{"id":1010,"date":"2021-01-27T00:59:39","date_gmt":"2021-01-27T00:59:39","guid":{"rendered":"https:\/\/pressbooks.publishdot.com\/anatomyphysiology\/chapter\/14-5-the-parathyroid-glands\/"},"modified":"2021-12-07T10:19:59","modified_gmt":"2021-12-07T10:19:59","slug":"14-5-the-parathyroid-glands","status":"publish","type":"chapter","link":"https:\/\/pressbooks.publishdot.com\/anatomyphysiology\/chapter\/14-5-the-parathyroid-glands\/","title":{"raw":"14.5 The Parathyroid Glands","rendered":"14.5 The Parathyroid Glands"},"content":{"raw":"
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Learning Objectives<\/strong><\/p>\n\n<\/header>\n

\n\nBy the end of this section, you will be able to:\n
    \n \t
  • Describe the location and structure of the parathyroid glands<\/li>\n \t
  • Describe the hormonal control of blood calcium levels<\/li>\n \t
  • Discuss the physiological response of parathyroid dysfunction<\/li>\n<\/ul>\n<\/div>\n<\/div>\nThe parathyroid glands<\/strong> are tiny, round structures usually found embedded in the posterior surface of the thyroid gland (Figure 14.5.1). A thick connective tissue capsule separates the glands from the thyroid tissue. Most people have four parathyroid glands, but occasionally there are more in tissues of the neck or chest. The function of one type of parathyroid cells, the oxyphil cells, is not clear. The primary functional cells of the parathyroid glands are the chief cells. These epithelial cells produce and secrete the parathyroid hormone (PTH)<\/strong>, the major hormone involved in the regulation of blood calcium levels.\n\n \n\n[caption id=\"attachment_1615\" align=\"aligncenter\" width=\"1024\"]\"Diagram Figure 14.5.1. Parathyroid glands.<\/strong> The small parathyroid glands are embedded in the posterior surface of the thyroid gland. LM \u00d7 760. (Micrograph provided by the Regents of University of Michigan Medical School \u00a9 2012).[\/caption]\n\nThe parathyroid glands produce and secrete PTH, a peptide hormone, in response to low blood calcium concentrations (Figure 14.5.2). PTH secretion causes the release of calcium from the bones by stimulating osteoclasts, which secrete enzymes that degrade bone and release calcium into the interstitial fluid. PTH also inhibits osteoblasts, the cells involved in bone deposition, thereby sparing blood calcium. PTH causes increased reabsorption of calcium (and magnesium) in the kidney tubules from the urine filtrate. In addition, PTH initiates the production of the steroid hormone calcitriol (also known as 1,25-dihydroxyvitamin D), which is the active form of vitamin D3, in the kidneys. Calcitriol then stimulates increased absorption of dietary calcium by the intestines. A negative feedback loop regulates the levels of PTH, with rising blood calcium levels inhibiting further release of PTH.\n\n[caption id=\"attachment_1617\" align=\"aligncenter\" width=\"852\"]\"Diagram Figure 14.5.2. Parathyroid hormone in maintaining blood calcium homeostasis.<\/strong> Parathyroid hormone increases blood calcium levels when they drop too low. Conversely, calcitonin, which is released from the thyroid gland, decreases blood calcium levels when they become too high. These two mechanisms constantly maintain blood calcium concentration at homeostasis.[\/caption]\n\nAbnormally high activity of the parathyroid gland can cause hyperparathyroidism<\/strong>, a disorder caused by an overproduction of PTH that results in excessive calcium reabsorption from bone. Hyperparathyroidism can significantly decrease bone density, leading to spontaneous fractures or deformities. As blood calcium levels rise, cell membrane permeability to sodium is decreased, and the responsiveness of the nervous system is reduced. At the same time, calcium deposits may collect in the body\u2019s tissues and organs, impairing their functioning.\n\nIn contrast, abnormally low blood calcium levels may be caused by parathyroid hormone deficiency, called hypoparathyroidism<\/strong>, which may develop following injury or surgery involving the thyroid gland. Low blood calcium increases membrane permeability to sodium, resulting in muscle twitching, cramping, spasms, or convulsions. Severe deficits can paralyse muscles, including those involved in breathing, and can be fatal.\n\nWhen blood calcium levels are elevated, calcitonin is produced and secreted by the parafollicular cells of the thyroid gland. As discussed earlier, calcitonin inhibits the activity of osteoclasts, reduces the absorption of dietary calcium in the intestine, and signals the kidneys to reabsorb less calcium, resulting in larger amounts of calcium excreted in the urine.\n
    \n

    Section Review<\/strong><\/p>\n\n<\/header>\n

    \n\nCalcium is required for a variety of important physiologic processes, including neuromuscular functioning; thus, blood calcium levels are closely regulated. The parathyroid glands are small structures located on the posterior thyroid gland that produce parathyroid hormone (PTH), which regulates blood calcium levels. Low blood calcium levels cause the production and secretion of PTH. In contrast, elevated blood calcium levels inhibit secretion of PTH and trigger secretion of the thyroid hormone calcitonin. Underproduction of PTH can result in hypoparathyroidism. In contrast, overproduction of PTH can result in hyperparathyroidism.\n\n<\/div>\n<\/div>\n
    \n

    Review Questions<\/strong><\/p>\n\n<\/header>\n

    \n\n[h5p id=\"539\"]\n\n<\/div>\n<\/div>\n
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    Critical Thinking Questions<\/strong><\/p>\n\n<\/header>\n

    \n\n[h5p id=\"540\"]\n\n[h5p id=\"541\"]\n\n<\/div>\n<\/div>\nClick the drop down below to review the terms learned from this chapter.\n\n[h5p id=\"542\"]","rendered":"
    \n
    \n

    Learning Objectives<\/strong><\/p>\n<\/header>\n

    \n

    By the end of this section, you will be able to:<\/p>\n

      \n
    • Describe the location and structure of the parathyroid glands<\/li>\n
    • Describe the hormonal control of blood calcium levels<\/li>\n
    • Discuss the physiological response of parathyroid dysfunction<\/li>\n<\/ul>\n<\/div>\n<\/div>\n

      The parathyroid glands<\/strong> are tiny, round structures usually found embedded in the posterior surface of the thyroid gland (Figure 14.5.1). A thick connective tissue capsule separates the glands from the thyroid tissue. Most people have four parathyroid glands, but occasionally there are more in tissues of the neck or chest. The function of one type of parathyroid cells, the oxyphil cells, is not clear. The primary functional cells of the parathyroid glands are the chief cells. These epithelial cells produce and secrete the parathyroid hormone (PTH)<\/strong>, the major hormone involved in the regulation of blood calcium levels.<\/p>\n

       <\/p>\n

      \"Diagram
      Figure 14.5.1. Parathyroid glands.<\/strong> The small parathyroid glands are embedded in the posterior surface of the thyroid gland. LM \u00d7 760. (Micrograph provided by the Regents of University of Michigan Medical School \u00a9 2012).<\/figcaption><\/figure>\n

      The parathyroid glands produce and secrete PTH, a peptide hormone, in response to low blood calcium concentrations (Figure 14.5.2). PTH secretion causes the release of calcium from the bones by stimulating osteoclasts, which secrete enzymes that degrade bone and release calcium into the interstitial fluid. PTH also inhibits osteoblasts, the cells involved in bone deposition, thereby sparing blood calcium. PTH causes increased reabsorption of calcium (and magnesium) in the kidney tubules from the urine filtrate. In addition, PTH initiates the production of the steroid hormone calcitriol (also known as 1,25-dihydroxyvitamin D), which is the active form of vitamin D3, in the kidneys. Calcitriol then stimulates increased absorption of dietary calcium by the intestines. A negative feedback loop regulates the levels of PTH, with rising blood calcium levels inhibiting further release of PTH.<\/p>\n

      \"Diagram
      Figure 14.5.2. Parathyroid hormone in maintaining blood calcium homeostasis.<\/strong> Parathyroid hormone increases blood calcium levels when they drop too low. Conversely, calcitonin, which is released from the thyroid gland, decreases blood calcium levels when they become too high. These two mechanisms constantly maintain blood calcium concentration at homeostasis.<\/figcaption><\/figure>\n

      Abnormally high activity of the parathyroid gland can cause hyperparathyroidism<\/strong>, a disorder caused by an overproduction of PTH that results in excessive calcium reabsorption from bone. Hyperparathyroidism can significantly decrease bone density, leading to spontaneous fractures or deformities. As blood calcium levels rise, cell membrane permeability to sodium is decreased, and the responsiveness of the nervous system is reduced. At the same time, calcium deposits may collect in the body\u2019s tissues and organs, impairing their functioning.<\/p>\n

      In contrast, abnormally low blood calcium levels may be caused by parathyroid hormone deficiency, called hypoparathyroidism<\/strong>, which may develop following injury or surgery involving the thyroid gland. Low blood calcium increases membrane permeability to sodium, resulting in muscle twitching, cramping, spasms, or convulsions. Severe deficits can paralyse muscles, including those involved in breathing, and can be fatal.<\/p>\n

      When blood calcium levels are elevated, calcitonin is produced and secreted by the parafollicular cells of the thyroid gland. As discussed earlier, calcitonin inhibits the activity of osteoclasts, reduces the absorption of dietary calcium in the intestine, and signals the kidneys to reabsorb less calcium, resulting in larger amounts of calcium excreted in the urine.<\/p>\n

      \n
      \n

      Section Review<\/strong><\/p>\n<\/header>\n

      \n

      Calcium is required for a variety of important physiologic processes, including neuromuscular functioning; thus, blood calcium levels are closely regulated. The parathyroid glands are small structures located on the posterior thyroid gland that produce parathyroid hormone (PTH), which regulates blood calcium levels. Low blood calcium levels cause the production and secretion of PTH. In contrast, elevated blood calcium levels inhibit secretion of PTH and trigger secretion of the thyroid hormone calcitonin. Underproduction of PTH can result in hypoparathyroidism. In contrast, overproduction of PTH can result in hyperparathyroidism.<\/p>\n<\/div>\n<\/div>\n

      \n
      \n

      Review Questions<\/strong><\/p>\n<\/header>\n

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