THE ENDOCRINE SYSTEM:
CHEMICAL CONTROL CENTRE
1. What are the characteristics of hormones?
§ A hormone is a chemical secreted by cells in one part of the body that is transported in the bloodstream to another part of the body.
§ Hormones are synthesized either from amino acids (i.e. peptides) or from lipids (steroids and prostagladins).
§ Hormone action is commonly regulated through negative feedback, a process in which a hormone causes changes that inhibit further secretion of that hormone. Such contrary hormonal substances are referred to as antagonistic hormones. (Recall the opposite effects of insulin and glucagon on the levels of blood glucose.)
§ The endocrine system produces two main types of hormone product. These hormone types can be differentiated by their chemical composition and their mode of action in target cells and tissues:
1. Non-steroid Hormones (refer to diagram):
§ Are composed of proteins, peptides, or amino acids. These hormone molecules are not fat-soluble, so they usually do not enter cells to exert their effect
§ Bind to the receptors on the surfaces of target cells and activate intracellular second messengers, such as cyclic AMP (cAMP).
§ The second messengers then alter the metabolism of the cell.
§ e.g. Regulation of blood glucose by the hormones glucagons and insulin. Other examples include adrenaline.
2. Steroid Hormones (refer to diagram):
§ These hormones are manufactured from cholesterol. Each type of steroid hormone is composed of a central structure of four carbon rings attached to distinctive side chains that determine the hormone’s specific and unique properties.
§ Bind to the surface receptors or diffuse through the plasma membranes of their target cells and bind with receptors in the cytoplasm.
§ The hormone-receptor complex travels to the nucleus and promotes transcription of specific genes.
§ e.g. Sex hormones also penetrate the plasma membrane but diffuse into the nucleus, where they bind to receptors associated with the chromosomes and influence gene transcription.
2. What are the structures and hormones of the Endocrine System?
§ The endocrine system is a body control system composed of a group of glands that maintain a stable internal environment by producing chemical regulatory substances called hormones. The endocrine system includes the pituitary gland, thyroid gland, parathyroid glands, adrenal gland, pancreas, ovaries, and testes (see testis). The thymus gland, pineal gland, and kidney are also sometimes considered endocrine organs. (Refer to diagram)
§ The endocrine glands appear unique in that the hormones they produce do not pass through tubes or ducts. The hormones are secreted directly into the internal environment, where they are transmitted via the bloodstream or by diffusion and act at distant points in the body. In contrast, other exocrine glands including sweat glands, salivary glands, and glands of the gastrointestinal system secrete the substances they produce through ducts, and those substances are used in the vicinity of the gland.
§ The regulation of body functions by the endocrine system depends on the existence of specific receptor cells in target organs that respond in specialized ways to the minute quantities of the hormonal messengers.
§ Some endocrine hormones, such as thyroxine from the thyroid gland, affect nearly all body cells; others, such as progesterone from the female ovary, which regulates the uterine lining, affect only a single organ. (Refer to table).
§ The amounts of hormones are maintained by feedback mechanisms that depend on interactions between the endocrine glands, the blood levels of the various hormones, and activities of the target organ. Hormones act by regulating cell metabolism. By accelerating, slowing, or maintaining enzyme activity in receptor cells, hormones control growth and development, metabolic rate, sexual rhythms, and reproduction.
Your endocrine system is a collection of glands that produce hormones that regulate your body's growth, metabolism, and sexual development and function. The hormones are released into the bloodstream and transported to tissues and organs throughout your body. The Table below the illustration describes the function of these glands.
Difference between Exocrine Gland and Endocrine Gland:
FUNCTION OF THE ENDOCRINE GLANDS
Adrenal glands / Divided into 2 regions; secrete hormones that influence the body's metabolism, blood chemicals, and body characteristics, as well as influence the part of the nervous system that is involved in the response and defense against stress. The hormones are amino acid derivatives known as adrenaline or noradrenaline.Hypothalamus / Activates and controls the part of the nervous system that controls involuntary body functions, the hormonal system, and many body functions, such as regulating sleep and stimulating appetite..
Ovaries and testicles / Secrete hormones that influence female and male characteristics, respectively. The ovaries release a steroid hormone called estrogen and progesterone. And, in the testes release a steroid hormone called testosterone.
Pancreas / Secretes peptide hormones (insulin and glucagon) that controls the use of glucose by the body.
Parathyroid glands / Secrete a hormone that maintains the calcium level in the blood by releasing a peptide hormone called parathormone.
Pineal body / Involved with daily biological cycles controls sleep-wake patterns. It will release an amino acid derivative hormone called melatonin
Pituitary gland
(Anterior and Posterior) / Produces a number of different hormones that influence various other endocrine glands. The posterior pituitary will release anti-diuretic hormone and oxytocin. The anterior pituitary will release thyroid stimulating hormone (TSH) to stimulate the thyroid, adrenocorticopic hormone (ACTH) stimulate the adrenal glands, growth hormone stimulates growth, prolactin stimulates milk production in females, endorphins to reduce the perception of pain, melanocyte-stimulating hormone (MSH) stimulates skin pigmentation via melanin. All of these are peptide hormones.
Thymus gland / Plays a role in the body's immune system. A peptide hormone, thymosin stimulates the maturation of immune cells.
Thyroid gland / Produces hormones that stimulate body heat production, bone growth, and the body's metabolism. These functions are stimulated by a peptide hormone called thyroxine.
Things That Can Go Wrong With the Endocrine System
Too much or too little of any hormone can be harmful to the body. For example, if the pituitary gland produces too much growth hormone, a child may grow excessively tall. If it produces too little, a child may be abnormally short. Controlling the production of or replacing specific hormones can treat many endocrine disorders in children and adolescents, some of which include:
Adrenal insufficiency. This condition is characterized by decreased function of the adrenal cortex and the consequent underproduction of adrenal corticosteroid hormones. The symptoms of adrenal insufficiency may include weakness, fatigue, abdominal pain, nausea, dehydration, and skin changes. Doctors treat adrenal insufficiency by giving replacement corticosteroid hormones.
Cushing syndrome. Excessive amounts of glucocorticoid hormones in the body can lead to Cushing syndrome. In children, it most often results when a child takes large doses of synthetic corticosteroid drugs (such as prednisone) to treat autoimmune diseases such as lupus. If the condition is due to a tumor in the pituitary gland that produces excessive amounts of corticotropin and stimulates the adrenals to overproduce corticosteroids, it's known as Cushing disease. Symptoms may take years to develop and include obesity, growth failure, muscle weakness, easy bruising of the skin, acne, high blood pressure, and psychological changes. Depending on the specific cause, doctors may treat this condition with surgery, radiation therapy, chemotherapy, or drugs that block the production of hormones.
Growth hormone problems. Too muchgrowth hormone in children who are still growing will make their bones and other body parts grow excessively, resulting in gigantism. This rare condition is usually caused by a pituitary tumor and can be treated by removing the tumor. In contrast, when the pituitary gland fails to produce adequate amounts of growth hormone, a child's growth in height is impaired. Hypoglycemia (low blood sugar) may also occur in children with growth hormone deficiency, particularly in infants and young children with the condition.
Hyperthyroidism.Hyperthyroidism (pronounced: hi-per-thigh-roy-dih-zum) is a condition in which the levels of thyroid hormones in the blood are excessively high. Symptoms may include weight loss, nervousness, tremors, excessive sweating, increased heart rate and blood pressure, protruding eyes, and a swelling in the neck from an enlarged thyroid gland (goiter). In children and teens the condition is usually caused by Graves' disease, an autoimmune disorder in which specific antibodies produced by the child's immune system stimulate the thyroid gland to become overactive. The disease may be controlled with medications or by removal or destruction of the thyroid gland through surgery or radiation treatments.
Hypothyroidism. Hypothyroidism (pronounced: hi-po-thigh-roy-dih-zum) is a condition in which the levels of thyroid hormones in the blood are abnormally low. Thyroid hormone deficiency slows body processes and may lead to fatigue, a slow heart rate, dry skin, weight gain, constipation, and, in children, slowing of growth and delayed puberty. Hashimoto thyroiditis, which results from an autoimmune process that damages the thyroid and blocks thyroid hormone production, is the most common cause of hypothyroidism in children. Infants can also be born with an absent or underdeveloped thyroid gland, resulting in hypothyroidism. The condition can be treated with oral thyroid hormone replacement.
Precocious puberty. Body changes associated with puberty may occur at an abnormally young age in some children if the pituitary hormones that stimulate the gonads to produce sex hormones rise prematurely. An injectable medication is available that can suppress the secretion of these pituitary hormones (known as gonadotropins) and arrest the progression of sexual development in most of these children.
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