Iodine, symbol I, chemically reactive element, a blue-black solid at room temperature. In group 17 (or VIIa) of the periodic table (see Periodic Law), iodine is one of the halogens. Its atomic number is 53. Iodine was first isolated from seaweed residues in 1811 by Bernard Courtois, a French manufacturer of saltpeter. The discovery was confirmed and announced by the French chemists Charles Desormes and Nicholas Clément. The nature of the element was further established in 1813 by the French chemist Joseph Louis Gay-Lussac, who also gave iodine its name.
PROPERTIES AND OCCURRENCE
The atomic weight of iodine is 126.9. Unlike the lighter halogens, iodine is a crystalline solid at room temperature. The lustrous, blue-black, soft substance sublimes when heated, giving off a violet vapor with a stinging odor like that of chlorine. The element’s name comes from its color; iodes is a Greek word for violet. The vapor rapidly condenses again on a cold surface. Iodine melts at 113.6°C (236°F) and boils at 185°C (365°F). The only isotope that occurs in nature is stable, but several radioactive ones have been produced artificially. The element, in pure form, is poisonous.
Iodine, like all halogens, is chemically active. It is only slightly soluble in water, but it dissolves readily in an aqueous solution of potassium iodide. It is also soluble in alcohol, chloroform, and other organic reagents. With seven electrons in the outer shell of its atom, iodine has several oxidation states, of which the main ones are -1, +1, +3, +5, and +7. It combines readily with most metals to form iodides, and it also combines with other halides (chemical compounds made up of a halogen atom and a metal atom). Reactions with oxygen, nitrogen, and carbon are accomplished with more difficulty.
Iodine is a relatively rare element, ranking about 62nd in abundance on Earth, but its compounds are widespread in seawater, soil, and rocks. Iodine is obtained from brines and from Chilean nitrate ores in which it occurs as an impurity. To a lesser extent, iodine is also derived from sea organisms, such as brown seaweeds, that concentrate iodine in their tissues
Uses
Iodine is medicinally very important because it is an essential trace element, present in a hormone of the thyroid gland that is involved in growth-controlling and other metabolic functions. Without iodine, stunted growth and conditions such as goiter can result. Thus in areas where iodine is not sufficiently abundant naturally, iodine-containing salt serves to make up the deficit. In medicine, iodine-alcohol solutions and iodine complexes have been used as antiseptics and disinfectants. Radioisotopes of iodine are used in medical and other fields of research. More broadly, various iodine compounds find use in photography, the making of dyes, and cloud-seeding operations. In chemistry, various iodine compounds serve as strong oxidizing agents, among other uses
Saturday, June 27, 2009
Hormone
Hormone, chemical that transfers information and instructions between cells in animals and plants. Often described as the body’s chemical messengers, hormones regulate growth and development, control the function of various tissues, support reproductive functions, and regulate metabolism (the process used to break down food to create energy). Unlike information sent by the nervous system, which is transmitted via electronic impulses that travel quickly and have an almost immediate and short-term effect, hormones act more slowly, and their effects typically are maintained over a longer period of time.
Hormones were first identified in 1902 by British physiologists William Bayliss and Ernest Starling. These researchers showed that a substance taken from the lining of the intestine could be injected into a dog to stimulate the pancreas to secrete fluid. They called the substance secretin and coined the term hormone from the Greek word hormo, which means “to set in motion.” Today more than 100 hormones have been identified.
Hormones are made by specialized glands or tissues that manufacture and secrete these chemicals as the body needs them. The majority of hormones are produced by the glands of the endocrine system, such as the pituitary, thyroid, adrenal glands, and the ovaries or testes. These endocrine glands produce and secrete hormones directly into the bloodstream. However, not all hormones are produced by endocrine glands. The mucous membranes of the small intestine secrete hormones that stimulate secretion of digestive juices from the pancreas. Other hormones are produced in the placenta, an organ formed during pregnancy, to regulate some aspects of fetal development.
Hormones are classified into two basic types based on their chemical makeup. The majority of hormones are peptides, or amino acid derivatives that include the hormones produced by the anterior pituitary, thyroid, parathyroid, placenta, and pancreas. Peptide hormones are typically produced as larger proteins. When they are called into action, these peptides are broken down into biologically active hormones and secreted into the blood to be circulated throughout the body. The second type of hormones is steroid hormones, which include those hormones secreted by the adrenal glands and ovaries or testes. Steroid hormones are synthesized from cholesterol (a fatty substance produced by the body) and modified by a series of chemical reactions to form a hormone ready for immediate action
Hormones were first identified in 1902 by British physiologists William Bayliss and Ernest Starling. These researchers showed that a substance taken from the lining of the intestine could be injected into a dog to stimulate the pancreas to secrete fluid. They called the substance secretin and coined the term hormone from the Greek word hormo, which means “to set in motion.” Today more than 100 hormones have been identified.
Hormones are made by specialized glands or tissues that manufacture and secrete these chemicals as the body needs them. The majority of hormones are produced by the glands of the endocrine system, such as the pituitary, thyroid, adrenal glands, and the ovaries or testes. These endocrine glands produce and secrete hormones directly into the bloodstream. However, not all hormones are produced by endocrine glands. The mucous membranes of the small intestine secrete hormones that stimulate secretion of digestive juices from the pancreas. Other hormones are produced in the placenta, an organ formed during pregnancy, to regulate some aspects of fetal development.
Hormones are classified into two basic types based on their chemical makeup. The majority of hormones are peptides, or amino acid derivatives that include the hormones produced by the anterior pituitary, thyroid, parathyroid, placenta, and pancreas. Peptide hormones are typically produced as larger proteins. When they are called into action, these peptides are broken down into biologically active hormones and secreted into the blood to be circulated throughout the body. The second type of hormones is steroid hormones, which include those hormones secreted by the adrenal glands and ovaries or testes. Steroid hormones are synthesized from cholesterol (a fatty substance produced by the body) and modified by a series of chemical reactions to form a hormone ready for immediate action
HOW THE ENDOCRINE SYSTEM WORKS
Hormones from the endocrine organs are secreted directly into the bloodstream, where special proteins usually bind to them, helping to keep the hormones intact as they travel throughout the body. The proteins also act as a reservoir, allowing only a small fraction of the hormone circulating in the blood to affect the target tissue. Specialized proteins in the target tissue, called receptors, bind with the hormones in the bloodstream, inducing chemical changes in response to the body’s needs. Typically, only minute concentrations of a hormone are needed to achieve the desired effect.
Too much or too little hormone can be harmful to the body, so hormone levels are regulated by a feedback mechanism. Feedback works something like a household thermostat. When the heat in a house falls, the thermostat responds by switching the furnace on, and when the temperature is too warm, the thermostat switches the furnace off. Usually, the change that a hormone produces also serves to regulate that hormone's secretion. For example, parathyroid hormone causes the body to increase the level of calcium in the blood. As calcium levels rise, the secretion of parathyroid hormone then decreases. This feedback mechanism allows for tight control over hormone levels, which is essential for ideal body function. Other mechanisms may also influence feedback relationships. For example, if an individual becomes ill, the adrenal glands increase the secretions of certain hormones that help the body deal with the stress of illness. The adrenal glands work in concert with the pituitary gland and the brain to increase the body’s tolerance of these hormones in the blood, preventing the normal feedback mechanism from decreasing secretion levels until the illness is gone.
Long-term changes in hormone levels can influence the endocrine glands themselves. For example, if hormone secretion is chronically low, the increased stimulation by the feedback mechanism leads to growth of the gland. This can occur in the thyroid if a person's diet has insufficient iodine, which is essential for thyroid hormone production. Constant stimulation from the pituitary gland to produce the needed hormone causes the thyroid to grow, eventually producing a medical condition known as goiter.
Too much or too little hormone can be harmful to the body, so hormone levels are regulated by a feedback mechanism. Feedback works something like a household thermostat. When the heat in a house falls, the thermostat responds by switching the furnace on, and when the temperature is too warm, the thermostat switches the furnace off. Usually, the change that a hormone produces also serves to regulate that hormone's secretion. For example, parathyroid hormone causes the body to increase the level of calcium in the blood. As calcium levels rise, the secretion of parathyroid hormone then decreases. This feedback mechanism allows for tight control over hormone levels, which is essential for ideal body function. Other mechanisms may also influence feedback relationships. For example, if an individual becomes ill, the adrenal glands increase the secretions of certain hormones that help the body deal with the stress of illness. The adrenal glands work in concert with the pituitary gland and the brain to increase the body’s tolerance of these hormones in the blood, preventing the normal feedback mechanism from decreasing secretion levels until the illness is gone.
Long-term changes in hormone levels can influence the endocrine glands themselves. For example, if hormone secretion is chronically low, the increased stimulation by the feedback mechanism leads to growth of the gland. This can occur in the thyroid if a person's diet has insufficient iodine, which is essential for thyroid hormone production. Constant stimulation from the pituitary gland to produce the needed hormone causes the thyroid to grow, eventually producing a medical condition known as goiter.
COMPONENTS OF THE ENDOCRINE SYSTEM
The primary glands that make up the human endocrine system are the hypothalamus, pituitary, thyroid, parathyroid, adrenal, pineal body, and reproductive glands—the ovary and testis. The pancreas, an organ often associated with the digestive system, is also considered part of the endocrine system. In addition, some nonendocrine organs are known to actively secrete hormones. These include the brain, heart, lungs, kidneys, liver, thymus, skin, and placenta. Almost all body cells can either produce or convert hormones, and some secrete hormones. For example, glucagon, a hormone that raises glucose levels in the blood when the body needs extra energy, is made in the pancreas but also in the wall of the gastrointestinal tract. However, it is the endocrine glands that are specialized for hormone production. They efficiently manufacture chemically complex hormones from simple chemical substances—for example, amino acids and carbohydrates—and they regulate their secretion more efficiently than any other tissues.
The hypothalamus, found deep within the brain, directly controls the pituitary gland. It is sometimes described as the coordinator of the endocrine system. When information reaching the brain indicates that changes are needed somewhere in the body, nerve cells in the hypothalamus secrete body chemicals that either stimulate or suppress hormone secretions from the pituitary gland. Acting as liaison between the brain and the pituitary gland, the hypothalamus is the primary link between the endocrine and nervous systems.
Located in a bony cavity just below the base of the brain is one of the endocrine system's most important members: the pituitary gland. Often described as the body’s master gland, the pituitary secretes several hormones that regulate the function of the other endocrine glands. Structurally, the pituitary gland is divided into two parts, the anterior and posterior lobes, each having separate functions. The anterior lobe regulates the activity of the thyroid and adrenal glands as well as the reproductive glands. It also regulates the body's growth and stimulates milk production in women who are breast-feeding. Hormones secreted by the anterior lobe include adrenocorticotropic hormone (ACTH), thyrotropic hormone (TSH), luteinizing hormone (LH), follicle-stimulating hormone (FSH), growth hormone (GH), and prolactin. The anterior lobe also secretes endorphins, chemicals that act on the nervous system to reduce sensitivity to pain.
The posterior lobe of the pituitary gland contains the nerve endings (axons) from the hypothalamus, which stimulate or suppress hormone production. This lobe secretes antidiuretic hormones (ADH), which control water balance in the body, and oxytocin, which controls muscle contractions in the uterus. The thyroid gland, located in the neck, secretes hormones in response to stimulation by TSH from the pituitary gland. The thyroid secretes hormones—for example, thyroxine and three-iodothyronine—that regulate growth and metabolism, and play a role in brain development during childhood.
The parathyroid glands are four small glands located at the four corners of the thyroid gland. The hormone they secrete, parathyroid hormone, regulates the level of calcium in the blood. Located on top of the kidneys, the adrenal glands have two distinct parts. The outer part, called the adrenal cortex, produces a variety of hormones called corticosteroids, which include cortisol. These hormones regulate salt and water balance in the body, prepare the body for stress, regulate metabolism, interact with the immune system, and influence sexual function. The inner part, the adrenal medulla, produces catecholamines, such as epinephrine, also called adrenaline, which increase the blood pressure and heart rate during times of stress.
The reproductive components of the endocrine system, called the gonads, secrete sex hormones in response to stimulation from the pituitary gland. Located in the pelvis, the female gonads, the ovaries, produce eggs. They also secrete a number of female sex hormones, including estrogen and progesterone, which control development of the reproductive organs, stimulate the appearance of female secondary sex characteristics, and regulate menstruation and pregnancy.
Located in the scrotum, the male gonads, the testes, produce sperm and also secrete a number of male sex hormones, or androgens. The androgens, the most important of which is testosterone, regulate development of the reproductive organs, stimulate male secondary sex characteristics, and stimulate muscle growth.
The pancreas is positioned in the upper abdomen, just under the stomach. The major part of the pancreas, called the exocrine pancreas, functions as an exocrine gland, secreting digestive enzymes into the gastrointestinal tract. Distributed through the pancreas are clusters of endocrine cells that secrete insulin, glucagon, and somastatin. These hormones all participate in regulating energy and metabolism in the body.
The pineal body, also called the pineal gland, is located in the middle of the brain. It secretes melatonin, a hormone that may help regulate the wake-sleep cycle. Research has shown that disturbances in the secretion of melatonin are responsible, in part, for the jet lag associated with long-distance air travel
The hypothalamus, found deep within the brain, directly controls the pituitary gland. It is sometimes described as the coordinator of the endocrine system. When information reaching the brain indicates that changes are needed somewhere in the body, nerve cells in the hypothalamus secrete body chemicals that either stimulate or suppress hormone secretions from the pituitary gland. Acting as liaison between the brain and the pituitary gland, the hypothalamus is the primary link between the endocrine and nervous systems.
Located in a bony cavity just below the base of the brain is one of the endocrine system's most important members: the pituitary gland. Often described as the body’s master gland, the pituitary secretes several hormones that regulate the function of the other endocrine glands. Structurally, the pituitary gland is divided into two parts, the anterior and posterior lobes, each having separate functions. The anterior lobe regulates the activity of the thyroid and adrenal glands as well as the reproductive glands. It also regulates the body's growth and stimulates milk production in women who are breast-feeding. Hormones secreted by the anterior lobe include adrenocorticotropic hormone (ACTH), thyrotropic hormone (TSH), luteinizing hormone (LH), follicle-stimulating hormone (FSH), growth hormone (GH), and prolactin. The anterior lobe also secretes endorphins, chemicals that act on the nervous system to reduce sensitivity to pain.
The posterior lobe of the pituitary gland contains the nerve endings (axons) from the hypothalamus, which stimulate or suppress hormone production. This lobe secretes antidiuretic hormones (ADH), which control water balance in the body, and oxytocin, which controls muscle contractions in the uterus. The thyroid gland, located in the neck, secretes hormones in response to stimulation by TSH from the pituitary gland. The thyroid secretes hormones—for example, thyroxine and three-iodothyronine—that regulate growth and metabolism, and play a role in brain development during childhood.
The parathyroid glands are four small glands located at the four corners of the thyroid gland. The hormone they secrete, parathyroid hormone, regulates the level of calcium in the blood. Located on top of the kidneys, the adrenal glands have two distinct parts. The outer part, called the adrenal cortex, produces a variety of hormones called corticosteroids, which include cortisol. These hormones regulate salt and water balance in the body, prepare the body for stress, regulate metabolism, interact with the immune system, and influence sexual function. The inner part, the adrenal medulla, produces catecholamines, such as epinephrine, also called adrenaline, which increase the blood pressure and heart rate during times of stress.
The reproductive components of the endocrine system, called the gonads, secrete sex hormones in response to stimulation from the pituitary gland. Located in the pelvis, the female gonads, the ovaries, produce eggs. They also secrete a number of female sex hormones, including estrogen and progesterone, which control development of the reproductive organs, stimulate the appearance of female secondary sex characteristics, and regulate menstruation and pregnancy.
Located in the scrotum, the male gonads, the testes, produce sperm and also secrete a number of male sex hormones, or androgens. The androgens, the most important of which is testosterone, regulate development of the reproductive organs, stimulate male secondary sex characteristics, and stimulate muscle growth.
The pancreas is positioned in the upper abdomen, just under the stomach. The major part of the pancreas, called the exocrine pancreas, functions as an exocrine gland, secreting digestive enzymes into the gastrointestinal tract. Distributed through the pancreas are clusters of endocrine cells that secrete insulin, glucagon, and somastatin. These hormones all participate in regulating energy and metabolism in the body.
The pineal body, also called the pineal gland, is located in the middle of the brain. It secretes melatonin, a hormone that may help regulate the wake-sleep cycle. Research has shown that disturbances in the secretion of melatonin are responsible, in part, for the jet lag associated with long-distance air travel
Endocrine System
Endocrine System, group of specialized organs and body tissues that produce, store, and secrete chemical substances known as hormones. As the body's chemical messengers, hormones transfer information and instructions from one set of cells to another. Because of the hormones they produce, endocrine organs have a great deal of influence over the body. Among their many jobs are regulating the body's growth and development, controlling the function of various tissues, supporting pregnancy and other reproductive functions, and regulating metabolism.
Endocrine organs are sometimes called ductless glands because they have no ducts connecting them to specific body parts. The hormones they secrete are released directly into the bloodstream. In contrast, the exocrine glands, such as the sweat glands or the salivary glands, release their secretions directly to target areas—for example, the skin or the inside of the mouth. Some of the body's glands are described as endo-exocrine glands because they secrete hormones as well as other types of substances. Even some nonglandular tissues produce hormone-like substances—nerve cells produce chemical messengers called neurotransmitters, for example.
The earliest reference to the endocrine system comes from ancient Greece, in about 400 bc. However, it was not until the 16th century that accurate anatomical descriptions of many of the endocrine organs were published. Research during the 20th century and in the early 21st century vastly improved our understanding of hormones and how they function in the body. Today, endocrinology, the study of the endocrine glands, is an important branch of modern medicine. Endocrinologists are medical doctors who specialize in researching and treating disorders and diseases of the endocrine system.
Endocrine organs are sometimes called ductless glands because they have no ducts connecting them to specific body parts. The hormones they secrete are released directly into the bloodstream. In contrast, the exocrine glands, such as the sweat glands or the salivary glands, release their secretions directly to target areas—for example, the skin or the inside of the mouth. Some of the body's glands are described as endo-exocrine glands because they secrete hormones as well as other types of substances. Even some nonglandular tissues produce hormone-like substances—nerve cells produce chemical messengers called neurotransmitters, for example.
The earliest reference to the endocrine system comes from ancient Greece, in about 400 bc. However, it was not until the 16th century that accurate anatomical descriptions of many of the endocrine organs were published. Research during the 20th century and in the early 21st century vastly improved our understanding of hormones and how they function in the body. Today, endocrinology, the study of the endocrine glands, is an important branch of modern medicine. Endocrinologists are medical doctors who specialize in researching and treating disorders and diseases of the endocrine system.
Thyroid Gland
Thyroid Gland, endocrine gland found in almost all vertebrate animals and so called because it is located in front of and on each side of the thyroid cartilage of the larynx (see Endocrine System). It secretes a hormone that controls metabolism and growth.
SECRETION
The thyroid gland in human beings is a brownish-red organ having two lobes connected by an isthmus; it normally weighs about 28 g (about 1 oz) and consists of cuboidal epithelial cells arranged to form small sacs known as vesicles or follicles. The vesicles are supported by connective tissue that forms a framework for the entire gland. In the normal thyroid gland, the vesicles are usually filled with a colloid substance containing the protein thyroglobulin in combination with the two thyroid hormones thyroxine, also called tetraiodothyronine (T4), and triiodothyronine (T3). These hormones are composed of the amino acid tyrosine, containing four and three iodine atoms, respectively.
The amount of thyroglobulin secreted by the thyroid is controlled by the thyroid-stimulating hormone (TSH) of the pituitary gland. Pituitary TSH, in turn, is regulated by a substance called thyroid-stimulating hormone releasing factor (TRF), which is secreted by the hypothalamus. Thyroglobulin is especially rich in iodine. Although the thyroid gland constitutes about 0.5 percent of the total human body weight, it holds about 25 percent of the total iodine in the body, which is obtained from food and water in the diet. Iodine usually circulates in the blood as an inorganic iodide and is concentrated in the thyroid to as much as 500 times the iodide level of the blood.
DISEASES OF THE THYROID
Many different laboratory tests, including direct measurement of thyroxine and triiodothyronine, are used to test the activity of the thyroid gland. Thyroid scanning with radioiodine or technetium-99m is especially useful for detecting or ruling out cancer of the thyroid in persons who have a palpable nodule, or thyroid lump. In most cases thyroid cancers are slow-growing and not fatal. The thyroid gland appears to be quite sensitive to irradiation: during the 1970s an increased incidence of thyroid cancer was found among people who had been treated early in life with X rays for such conditions as acne, ringworm, and tonsilitis.
Excessive production of thyroid hormones, called hyperthyroidism or Graves' disease, results in elevated metabolism and activity. Sometimes this condition is associated with abnormalities of the eye, including bulging eyes. The usual treatment is to administer an antithyroid drug, such as propylthiouracil, or a dose of radioactive iodine, which is concentrated in the thyroid gland and destroys some of the tissue. Apparent hyperthyroidism can result from destruction of thyroid cells with release of large amounts of hormone. In one condition, called Hashimoto's thyroiditis, this destruction results from production of an antibody against thyroid tissue (see Autoimmune Diseases).
Deficiency of thyroid hormones, or hypothyroidism, is characterized by lethargy and lowering of metabolism. This condition can result from disorders of the pituitary or of the thyroid gland itself. Formerly a major cause of hypothyroidism in the Great Lakes and inland mountain areas of the U.S. was a deficiency of iodine in the diet, which caused a condition called goiter. Iodide is now added to table salt to prevent this. The condition called cretinism, more properly known as congenital hypothyroidism, is an inherited deficiency to thyroid function that occurs in about one in every 6000 births. In most instances, but not all, these infants grow up to be mentally retarded. Since early treatment can prevent retardation, Canadian researchers developed a test to detect the condition in newborns. Programs in Québec and in the northeastern U.S. have been effective in limiting the effects of this abnormality.
SECRETION
The thyroid gland in human beings is a brownish-red organ having two lobes connected by an isthmus; it normally weighs about 28 g (about 1 oz) and consists of cuboidal epithelial cells arranged to form small sacs known as vesicles or follicles. The vesicles are supported by connective tissue that forms a framework for the entire gland. In the normal thyroid gland, the vesicles are usually filled with a colloid substance containing the protein thyroglobulin in combination with the two thyroid hormones thyroxine, also called tetraiodothyronine (T4), and triiodothyronine (T3). These hormones are composed of the amino acid tyrosine, containing four and three iodine atoms, respectively.
The amount of thyroglobulin secreted by the thyroid is controlled by the thyroid-stimulating hormone (TSH) of the pituitary gland. Pituitary TSH, in turn, is regulated by a substance called thyroid-stimulating hormone releasing factor (TRF), which is secreted by the hypothalamus. Thyroglobulin is especially rich in iodine. Although the thyroid gland constitutes about 0.5 percent of the total human body weight, it holds about 25 percent of the total iodine in the body, which is obtained from food and water in the diet. Iodine usually circulates in the blood as an inorganic iodide and is concentrated in the thyroid to as much as 500 times the iodide level of the blood.
DISEASES OF THE THYROID
Many different laboratory tests, including direct measurement of thyroxine and triiodothyronine, are used to test the activity of the thyroid gland. Thyroid scanning with radioiodine or technetium-99m is especially useful for detecting or ruling out cancer of the thyroid in persons who have a palpable nodule, or thyroid lump. In most cases thyroid cancers are slow-growing and not fatal. The thyroid gland appears to be quite sensitive to irradiation: during the 1970s an increased incidence of thyroid cancer was found among people who had been treated early in life with X rays for such conditions as acne, ringworm, and tonsilitis.
Excessive production of thyroid hormones, called hyperthyroidism or Graves' disease, results in elevated metabolism and activity. Sometimes this condition is associated with abnormalities of the eye, including bulging eyes. The usual treatment is to administer an antithyroid drug, such as propylthiouracil, or a dose of radioactive iodine, which is concentrated in the thyroid gland and destroys some of the tissue. Apparent hyperthyroidism can result from destruction of thyroid cells with release of large amounts of hormone. In one condition, called Hashimoto's thyroiditis, this destruction results from production of an antibody against thyroid tissue (see Autoimmune Diseases).
Deficiency of thyroid hormones, or hypothyroidism, is characterized by lethargy and lowering of metabolism. This condition can result from disorders of the pituitary or of the thyroid gland itself. Formerly a major cause of hypothyroidism in the Great Lakes and inland mountain areas of the U.S. was a deficiency of iodine in the diet, which caused a condition called goiter. Iodide is now added to table salt to prevent this. The condition called cretinism, more properly known as congenital hypothyroidism, is an inherited deficiency to thyroid function that occurs in about one in every 6000 births. In most instances, but not all, these infants grow up to be mentally retarded. Since early treatment can prevent retardation, Canadian researchers developed a test to detect the condition in newborns. Programs in Québec and in the northeastern U.S. have been effective in limiting the effects of this abnormality.
skin
Skin, outer body covering of an animal. The term skin is commonly used to describe the body covering of any animal but technically refers only to the body covering of vertebrates (animals that have a backbone). The skin has the same basic structure in all vertebrates, including fish, reptiles, birds, and humans and other mammals. This article focuses primarily on human skin.
The skin is essential to a person’s survival. It forms a barrier that helps prevent harmful microorganisms and chemicals from entering the body, and it also prevents the loss of life-sustaining body fluids. It protects the vital structures inside the body from injury and from the potentially damaging ultraviolet rays of the sun. The skin also helps regulate body temperature, excretes some waste products, and is an important sensory organ. It contains various types of specialized nerve cells responsible for the sense of touch.
The skin is the body’s largest organ—that of an average adult male weighs 4.5 to 5 kg (10 to 11 lb) and measures about 2 sq m (22 sq ft) in area. It covers the surface of the body at a thickness of just 1.4 to 4.0 mm (0.06 to 0.16 in). The skin is thickest on areas of the body that regularly rub against objects, such as the palms of the hands and the soles of the feet. Both delicate and resilient, the skin constantly renews itself and has a remarkable ability to repair itself after injury
The skin is essential to a person’s survival. It forms a barrier that helps prevent harmful microorganisms and chemicals from entering the body, and it also prevents the loss of life-sustaining body fluids. It protects the vital structures inside the body from injury and from the potentially damaging ultraviolet rays of the sun. The skin also helps regulate body temperature, excretes some waste products, and is an important sensory organ. It contains various types of specialized nerve cells responsible for the sense of touch.
The skin is the body’s largest organ—that of an average adult male weighs 4.5 to 5 kg (10 to 11 lb) and measures about 2 sq m (22 sq ft) in area. It covers the surface of the body at a thickness of just 1.4 to 4.0 mm (0.06 to 0.16 in). The skin is thickest on areas of the body that regularly rub against objects, such as the palms of the hands and the soles of the feet. Both delicate and resilient, the skin constantly renews itself and has a remarkable ability to repair itself after injury
Thursday, June 25, 2009
GOITER
Goiter
Goiter, disease of the thyroid gland, characterized by an enlargement of the gland, visible externally as a swelling on the front of the neck. In simple goiter the basal metabolic rate is somewhat lowered, and in toxic goiter it is elevated.
Simple Goiter
Simple goiter is characterized by an enlargement of the entire thyroid gland or one of its two lobes. It is associated with hypothyroidism, a condition caused by insufficient production of thyroid hormone. Because the body needs iodine to produce thyroid hormone, inadequate amounts of iodine in the diet may result in simple goiters. Simple goiters may be classified as either endemic or nontoxic.
Endemic goiters are caused by a deficiency of iodine in the diet and usually occur in populations living in areas with iodine-depleted soil. It once was common in what was referred to as the goiter belt of the United States, largely the Midwestern states. Public health measures, including the addition of iodine to water supplies and to table salt, have helped the prevention of endemic goiters in the United States. Ingestion of iodine supplements during pregnancy prevents development of the disease in the infant as well as in the mother.
The cause of most nontoxic goiters is unknown, but researchers suspect that environmental factors and heredity play a role. In some areas of the world, certain chemical compounds in food or water may block the body’s production of thyroid hormones and lead to nontoxic goiter formation. These compounds, known as goitrogens, also include certain drugs, such as aminoglutethimide and lithium.
Most simple goiters do not produce symptoms. The administration of thyroid hormone will lead to a reduction in size of simple goiters in most cases. Surgical removal of a simple goiter is usually unnecessary unless the goiter is very large and is impingeing on vital body organs.
Toxic goiter
This disease, also called exophthalmic goiter, hyperthyroidism, thyrotoxicosis, or Graves' disease, for the Irish physician Robert James Graves, is caused by an excess of thyroxine secretion. The cause of the excessive secretion is obscure. In some cases it may result from excessive stimulation by the pituitary gland. The symptoms of toxic goiter may include a rapid heartbeat, tremor, increased sweating, increased appetite, weight loss, weakness, and fatigue. Some patients have eye problems, such as staring or protrusion. Toxic goiter is commonly treated with radioactive iodine, which is taken up by the gland and destroys the cells by irradiation. Drugs also can be used to suppress hormone production, or most of the toxic goiter can be removed surgically.
Goiter, disease of the thyroid gland, characterized by an enlargement of the gland, visible externally as a swelling on the front of the neck. In simple goiter the basal metabolic rate is somewhat lowered, and in toxic goiter it is elevated.
Simple Goiter
Simple goiter is characterized by an enlargement of the entire thyroid gland or one of its two lobes. It is associated with hypothyroidism, a condition caused by insufficient production of thyroid hormone. Because the body needs iodine to produce thyroid hormone, inadequate amounts of iodine in the diet may result in simple goiters. Simple goiters may be classified as either endemic or nontoxic.
Endemic goiters are caused by a deficiency of iodine in the diet and usually occur in populations living in areas with iodine-depleted soil. It once was common in what was referred to as the goiter belt of the United States, largely the Midwestern states. Public health measures, including the addition of iodine to water supplies and to table salt, have helped the prevention of endemic goiters in the United States. Ingestion of iodine supplements during pregnancy prevents development of the disease in the infant as well as in the mother.
The cause of most nontoxic goiters is unknown, but researchers suspect that environmental factors and heredity play a role. In some areas of the world, certain chemical compounds in food or water may block the body’s production of thyroid hormones and lead to nontoxic goiter formation. These compounds, known as goitrogens, also include certain drugs, such as aminoglutethimide and lithium.
Most simple goiters do not produce symptoms. The administration of thyroid hormone will lead to a reduction in size of simple goiters in most cases. Surgical removal of a simple goiter is usually unnecessary unless the goiter is very large and is impingeing on vital body organs.
Toxic goiter
This disease, also called exophthalmic goiter, hyperthyroidism, thyrotoxicosis, or Graves' disease, for the Irish physician Robert James Graves, is caused by an excess of thyroxine secretion. The cause of the excessive secretion is obscure. In some cases it may result from excessive stimulation by the pituitary gland. The symptoms of toxic goiter may include a rapid heartbeat, tremor, increased sweating, increased appetite, weight loss, weakness, and fatigue. Some patients have eye problems, such as staring or protrusion. Toxic goiter is commonly treated with radioactive iodine, which is taken up by the gland and destroys the cells by irradiation. Drugs also can be used to suppress hormone production, or most of the toxic goiter can be removed surgically.
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