Heart, in anatomy, hollow muscular organ that pumps blood through the body. The heart, blood, and blood vessels make up the circulatory system, which is responsible for distributing oxygen and nutrients to the body and carrying away carbon dioxide and other waste products. The heart is the circulatory system’s power supply. It must beat ceaselessly because the body’s tissues—especially the brain and the heart itself—depend on a constant supply of oxygen and nutrients delivered by the flowing blood. If the heart stops pumping blood for more than a few minutes, death will result.

The human heart is shaped like an upside-down pear and is located slightly to the left of center inside the chest cavity. About the size of a closed fist, the heart is made primarily of muscle tissue that contracts rhythmically to propel blood to all parts of the body. This rhythmic contraction begins in the developing embryo about three weeks after conception and continues throughout an individual’s life. The muscle rests only for a fraction of a second between beats. Over a typical life span of 76 years, the heart will beat nearly 2.8 billion times and move 169 million liters (179 million quarts) of blood.

Since prehistoric times people have had a sense of the heart’s vital importance. Cave paintings from 20,000 years ago depict a stylized heart inside the outline of hunted animals such as bison and elephant. The ancient Greeks believed the heart was the seat of intelligence. Others believed the heart to be the source of the soul or of the emotions—an idea that persists in popular culture and various verbal expressions, such as heartbreak, to the present day.

STRUCTURE OF THE HEART

The human heart has four chambers. The upper two chambers, the right and left atria, are receiving chambers for blood. The atria are sometimes known as auricles. They collect blood that pours in from veins, blood vessels that return blood to the heart. The heart’s lower two chambers, the right and left ventricles, are the powerful pumping chambers. The ventricles propel blood into arteries, blood vessels that carry blood away from the heart.

A wall of tissue separates the right and left sides of the heart. Each side pumps blood through a different circuit of blood vessels: The right side of the heart pumps oxygen-poor blood to the lungs, while the left side of the heart pumps oxygen-rich blood to the body. Blood returning from a trip around the body has given up most of its oxygen and picked up carbon dioxide in the body’s tissues. This oxygen-poor blood feeds into two large veins, the superior vena cava and inferior vena cava, which empty into the right atrium of the heart.The right atrium conducts blood to the right ventricle, and the right ventricle pumps blood into the pulmonary artery. The pulmonary artery carries the blood to the lungs, where it picks up a fresh supply of oxygen and eliminates carbon dioxide. The blood, now oxygen-rich, returns to the heart through the pulmonary veins, which empty into the left atrium. Blood passes from the left atrium into the left ventricle, from where it is pumped out of the heart into the aorta, the body’s largest artery. Smaller arteries that branch off the aorta distribute blood to various parts of the body.
Heart Valves
Four valves within the heart prevent blood from flowing backward in the heart. The valves open easily in the direction of blood flow, but when blood pushes against the valves in the opposite direction, the valves close. Two valves, known as atrioventricular valves, are located between the atria and ventricles. The right atrioventricular valve is formed from three flaps of tissue and is called the tricuspid valve. The left atrioventricular valve has two flaps and is called the bicuspid or mitral valve. The other two heart valves are located between the ventricles and arteries. They are called semilunar valves because they each consist of three half-moon-shaped flaps of tissue. The right semilunar valve, between the right ventricle and pulmonary artery, is also called the pulmonary valve. The left semilunar valve, between the left ventricle and aorta, is also called the aortic valve.

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Heart, in anatomy, hollow muscular organ that pumps blood through the body. The heart, blood, and blood vessels make up the circulatory system, which is responsible for distributing oxygen and nutrients to the body and carrying away carbon dioxide and other waste products. The heart is the circulatory system’s power supply. It must beat ceaselessly because the body’s tissues—especially the brain and the heart itself—depend on a constant supply of oxygen and nutrients delivered by the flowing blood. If the heart stops pumping blood for more than a few minutes, death will result.

The human heart is shaped like an upside-down pear and is located slightly to the left of center inside the chest cavity. About the size of a closed fist, the heart is made primarily of muscle tissue that contracts rhythmically to propel blood to all parts of the body. This rhythmic contraction begins in the developing embryo about three weeks after conception and continues throughout an individual’s life. The muscle rests only for a fraction of a second between beats. Over a typical life span of 76 years, the heart will beat nearly 2.8 billion times and move 169 million liters (179 million quarts) of blood.

Since prehistoric times people have had a sense of the heart’s vital importance. Cave paintings from 20,000 years ago depict a stylized heart inside the outline of hunted animals such as bison and elephant. The ancient Greeks believed the heart was the seat of intelligence. Others believed the heart to be the source of the soul or of the emotions—an idea that persists in popular culture and various verbal expressions, such as heartbreak, to the present day.

STRUCTURE OF THE HEART

The human heart has four chambers. The upper two chambers, the right and left atria, are receiving chambers for blood. The atria are sometimes known as auricles. They collect blood that pours in from veins, blood vessels that return blood to the heart. The heart’s lower two chambers, the right and left ventricles, are the powerful pumping chambers. The ventricles propel blood into arteries, blood vessels that carry blood away from the heart.

A wall of tissue separates the right and left sides of the heart. Each side pumps blood through a different circuit of blood vessels: The right side of the heart pumps oxygen-poor blood to the lungs, while the left side of the heart pumps oxygen-rich blood to the body. Blood returning from a trip around the body has given up most of its oxygen and picked up carbon dioxide in the body’s tissues. This oxygen-poor blood feeds into two large veins, the superior vena cava and inferior vena cava, which empty into the right atrium of the heart.The right atrium conducts blood to the right ventricle, and the right ventricle pumps blood into the pulmonary artery. The pulmonary artery carries the blood to the lungs, where it picks up a fresh supply of oxygen and eliminates carbon dioxide. The blood, now oxygen-rich, returns to the heart through the pulmonary veins, which empty into the left atrium. Blood passes from the left atrium into the left ventricle, from where it is pumped out of the heart into the aorta, the body’s largest artery. Smaller arteries that branch off the aorta distribute blood to various parts of the body.
Heart Valves
Four valves within the heart prevent blood from flowing backward in the heart. The valves open easily in the direction of blood flow, but when blood pushes against the valves in the opposite direction, the valves close. Two valves, known as atrioventricular valves, are located between the atria and ventricles. The right atrioventricular valve is formed from three flaps of tissue and is called the tricuspid valve. The left atrioventricular valve has two flaps and is called the bicuspid or mitral valve. The other two heart valves are located between the ventricles and arteries. They are called semilunar valves because they each consist of three half-moon-shaped flaps of tissue. The right semilunar valve, between the right ventricle and pulmonary artery, is also called the pulmonary valve. The left semilunar valve, between the left ventricle and aorta, is also called the aortic valve.

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nervous system

The nervous system has two divisions: the central nervous system and the peripheral nervous system. The central nervous system includes the brain and spinal cord. It processes incoming sensory information and sends outgoing motor commands. The peripheral nervous system includes all neural tissue outside the central nervous system. It is divided into motor and sensory systems. Impulses go to the central nervous system through sensory nerves and are carried away from it by the motor nerves. The motor system is further divided into the somatic (or skeletal) nervous system and the autonomic nervous system. The somatic, or skeletal, motor system allows voluntary control over skeletal muscle with a few exceptions. The autonomic nervous system is largely involuntary and controls cardiac and smooth muscles and glands.The autonomic nervous system has three divisions: the enteric, the sympathetic, and the parasympathetic. The enteric nervous system is a system of nerves in the gastrointestinal tract, pancreas, and gallbladder that influences all digestive processes. The enteric system operates without input from the brain or spinal cord.
The sympathetic and parasympathetic divisions may operate together or in opposition. Many, but not all, of the muscles and glands that distribute nerve impulses to the larger interior organs have both sympathetic and parasympathetic nerve systems. 
In such cases the two divisions may exert opposing effects. Thus, the sympathetic system increases heartbeat, and the parasympathetic system decreases heartbeat. The two nervous systems are not always in opposition, however. For example, both nerve supplies to the salivary glands excite the cells of secretion. Furthermore, a single division of the autonomic nervous system may both stimulate and inhibit, as in the sympathetic supply to the blood vessels of skeletal muscle. Finally, the sweat glands, the muscles that cause involuntary erection or bristling of the hair, the smooth muscle of the spleen, and the blood vessels of the skin and skeletal muscle are actuated only by the sympathetic division.

Voluntary movement of head, limbs, and body is caused by nerve impulses arising in the motor area of the cortex of the brain and carried by cranial nerves or by nerves that emerge from the spinal cord to connect with skeletal muscles. The reaction involves both excitation of nerve cells stimulating the muscles involved and inhibition of the cells that stimulate opposing muscles. A nerve impulse is an electrical change within a nerve cell or fiber; measured in millivolts, it lasts a few milliseconds and can be recorded by electrodes.
Movement may occur also in direct response to an outside stimulus; thus, a tap on the knee causes a jerk, and a light shone into the eye makes the pupil contract. These involuntary responses are called reflexes. Various nerve terminals called receptors constantly send impulses into the central nervous system. These are of three classes: exteroceptors, which are sensitive to pain, temperature, touch, and pressure; interoceptors, which react to changes in the internal environment; and proprioceptors, which respond to variations in movement, position, and tension. These impulses terminate in special areas of the brain, as do those of special receptors concerned with sight, hearing, smell, and taste.
Muscular contractions do not always cause actual movement. A small fraction of the total number of fibers in most muscles are usually contracting. This serves to maintain the posture of a limb and enables the limb to resist passive elongation or stretch. This slight continuous contraction is called muscle tone.

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HUMAN EMBRYOLOGY

The human ovum, fertilized high in a fallopian tube, is brushed by the hairlike cilia in the tube toward the uterus, where it becomes implanted, that is, attached to and enclosed by decidual tissue of the uterine lining. Studies of primate embryos indicate that, in humans as well as in apes, cell multiplication begins during the journey of the ovum through the tube. The implanted embryo consists of a hollow sphere, the blastocyst, containing a mass of cells, called the embryonic mass, attached by a stalk to one side of the encircling membrane. In a blastocyst less than two weeks old and measuring 1 mm (0.04 in) in diameter, the microscope reveals the amnion (a sac surrounding the embryo), chorion (a membrane that develops around the amnion and lines the uterine wall), yolk sac, and distinct germ layers.

In the third week a closed tube appears in which the brain and spinal cord are to develop. Another tube, folding on itself, is developing into the heart, and at about this stage a portion of the minute yolk sac is enclosed in the body of the embryo to form a part of the embryonic alimentary canal. At the beginning of its fourth week the embryo, now about 4 to 5 mm (about 0.16 to 0.2 in) long, has the rudiments of eyes and ears, and each side of the neck shows four gill clefts. A tail is also present.

Early in the second month the buds of the arms and legs appear. The major internal organs begin to take shape, and in about the sixth week bones and muscles begin to form. By the third month the embryo is recognizable as that of a primate, and is now called a fetus. It has a definite face, with the mouth and nostrils distinct, and the external ears are forming. By the end of the eighth week the tail has usually been incorporated in the body, and in the 11th or 12th week the external genitals become evident. The human embryo is especially vulnerable to the damaging effects of X rays, of disease viruses such as measles, and of certain drugs during the fourth to the eighth week of gestation. These agents can result in the death of the embryo or in the birth of a child with deformed limbs or other abnormalities. By the fourth month an embryo has developed obvious human features. For development in the fetal stage, see Fetus. For abnormalities due to anomalous development, see Birth Defects. See alsoDevelopment; Multiple Birth; Obstetrics.

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HUMAN EMBRYOLOGY

The human ovum, fertilized high in a fallopian tube, is brushed by the hairlike cilia in the tube toward the uterus, where it becomes implanted, that is, attached to and enclosed by decidual tissue of the uterine lining. Studies of primate embryos indicate that, in humans as well as in apes, cell multiplication begins during the journey of the ovum through the tube. The implanted embryo consists of a hollow sphere, the blastocyst, containing a mass of cells, called the embryonic mass, attached by a stalk to one side of the encircling membrane. In a blastocyst less than two weeks old and measuring 1 mm (0.04 in) in diameter, the microscope reveals the amnion (a sac surrounding the embryo), chorion (a membrane that develops around the amnion and lines the uterine wall), yolk sac, and distinct germ layers.

In the third week a closed tube appears in which the brain and spinal cord are to develop. Another tube, folding on itself, is developing into the heart, and at about this stage a portion of the minute yolk sac is enclosed in the body of the embryo to form a part of the embryonic alimentary canal. At the beginning of its fourth week the embryo, now about 4 to 5 mm (about 0.16 to 0.2 in) long, has the rudiments of eyes and ears, and each side of the neck shows four gill clefts. A tail is also present.

Early in the second month the buds of the arms and legs appear. The major internal organs begin to take shape, and in about the sixth week bones and muscles begin to form. By the third month the embryo is recognizable as that of a primate, and is now called a fetus. It has a definite face, with the mouth and nostrils distinct, and the external ears are forming. By the end of the eighth week the tail has usually been incorporated in the body, and in the 11th or 12th week the external genitals become evident. The human embryo is especially vulnerable to the damaging effects of X rays, of disease viruses such as measles, and of certain drugs during the fourth to the eighth week of gestation. These agents can result in the death of the embryo or in the birth of a child with deformed limbs or other abnormalities. By the fourth month an embryo has developed obvious human features. For development in the fetal stage, see Fetus. For abnormalities due to anomalous development, see Birth Defects. See alsoDevelopment; Multiple Birth; Obstetrics.

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HUMAN EMBRYOLOGY

The human ovum, fertilized high in a fallopian tube, is brushed by the hairlike cilia in the tube toward the uterus, where it becomes implanted, that is, attached to and enclosed by decidual tissue of the uterine lining. Studies of primate embryos indicate that, in humans as well as in apes, cell multiplication begins during the journey of the ovum through the tube. The implanted embryo consists of a hollow sphere, the blastocyst, containing a mass of cells, called the embryonic mass, attached by a stalk to one side of the encircling membrane. In a blastocyst less than two weeks old and measuring 1 mm (0.04 in) in diameter, the microscope reveals the amnion (a sac surrounding the embryo), chorion (a membrane that develops around the amnion and lines the uterine wall), yolk sac, and distinct germ layers.

	Developing Embryo's First Month

	Developing Embryo's First Month Thirty hours after conception, the fertilized egg undergoes its first cell division. The embryo, as it is now called, continues to divide as it travels down the fallopian tube. It implants in the uterine lining approximately six days after fertilization, a ball of cells with a disk-shaped embryonic mass. In the second week, the placenta begins to form, nourishing an embryo now composed of the three primary types of tissue: endoderm, ectoderm, and mesoderm. The third week sees the formation of the neural tube, precursor to the central nervous system. Blocks of muscle tissue called somites, from which major organs and glands will arise, form along the embryo’s dorsal surface. Blood vessels and the beginnings of the digestive cavity appear by the end of the week. At the close of the first month, all major organs have begun their development. The eyes are visible, the arms and legs begin to bud, and the four-chambered heart beats for the first time. Encarta Encyclopedia © Microsoft Corporation. All Rights Reserved.
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In the third week a closed tube appears in which the brain and spinal cord are to develop. Another tube, folding on itself, is developing into the heart, and at about this stage a portion of the minute yolk sac is enclosed in the body of the embryo to form a part of the embryonic alimentary canal. At the beginning of its fourth week the embryo, now about 4 to 5 mm (about 0.16 to 0.2 in) long, has the rudiments of eyes and ears, and each side of the neck shows four gill clefts. A tail is also present.

Early in the second month the buds of the arms and legs appear. The major internal organs begin to take shape, and in about the sixth week bones and muscles begin to form. By the third month the embryo is recognizable as that of a primate, and is now called a fetus. It has a definite face, with the mouth and nostrils distinct, and the external ears are forming. By the end of the eighth week the tail has usually been incorporated in the body, and in the 11th or 12th week the external genitals become evident. The human embryo is especially vulnerable to the damaging effects of X rays, of disease viruses such as measles, and of certain drugs during the fourth to the eighth week of gestation. These agents can result in the death of the embryo or in the birth of a child with deformed limbs or other abnormalities. By the fourth month an embryo has developed obvious human features. For development in the fetal stage, see Fetus. For abnormalities due to anomalous development, see Birth Defects. See also Development; Multiple Birth; Obstetrics.

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HUMAN EMBRYOLOGY

The human ovum, fertilized high in a fallopian tube, is brushed by the hairlike cilia in the tube toward the uterus, where it becomes implanted, that is, attached to and enclosed by decidual tissue of the uterine lining. Studies of primate embryos indicate that, in humans as well as in apes, cell multiplication begins during the journey of the ovum through the tube. The implanted embryo consists of a hollow sphere, the blastocyst, containing a mass of cells, called the embryonic mass, attached by a stalk to one side of the encircling membrane. In a blastocyst less than two weeks old and measuring 1 mm (0.04 in) in diameter, the microscope reveals the amnion (a sac surrounding the embryo), chorion (a membrane that develops around the amnion and lines the uterine wall), yolk sac, and distinct germ layers.

	Developing Embryo's First Month Thirty hours after conception, the fertilized egg undergoes its first cell division. The embryo, as it is now called, continues to divide as it travels down the fallopian tube. It implants in the uterine lining approximately six days after fertilization, a ball of cells with a disk-shaped embryonic mass. In the second week, the placenta begins to form, nourishing an embryo now composed of the three primary types of tissue: endoderm, ectoderm, and mesoderm. The third week sees the formation of the neural tube, precursor to the central nervous system. Blocks of muscle tissue called somites, from which major organs and glands will arise, form along the embryo’s dorsal surface. Blood vessels and the beginnings of the digestive cavity appear by the end of the week. At the close of the first month, all major organs have begun their development. The eyes are visible, the arms and legs begin to bud, and the four-chambered heart beats for the first time. Encarta Encyclopedia © Microsoft Corporation. All Rights Reserved.
	Full Size

In the third week a closed tube appears in which the brain and spinal cord are to develop. Another tube, folding on itself, is developing into the heart, and at about this stage a portion of the minute yolk sac is enclosed in the body of the embryo to form a part of the embryonic alimentary canal. At the beginning of its fourth week the embryo, now about 4 to 5 mm (about 0.16 to 0.2 in) long, has the rudiments of eyes and ears, and each side of the neck shows four gill clefts. A tail is also present.

Early in the second month the buds of the arms and legs appear. The major internal organs begin to take shape, and in about the sixth week bones and muscles begin to form. By the third month the embryo is recognizable as that of a primate, and is now called a fetus. It has a definite face, with the mouth and nostrils distinct, and the external ears are forming. By the end of the eighth week the tail has usually been incorporated in the body, and in the 11th or 12th week the external genitals become evident. The human embryo is especially vulnerable to the damaging effects of X rays, of disease viruses such as measles, and of certain drugs during the fourth to the eighth week of gestation. These agents can result in the death of the embryo or in the birth of a child with deformed limbs or other abnormalities. By the fourth month an embryo has developed obvious human features. For development in the fetal stage, see Fetus. For abnormalities due to anomalous development, see Birth Defects. See also Development; Multiple Birth; Obstetrics.

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HUMAN EMBRYOLOGY

The human ovum, fertilized high in a fallopian tube, is brushed by the hairlike cilia in the tube toward the uterus, where it becomes implanted, that is, attached to and enclosed by decidual tissue of the uterine lining. Studies of primate embryos indicate that, in humans as well as in apes, cell multiplication begins during the journey of the ovum through the tube. The implanted embryo consists of a hollow sphere, the blastocyst, containing a mass of cells, called the embryonic mass, attached by a stalk to one side of the encircling membrane. In a blastocyst less than two weeks old and measuring 1 mm (0.04 in) in diameter, the microscope reveals the amnion (a sac surrounding the embryo), chorion (a membrane that develops around the amnion and lines the uterine wall), yolk sac, and distinct germ layers.

	Developing Embryo's First Month Thirty hours after conception, the fertilized egg undergoes its first cell division. The embryo, as it is now called, continues to divide as it travels down the fallopian tube. It implants in the uterine lining approximately six days after fertilization, a ball of cells with a disk-shaped embryonic mass. In the second week, the placenta begins to form, nourishing an embryo now composed of the three primary types of tissue: endoderm, ectoderm, and mesoderm. The third week sees the formation of the neural tube, precursor to the central nervous system. Blocks of muscle tissue called somites, from which major organs and glands will arise, form along the embryo’s dorsal surface. Blood vessels and the beginnings of the digestive cavity appear by the end of the week. At the close of the first month, all major organs have begun their development. The eyes are visible, the arms and legs begin to bud, and the four-chambered heart beats for the first time. Encarta Encyclopedia © Microsoft Corporation. All Rights Reserved.
	Full Size

In the third week a closed tube appears in which the brain and spinal cord are to develop. Another tube, folding on itself, is developing into the heart, and at about this stage a portion of the minute yolk sac is enclosed in the body of the embryo to form a part of the embryonic alimentary canal. At the beginning of its fourth week the embryo, now about 4 to 5 mm (about 0.16 to 0.2 in) long, has the rudiments of eyes and ears, and each side of the neck shows four gill clefts. A tail is also present.

Early in the second month the buds of the arms and legs appear. The major internal organs begin to take shape, and in about the sixth week bones and muscles begin to form. By the third month the embryo is recognizable as that of a primate, and is now called a fetus. It has a definite face, with the mouth and nostrils distinct, and the external ears are forming. By the end of the eighth week the tail has usually been incorporated in the body, and in the 11th or 12th week the external genitals become evident. The human embryo is especially vulnerable to the damaging effects of X rays, of disease viruses such as measles, and of certain drugs during the fourth to the eighth week of gestation. These agents can result in the death of the embryo or in the birth of a child with deformed limbs or other abnormalities. By the fourth month an embryo has developed obvious human features. For development in the fetal stage, see Fetus. For abnormalities due to anomalous development, see Birth Defects. See also Development; Multiple Birth; Obstetrics.

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HUMAN EMBRYOLOGY

The human ovum, fertilized high in a fallopian tube, is brushed by the hairlike cilia in the tube toward the uterus, where it becomes implanted, that is, attached to and enclosed by decidual tissue of the uterine lining. Studies of primate embryos indicate that, in humans as well as in apes, cell multiplication begins during the journey of the ovum through the tube. The implanted embryo consists of a hollow sphere, the blastocyst, containing a mass of cells, called the embryonic mass, attached by a stalk to one side of the encircling membrane. In a blastocyst less than two weeks old and measuring 1 mm (0.04 in) in diameter, the microscope reveals the amnion (a sac surrounding the embryo), chorion (a membrane that develops around the amnion and lines the uterine wall), yolk sac, and distinct germ layers.

	Developing Embryo's First Month

	Developing Embryo's First Month Thirty hours after conception, the fertilized egg undergoes its first cell division. The embryo, as it is now called, continues to divide as it travels down the fallopian tube. It implants in the uterine lining approximately six days after fertilization, a ball of cells with a disk-shaped embryonic mass. In the second week, the placenta begins to form, nourishing an embryo now composed of the three primary types of tissue: endoderm, ectoderm, and mesoderm. The third week sees the formation of the neural tube, precursor to the central nervous system. Blocks of muscle tissue called somites, from which major organs and glands will arise, form along the embryo’s dorsal surface. Blood vessels and the beginnings of the digestive cavity appear by the end of the week. At the close of the first month, all major organs have begun their development. The eyes are visible, the arms and legs begin to bud, and the four-chambered heart beats for the first time. Encarta Encyclopedia © Microsoft Corporation. All Rights Reserved.
	Full Size

In the third week a closed tube appears in which the brain and spinal cord are to develop. Another tube, folding on itself, is developing into the heart, and at about this stage a portion of the minute yolk sac is enclosed in the body of the embryo to form a part of the embryonic alimentary canal. At the beginning of its fourth week the embryo, now about 4 to 5 mm (about 0.16 to 0.2 in) long, has the rudiments of eyes and ears, and each side of the neck shows four gill clefts. A tail is also present.

Early in the second month the buds of the arms and legs appear. The major internal organs begin to take shape, and in about the sixth week bones and muscles begin to form. By the third month the embryo is recognizable as that of a primate, and is now called a fetus. It has a definite face, with the mouth and nostrils distinct, and the external ears are forming. By the end of the eighth week the tail has usually been incorporated in the body, and in the 11th or 12th week the external genitals become evident. The human embryo is especially vulnerable to the damaging effects of X rays, of disease viruses such as measles, and of certain drugs during the fourth to the eighth week of gestation. These agents can result in the death of the embryo or in the birth of a child with deformed limbs or other abnormalities. By the fourth month an embryo has developed obvious human features. For development in the fetal stage, see Fetus. For abnormalities due to anomalous development, see Birth Defects. See also Development; Multiple Birth; Obstetrics.

Read More