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Transcript
Ireland 3e Chapte Podcast, Chapter 17 The Endocrine System The endocrine system is composed of a series of glands, connected by the cardiovascular system. Endocrine glands secrete hormones onto their surface, rather than through ducts like exocrine glands. Hormones are chemical messengers that allow the glands of the endocrine system to communicate with other parts of the body. The cells that hormones act upon are called target cells. One hormone may have a series of different target cells, causing different but related effects in each target cell. The two main classes of hormones are steroid hormones, which are structurally related to cholesterol and nonsteroid hormones which are mainly composed of amino acids. Depending on the type of hormone, its action on the target cell is slightly different. The receptors for steroid hormones lie within or near the DNA, and the hormone either causes an increase or decrease in transcription of a gene. These hormones cannot pass the cell membrane, therefore they are picked up by receptors on the cell’s surface. Binding the hormone on the surface begins a cascade of activity that results in the modification of a protein already present in the cell cytosol. The pituitary gland lies below the hypothalamus in the sella turcica of the sphenoid bone, and releases hormones that activate other endocrine glands. The hypothalamus releases factors that stimulate the pituitary gland. The posterior pituitary gland secretes two hormones, oxytocin and antidiuretic hormone. These are actually produced by neurons found in the hypothalamus whose axons extend into the posterior pituitary. The anterior pituitary is glandular in nature and secretes seven hormones. Over- or under-secretion of any of these hormones causes odd growth patterns or ion imbalances resulting in impaired functioning. Each of the endocrine glands secretes specialized hormones in response to drifting blood chemistry levels that threaten homeostasis. The adrenal glands secrete glucocorticoids, mineralocorticoids and androgens in response to ACTH released from the anterior pituitary gland. ACTH is adrenocorticotropic hormone. The pancreas performs both endocrine and exocrine functions. The islets of Langerhans secrete insulin and glucagons, the hormones that are responsible for maintaining blood glucose levels. Basal metabolic rate is governed by the thyroid gland. If it is not working properly, a goiter or exopthalmos and other signs of Grave’s disease may develop. Four small parathyroid glands lie buried, behind the thyroid gland. These glands are responsible for maintaining blood calcium levels. There are other endocrine organs in the body as well. The heart, kidneys and stomach produce hormones, as does the pineal gland in the brain. The development of the body is under the control of the endocrine system. Humans pass through recognized stages as they mature. Newborns, or neonates, are totally dependent on adults. The majority of their time is spent sleeping and gaining weight. Infancy is the stage from 2 to 15 months. During this time, body proportions begin to shift, teeth erupt, eating and sleep patterns begin to match the rest of the family, and the immune system begins to function. The years from age 2 to approximately age 12 are referred to as childhood. Growth occurs, muscle coordination improves, language skills develop, and socialization patterns appear. Once reproductive hormones begin to be produced by the anterior pituitary, puberty has begun. This is a relatively short stage of concentrated growth and development of the reproductive organs. Adulthood follows puberty, lasting until senescence and death. The inevitable result of reaching adulthood is aging.