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Ophthalmology => High Blood Pressure => Heart
Heart
INTRODUCTION
Heart, in anatomy, the hollow muscular organ that receives blood from the veins and propels it into and through the arteries. This article deals with the heart's anatomy only. For information on disorders of the heart, seeHeart Diseases.
HEART STRUCTURE
The human heart is about the size of a closed fist. It is situated behind the lower part of the breastbone, extending more to the left of the midline than to the right. It is roughly conical in shape, with the base directed upwards, to the right, and slightly backwards; the apex touches the chest wall between the fifth and sixth ribs. The heart is held in place principally by its attachment to the great arteries and veins, and by its confinement in the pericardium, a double-walled sac with one layer enveloping the heart and the other attached to the breastbone, the diaphragm, and the membranes of the thorax.
Within the adult heart are two parallel independent systems, each consisting of an auricle, or atrium, and a ventricle; from their anatomical positions these systems are often designated the right heart and the left heart.
CIRCULATION
Blood from the body returns to the right atrium through two large veins, the superior and inferior venae cavae; in addition the blood that has supplied the heart muscle is drained directly into the right atrium through the coronary sinus. Return of venous blood to the right atrium takes place during the entire heart cycle of contraction and relaxation, and to the right ventricle only during the relaxation part of the cycle, called diastole, when both right heart cavities constitute a common chamber; near the end of diastole, contraction of the right atrium completes the filling of the right ventricle with blood. Rhythmic contractions of the right ventricle expel the blood through the pulmonary arteries into the capillaries of the lung, where the blood receives oxygen (seeCirculatory System; Respiration). The lung capillaries then empty into the pulmonary veins, which in turn empty into the left atrium. Pulmonary venous return to the left atrium and left ventricle proceeds simultaneously in the same manner as the venous return to the right heart cavities. Contraction of the left ventricle rhythmically propels the blood into the aorta and from there to all arteries of the body, including the coronary arteries, which supply the heart muscle.
The blood forced from the ventricles during systole, or contraction, is prevented from returning during diastole by valves at the openings of the aortic and pulmonary arteries. These valves consist of three semi-lunar (half-moon-shaped) flaps of membrane, which are curved in the direction of blood flow and which open readily on pressure in that direction; when the original pressure subsides, back pressure forces the edges of the flaps together. The tricuspid valve, situated between the right atrium and ventricle, is composed of three triangular flaps of membrane, and the bicuspid or mitral valve, between the left atrium and ventricle, has two such flaps. The bases of the flaps in both these valves are attached in a groove at the junction of the atrium and ventricle; the free edge is anchored by tendon-like cords, known as the chordae tendinae, to the muscles of the heart wall. The flaps remain open until the ventricle fills with blood. When the ventricle begins to contract, the valve is closed by pressure. The chordae tendinae prevent inversion of the flaps during this period of systolic pressure.
HEARTBEAT
The rate of heartbeat is controlled by the autonomic nervous system, being accelerated by the sympathetic system and depressed by the parasympathetic system. Nervous impulses originate rhythmically in a special nervous node, the sinus, or sinoatrial node, located in the right atrium near the superior vena cava. The impulses travel along a slender bundle of neuromuscular fibres, called the bundle of His, or the atrioventricular bundle, which, after a relay at the atrioventricular node, branches into two bundles, one for each of the ventricles. Through these specialized fibres the nervous impulse stimulates, at rapid speed and in a known orderly sequence, contraction of the heart-muscle fibres.
The heart produces two sounds in each cycle of the beat. The first is rather dull and is caused by vibration of the auriculoventricular valves and by the contraction of the ventricular muscle fibres; the second is sharp and is caused by the sudden closing of the aortic and pulmonary valves. In cardiac disease these regular sounds may be replaced or accompanied by murmurs caused by turbulent blood rushing through abnormal valves or openings; detection of such murmurs is of great importance in diagnosis.
Although the nerves and certain drugs influence the rate of the heartbeat-as do certain chemicals, similar to hormones, that are secreted by the heart itself-heart muscle is inherently contractile and will continue beating at a regular rate even when all connection with the nervous system is severed.
PRENATAL DEVELOPMENT
In the embryo the heart develops from the fusion of the two ventral aortas, forming a single pulsating organ. Separation into right and left heart takes place later with the formation of an interauricular and an interventricular septum (partition). The separation is not completed, however, until the lungs begin their function at birth. Before birth, the blood is oxygenated in the placenta and returns to the right auricle through the inferior vena cava; it is then directed by the Eustachian valve through the foramen ovale, a persistent opening in the interauricular septum. After birth the Eustachian valve degenerates to a rudiment and the foramen ovale normally closes, but the opening in the interauricular septum may persist in varying degrees in about one fifth of the adult population.INTRODUCTION
Heart, in anatomy, the hollow muscular organ that receives blood from the veins and propels it into and through the arteries. This article deals with the heart's anatomy only. For information on disorders of the heart, seeHeart Diseases.
HEART STRUCTURE
The human heart is about the size of a closed fist. It is situated behind the lower part of the breastbone, extending more to the left of the midline than to the right. It is roughly conical in shape, with the base directed upwards, to the right, and slightly backwards; the apex touches the chest wall between the fifth and sixth ribs. The heart is held in place principally by its attachment to the great arteries and veins, and by its confinement in the pericardium, a double-walled sac with one layer enveloping the heart and the other attached to the breastbone, the diaphragm, and the membranes of the thorax.
Within the adult heart are two parallel independent systems, each consisting of an auricle, or atrium, and a ventricle; from their anatomical positions these systems are often designated the right heart and the left heart.
CIRCULATION
Blood from the body returns to the right atrium through two large veins, the superior and inferior venae cavae; in addition the blood that has supplied the heart muscle is drained directly into the right atrium through the coronary sinus. Return of venous blood to the right atrium takes place during the entire heart cycle of contraction and relaxation, and to the right ventricle only during the relaxation part of the cycle, called diastole, when both right heart cavities constitute a common chamber; near the end of diastole, contraction of the right atrium completes the filling of the right ventricle with blood. Rhythmic contractions of the right ventricle expel the blood through the pulmonary arteries into the capillaries of the lung, where the blood receives oxygen (seeCirculatory System; Respiration). The lung capillaries then empty into the pulmonary veins, which in turn empty into the left atrium. Pulmonary venous return to the left atrium and left ventricle proceeds simultaneously in the same manner as the venous return to the right heart cavities. Contraction of the left ventricle rhythmically propels the blood into the aorta and from there to all arteries of the body, including the coronary arteries, which supply the heart muscle.
The blood forced from the ventricles during systole, or contraction, is prevented from returning during diastole by valves at the openings of the aortic and pulmonary arteries. These valves consist of three semi-lunar (half-moon-shaped) flaps of membrane, which are curved in the direction of blood flow and which open readily on pressure in that direction; when the original pressure subsides, back pressure forces the edges of the flaps together. The tricuspid valve, situated between the right atrium and ventricle, is composed of three triangular flaps of membrane, and the bicuspid or mitral valve, between the left atrium and ventricle, has two such flaps. The bases of the flaps in both these valves are attached in a groove at the junction of the atrium and ventricle; the free edge is anchored by tendon-like cords, known as the chordae tendinae, to the muscles of the heart wall. The flaps remain open until the ventricle fills with blood. When the ventricle begins to contract, the valve is closed by pressure. The chordae tendinae prevent inversion of the flaps during this period of systolic pressure.
HEARTBEAT
The rate of heartbeat is controlled by the autonomic nervous system, being accelerated by the sympathetic system and depressed by the parasympathetic system. Nervous impulses originate rhythmically in a special nervous node, the sinus, or sinoatrial node, located in the right atrium near the superior vena cava. The impulses travel along a slender bundle of neuromuscular fibres, called the bundle of His, or the atrioventricular bundle, which, after a relay at the atrioventricular node, branches into two bundles, one for each of the ventricles. Through these specialized fibres the nervous impulse stimulates, at rapid speed and in a known orderly sequence, contraction of the heart-muscle fibres.
The heart produces two sounds in each cycle of the beat. The first is rather dull and is caused by vibration of the auriculoventricular valves and by the contraction of the ventricular muscle fibres; the second is sharp and is caused by the sudden closing of the aortic and pulmonary valves. In cardiac disease these regular sounds may be replaced or accompanied by murmurs caused by turbulent blood rushing through abnormal valves or openings; detection of such murmurs is of great importance in diagnosis.
Although the nerves and certain drugs influence the rate of the heartbeat-as do certain chemicals, similar to hormones, that are secreted by the heart itself-heart muscle is inherently contractile and will continue beating at a regular rate even when all connection with the nervous system is severed.
PRENATAL DEVELOPMENT
In the embryo the heart develops from the fusion of the two ventral aortas, forming a single pulsating organ. Separation into right and left heart takes place later with the formation of an interauricular and an interventricular septum (partition). The separation is not completed, however, until the lungs begin their function at birth. Before birth, the blood is oxygenated in the placenta and returns to the right auricle through the inferior vena cava; it is then directed by the Eustachian valve through the foramen ovale, a persistent opening in the interauricular septum. After birth the Eustachian valve degenerates to a rudiment and the foramen ovale normally closes, but the opening in the interauricular septum may persist in varying degrees in about one fifth of the adult population.INTRODUCTION
Heart, in anatomy, the hollow muscular organ that receives blood from the veins and propels it into and through the arteries. This article deals with the heart's anatomy only. For information on disorders of the heart, seeHeart Diseases.
HEART STRUCTURE
The human heart is about the size of a closed fist. It is situated behind the lower part of the breastbone, extending more to the left of the midline than to the right. It is roughly conical in shape, with the base directed upwards, to the right, and slightly backwards; the apex touches the chest wall between the fifth and sixth ribs. The heart is held in place principally by its attachment to the great arteries and veins, and by its confinement in the pericardium, a double-walled sac with one layer enveloping the heart and the other attached to the breastbone, the diaphragm, and the membranes of the thorax.
Within the adult heart are two parallel independent systems, each consisting of an auricle, or atrium, and a ventricle; from their anatomical positions these systems are often designated the right heart and the left heart.
CIRCULATION
Blood from the body returns to the right atrium through two large veins, the superior and inferior venae cavae; in addition the blood that has supplied the heart muscle is drained directly into the right atrium through the coronary sinus. Return of venous blood to the right atrium takes place during the entire heart cycle of contraction and relaxation, and to the right ventricle only during the relaxation part of the cycle, called diastole, when both right heart cavities constitute a common chamber; near the end of diastole, contraction of the right atrium completes the filling of the right ventricle with blood. Rhythmic contractions of the right ventricle expel the blood through the pulmonary arteries into the capillaries of the lung, where the blood receives oxygen (seeCirculatory System; Respiration). The lung capillaries then empty into the pulmonary veins, which in turn empty into the left atrium. Pulmonary venous return to the left atrium and left ventricle proceeds simultaneously in the same manner as the venous return to the right heart cavities. Contraction of the left ventricle rhythmically propels the blood into the aorta and from there to all arteries of the body, including the coronary arteries, which supply the heart muscle.
The blood forced from the ventricles during systole, or contraction, is prevented from returning during diastole by valves at the openings of the aortic and pulmonary arteries. These valves consist of three semi-lunar (half-moon-shaped) flaps of membrane, which are curved in the direction of blood flow and which open readily on pressure in that direction; when the original pressure subsides, back pressure forces the edges of the flaps together. The tricuspid valve, situated between the right atrium and ventricle, is composed of three triangular flaps of membrane, and the bicuspid or mitral valve, between the left atrium and ventricle, has two such flaps. The bases of the flaps in both these valves are attached in a groove at the junction of the atrium and ventricle; the free edge is anchored by tendon-like cords, known as the chordae tendinae, to the muscles of the heart wall. The flaps remain open until the ventricle fills with blood. When the ventricle begins to contract, the valve is closed by pressure. The chordae tendinae prevent inversion of the flaps during this period of systolic pressure.
HEARTBEAT
The rate of heartbeat is controlled by the autonomic nervous system, being accelerated by the sympathetic system and depressed by the parasympathetic system. Nervous impulses originate rhythmically in a special nervous node, the sinus, or sinoatrial node, located in the right atrium near the superior vena cava. The impulses travel along a slender bundle of neuromuscular fibres, called the bundle of His, or the atrioventricular bundle, which, after a relay at the atrioventricular node, branches into two bundles, one for each of the ventricles. Through these specialized fibres the nervous impulse stimulates, at rapid speed and in a known orderly sequence, contraction of the heart-muscle fibres.
The heart produces two sounds in each cycle of the beat. The first is rather dull and is caused by vibration of the auriculoventricular valves and by the contraction of the ventricular muscle fibres; the second is sharp and is caused by the sudden closing of the aortic and pulmonary valves. In cardiac disease these regular sounds may be replaced or accompanied by murmurs caused by turbulent blood rushing through abnormal valves or openings; detection of such murmurs is of great importance in diagnosis.
Although the nerves and certain drugs influence the rate of the heartbeat-as do certain chemicals, similar to hormones, that are secreted by the heart itself-heart muscle is inherently contractile and will continue beating at a regular rate even when all connection with the nervous system is severed.
PRENATAL DEVELOPMENT
In the embryo the heart develops from the fusion of the two ventral aortas, forming a single pulsating organ. Separation into right and left heart takes place later with the formation of an interauricular and an interventricular septum (partition). The separation is not completed, however, until the lungs begin their function at birth. Before birth, the blood is oxygenated in the placenta and returns to the right auricle through the inferior vena cava; it is then directed by the Eustachian valve through the foramen ovale, a persistent opening in the interauricular septum. After birth the Eustachian valve degenerates to a rudiment and the foramen ovale normally closes, but the opening in the interauricular septum may persist in varying degrees in about one fifth of the adult population.INTRODUCTION
Heart, in anatomy, the hollow muscular organ that receives blood from the veins and propels it into and through the arteries. This article deals with the heart's anatomy only. For information on disorders of the heart, seeHeart Diseases.
HEART STRUCTURE
The human heart is about the size of a closed fist. It is situated behind the lower part of the breastbone, extending more to the left of the midline than to the right. It is roughly conical in shape, with the base directed upwards, to the right, and slightly backwards; the apex touches the chest wall between the fifth and sixth ribs. The heart is held in place principally by its attachment to the great arteries and veins, and by its confinement in the pericardium, a double-walled sac with one layer enveloping the heart and the other attached to the breastbone, the diaphragm, and the membranes of the thorax.
Within the adult heart are two parallel independent systems, each consisting of an auricle, or atrium, and a ventricle; from their anatomical positions these systems are often designated the right heart and the left heart.
CIRCULATION
Blood from the body returns to the right atrium through two large veins, the superior and inferior venae cavae; in addition the blood that has supplied the heart muscle is drained directly into the right atrium through the coronary sinus. Return of venous blood to the right atrium takes place during the entire heart cycle of contraction and relaxation, and to the right ventricle only during the relaxation part of the cycle, called diastole, when both right heart cavities constitute a common chamber; near the end of diastole, contraction of the right atrium completes the filling of the right ventricle with blood. Rhythmic contractions of the right ventricle expel the blood through the pulmonary arteries into the capillaries of the lung, where the blood receives oxygen (seeCirculatory System; Respiration). The lung capillaries then empty into the pulmonary veins, which in turn empty into the left atrium. Pulmonary venous return to the left atrium and left ventricle proceeds simultaneously in the same manner as the venous return to the right heart cavities. Contraction of the left ventricle rhythmically propels the blood into the aorta and from there to all arteries of the body, including the coronary arteries, which supply the heart muscle.
The blood forced from the ventricles during systole, or contraction, is prevented from returning during diastole by valves at the openings of the aortic and pulmonary arteries. These valves consist of three semi-lunar (half-moon-shaped) flaps of membrane, which are curved in the direction of blood flow and which open readily on pressure in that direction; when the original pressure subsides, back pressure forces the edges of the flaps together. The tricuspid valve, situated between the right atrium and ventricle, is composed of three triangular flaps of membrane, and the bicuspid or mitral valve, between the left atrium and ventricle, has two such flaps. The bases of the flaps in both these valves are attached in a groove at the junction of the atrium and ventricle; the free edge is anchored by tendon-like cords, known as the chordae tendinae, to the muscles of the heart wall. The flaps remain open until the ventricle fills with blood. When the ventricle begins to contract, the valve is closed by pressure. The chordae tendinae prevent inversion of the flaps during this period of systolic pressure.
HEARTBEAT
The rate of heartbeat is controlled by the autonomic nervous system, being accelerated by the sympathetic system and depressed by the parasympathetic system. Nervous impulses originate rhythmically in a special nervous node, the sinus, or sinoatrial node, located in the right atrium near the superior vena cava. The impulses travel along a slender bundle of neuromuscular fibres, called the bundle of His, or the atrioventricular bundle, which, after a relay at the atrioventricular node, branches into two bundles, one for each of the ventricles. Through these specialized fibres the nervous impulse stimulates, at rapid speed and in a known orderly sequence, contraction of the heart-muscle fibres.
The heart produces two sounds in each cycle of the beat. The first is rather dull and is caused by vibration of the auriculoventricular valves and by the contraction of the ventricular muscle fibres; the second is sharp and is caused by the sudden closing of the aortic and pulmonary valves. In cardiac disease these regular sounds may be replaced or accompanied by murmurs caused by turbulent blood rushing through abnormal valves or openings; detection of such murmurs is of great importance in diagnosis.
Although the nerves and certain drugs influence the rate of the heartbeat-as do certain chemicals, similar to hormones, that are secreted by the heart itself-heart muscle is inherently contractile and will continue beating at a regular rate even when all connection with the nervous system is severed.
PRENATAL DEVELOPMENT
In the embryo the heart develops from the fusion of the two ventral aortas, forming a single pulsating organ. Separation into right and left heart takes place later with the formation of an interauricular and an interventricular septum (partition). The separation is not completed, however, until the lungs begin their function at birth. Before birth, the blood is oxygenated in the placenta and returns to the right auricle through the inferior vena cava; it is then directed by the Eustachian valve through the foramen ovale, a persistent opening in the interauricular septum. After birth the Eustachian valve degenerates to a rudiment and the foramen ovale normally closes, but the opening in the interauricular septum may persist in varying degrees in about one fifth of the adult population.
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