid muscle that makes up about two-thirds of its thickness, while its inner third is trabeculated.In the heart of the fetus and the newborn infant the left ventricular wall is no thicker than the right, and the interventricular septum forms a nearly straight partition between the two ventricular cavities. However, after birth, with the complete separation of the pulmonary and the systemic vascular circuits, the left ventricular myocardium begins to assume its characteristic preponderance. By the fourth year the adult proportions are attained, and the left ventricular wall has approximately twice the thickness and three times the mass of the right (Miller, 1883).

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Atrioventricular Valves

The atrioventricular valves are attached around the orifices leading from the atria into the ventricles and their leaflets extend into the cavities of the ventricles. Each valve has a continuous line of attachment, but its free edge is notched, partially subdividing it into leaflets. The right atrioventricular valve is usually divided into three leaflets and is, therefore, called the tricuspid valve. The left atrioventricular valve is similarly divided, but into two leaflets and is called the bicuspid or, from its fancied resemblance to a bishop's miter, the mitral valve.

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Semilunar Valves

The outlet of each ventricle is guarded by three semilunar valve cusps, each of which is a pocketlike flap of connective tissue, covered by endothelium and attached to the annulus fibrosus of the aorta or the pulmonary trunk. The free edges of these cusps are directed away from the ventricle, and in the center of each there is a small fibrocartilagino module, the corpus arantii. Radiating from this module over the fundus of the cusp and extending to its attached margin are fibros thickenings. On either side of the nodue the free edge of each cusp is thin, forming a pair of crescentic areas called the lunuli.

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The left ventricle is thick walled and is circular in cross section. Again, the blood must take a U-shaped course between the entrance behind and the exit above. It is also equipped with muscular ridges and bridges; it possesses papillary muscles, tendinous cords, and cusps. The cusps are two in number (hence the obsolete name "bi-cuspid") and their bases are attached to the fibrous ring surrounding the mitral orifice - the entrance to the chamber. The aorta is the great artery leaving the left ventricle above, and it lies in close apposition with the right side of the pulmonary trunk and somewhat behind it. It is guarded at its entrance by the aortic valve, exactly similar to the valve of the pulmonary trunk.

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The Circulation of Blood

The circulatory system is actually made up of two circles. The left ventricle of the heart sends blood out through the big artery called the aorta. From there it flows into the branching arteries that carry it to various parts of the body. The veins that carry blood back from the body parts empty the blood into the right atrium of the heart. The blood then flows down into the right ventricle, which pumps it into a different circle of blood vessels. This circle goes from the right ventricle into the lungs and back to the heart, into the left atrium. From there the blood flows into the left ventricle. Then, in the next heartbeat, this blood is pumped out into the body again by the left ventricle. The two circles of blood vessels are separate, but the heart is in the middle of each of them.Why does the heart pump blood in two separate circles? The answer involves two important gases, oxygen (02) and carbon dioxide (C02). All the cells of the body need energy to do their work. They get energy by combining sugars or other food materials with oxygen. This chemical reaction is something like burning. The chemical reaction in a burning fire gives off heat and light. The chemical reaction inside body cells gives off heat and other forms of energy. This energy provides the power we need to talk and move and think.

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The Heartbeat

The heart never rests. Your heart started beating about eight months before you were born. It will beat over 2 1/2 billion times in your lifetime. In all those years it will never take a day off -- not even a minute off. Your body needs the flow of fresh blood that the heart keeps pumping.What is a heartbeat? It has two parts. First the heart muscle contracts. The contraction starts with the atria, which push blood into the ventricles. Then the walls of the ventricles squeeze together and force the blood out into the arteries. The contraction is the first part of the heartbeat. After that, the heart muscle relaxes. The heart gets larger again, and blood can flow in from the veins.Why doesn't blood flow back in from the arteries, the same way it went out? It can't, because the heart has some little trapdoors to stop it. These trapdoors are called valves. There is a valve at the bottom of the aorta, and another on in the large artery -- called the pulmonary artery -- that leads to the lungs. Trapdoor valves also guard the openings from the atria into the ventricles. So there is always one-way traffic in the heart. The valves keep blood from flowing backward.If you listen to a person's chest with a stethoscope, you can hear the sounds of the heartbeat clearly. Each beat has two sounds, like 'lub-dub'. The 'lub' is a long, booming sound. The 'dub' is a short, snapping sound. The 'lub' is the sound of the ventricles contracting and the atria closing. When the ventricles start to relax, the valves to the arteries snap shut. That is the 'dub' sound.

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The Crimson Stream

The path of blood forms a double loop. One route, called systemic circulation, wends its way through all the muscles, organs and tissues of the body, then back to the heart. A second, shorter circuit, the pulmonary circulation, travels only through the lungs. Pulmonary circulation changes blood from a purplish, breathless fluid into a bright crimson stream rich in oxygen.The pulmonary artery sprouts from the right ventricle and branches into smaller left and right arteries which lead to each lung. Inside the lungs, the arteries divide into smaller and smaller vessels until finally they spread out into a bed of capillaries. Hundreds of miles of these microscopic vessels thread their way through the lungs and wrap around pockets of lung tissue called alveoli. The lungs contain about 750 million of these small air sacs, their combined surface area totaling more than 750 square feet, about the size of a racquetball court.Blood in the capillaries is separated from the air in the alveoli by two thin membranes, each only one cell thick, and by a thin film of fluid. As blood flows through the capillaries, the gases on either side of this infinitesimal divide strain to reach equilibrium. Since the pressure of oxygen in the alveolar air is higher than its pressure in the blood, molecules of oxygen diffuse across the membranes into the blood. The greater pressure of carbon dioxide in the blood forces the gas to flow from the capillaries into the alveoli.Blood makes a complete circuit from the right side of the heart through the lungs, back through the left side of the heart and out the aorta every two-and-a-half seconds when a person is at rest. During exercise, the blood can travel this short loop in about one second. A remarkable blood protein, hemoglobin, is largely responsible for the speed with which this vital exchange occurs. Rich in iron, hemoglobin can unload carbon dioxide and absorb oxygen sixty times faster than blood plasma, the fluid portion of blood. Each molecule of hemoglobin carries four molecules of oxygen to the tissues of the body. It is hemoglobin, when combined with oxygen, that gives blood its bright red color.Leaving the capillaries, the red blood journeys through progressively larger blood vessels until it flows into one of the four pulmonary veins that empty into the left atrium. From the left atrium, blood flows through the left ventricle and out the aorta to the body. When oxygen-rich blood reaches the tissues of the body, a process much like the transfer of gases in the lungs, but in reverse, occurs. Because the pressure of oxygen in the blood is higher than its pressure in the oxygen-starved cells, molecules of the gas diffuse across the capillary and cell membranes into the cell. Differing pressures move carbon dioxide in the opposite direction. As the blood is drained of oxygen, the molecules of hemoglobin lose their brilliant red stain, leaving the blood a dull purple, the color it wears on its long journey back through the heart to the lungs.

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The Interior of the Heart

The Interatrial Septum The interior of the atrial portion of the heart is divided by the interatrial septum into right and left chambers. This septum is a composite structure, being derived from two independent septa of the embryonic atrium, neither one of which was formed as a complete partition in itself. The openings in the two embryonic septa do not normally coincide in position, so that when fusion of the septa is completed, usually during the first year of postnatal life, the impervious partition characteristic of the adult heart is formed.Traces of the two originally independent parts of the interatrial septum are, however, clearly recognizable in the adult. The crescentic margin of the old valvula foraminis ovalis can be seen more or less firmly adherent to the left side of the septum. The area cephalic to this margin represents the location of ostium II of interatrial septum primum of embryologic descriptions. The main muscular part of the interatrial septum is derived from a septum (interatrial septum secundum) that forms somewhat later, immediately to the right of the septum primum. Septum secundum retains throughout fetal life an oval opening called the foramen ovale, the margin of which is seen on the right side of the adult interatrial septum as the limbus fossae ovalis. After the valve of the foramen ovale (septum primum) has fused to the left atrial side of the septum secundum, the foramen ovale becomes a more or less oval depression on the right side of the interatrial septum, called the fossa ovalis.In some 20 to 25 per cent of adult hearts the fusion of the valve of the foramen ovale with the septum secondum is not complete. By following the direction of the inferior vena cava, a slender probe may be slipped under the limbus fossae ovalis, between the valve of the foramen ovale and the muscular portion of the septum, into the left atrium. Such openings are vestiges of an important fetal blood route which is abandoned postnatally, after the lungs have become completely functional. Failure of complete fusion between the two parts of the embryonic interatrial septum with resulting probe-patency does not appear to be a handicap to an otherwise normal heart, and is of sufficient frequency to be regarded as a variant of the normal. Probe-patency should be sharply distinguished from a true valvular defect such as exists when the valve of the foramen ovale is incompetent to guard the foramen ovale.

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