January 20 Lecture - January 20, 2009 The Endocrine System...

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January 20, 2009 The Endocrine System The Thyroid Gland - The thyroid is found in the throat region - Butterfly shaped mass of tissue that straddles the larynx o Lower margin of the larynx - Isthmus of tissue connects the glands in the front - Weighs approximately 30 grams, but can vary in size (and secretory activity) due to o Gender o Age o Climate o Diet - Consists of a number of spherical structures known as thyroid follicles - Follicles are made up of a single layer of cuboidal cells o These cells represent the secretory fraction of the gland o The major hormonal secretions are produced by these cells o Known as follicle cells - Follicle cells produce primary hormone produced by the thyroid, thyroxin - Thyroxin combines with other compounds in the follicle and is stored in the central cavity of the follicle (stored as a colloid, can be mobilized with required, stored form of thyroxin is thyrogobulin ) - Follicle cavities store chemical secretions of the thyroid - Thyroxin functions to o Control basal metabolic rate Determines amount of metabolic activity or heat production in the body This is why thyroxin is found in higher levels in the winter - C cells produce a hormone known as calcitonin o Calcitonin regulates the levels of calcium ions within bodily fluids o If calcitonin levels increase, a number of effects Reduces release of calcium from bones Increases amount of calcium excreted by the renal system Increases calcium levels in the fluids of the body The Parathyroid Gland - Glands are embedded on the dorsal surface of the thyroid - Very small structures - 2-4 mm in diameter - All four parathyroid glands combined and weighed will be about 1-1.5 g of tissue - Glands went unnoticed for many years and wasn’t until they were accidentally removed with thyroid tissue were they discovered - Each gland contains two types of cells o Principal cells (Chief cells) Small, numerous cells found throughout the gland Site of origin for hormone known as parathormone
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Important for regulating calcium and phosphorous ratio throughout the body o Oxyphil cells More prominent, larger cells Less numerous than principal cells Not apparent at the time of birth, appear at time of puberty until the end of life Not sure of what their functional role is - When parathyroid glands are removed, calcium levels are reduced and phosphorous levels are elevated - Under normal circumstances, the levels of parathormone and calcium are inversely related o If calcium levels increase, the parathormone levels will decrease and vice versa - Control of parathormone levels is controlled at 3 sites o Parathormone will increase calcium that is absorbed from the diet Increases absorption of calcium across the gut wall o Parathormone will increase calcium release from the bone from the osteoclasts (in addition to phosphorous) > 99% of the body’s calcium storage is in the bone o Parathormone also acts on the renal system Acts on kidney by increasing amount of calcium absorbed from the filtrate
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January 20 Lecture - January 20, 2009 The Endocrine System...

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