15: Endocrine System
In order to survive, animals must constantly adapt to changes in the environment. The nervous and endocrine systems both work together to bring about this adaptation. In general the nervous system responds rapidly to short-term changes by sending electrical impulses along nerves and the endocrine system brings about longer-term adaptations by sending out chemical messengers called hormones into the bloodstream. (Thumbnail image credit: "The Endocrine System" by OpenStax College is licensed under CC BY 3.0 )
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- 15.2: An Overview of the Endocrine System
- Communication is a process in which a sender transmits signals to one or more receivers to control and coordinate actions. In the human body, two major organ systems participate in relatively “long distance” communication: the nervous system and the endocrine system. Together, these two systems are primarily responsible for maintaining homeostasis in the body.
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- 15.3: Pituitary Gland and Hypothalamus
- The hypothalamus–pituitary complex can be thought of as the “command center” of the endocrine system. This complex secretes several hormones that directly produce responses in target tissues, as well as hormones that regulate the synthesis and secretion of hormones of other glands. In addition, the hypothalamus–pituitary complex coordinates the messages of the endocrine and nervous systems.
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- 15.4: Thyroid Gland
- A butterfly-shaped organ, the thyroid gland is located anterior to the trachea, just inferior to the larynx. The medial region, called the isthmus, is flanked by wing-shaped left and right lobes. Each of the thyroid lobes are embedded with parathyroid glands, primarily on their posterior surfaces. The tissue of the thyroid gland is composed mostly of thyroid follicles. The follicles are made up of a central cavity filled with a sticky fluid called colloid.
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- 15.5: Parathyroid Glands
- The parathyroid glands are tiny, round structures usually found embedded in the posterior surface of the thyroid gland. Most people have four parathyroid glands. The primary functional cells of the parathyroid glands are the chief cells that release parathyroid hormone to help maintain calcium homeostasis.
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- 15.6: Adrenal Glands
- The adrenal glands, located on the superior surface of the kidneys, release the hormones aldosterone, which regulates blood sodium concentrations, glucocorticoid hormones such as cortisol that regulate blood glucose levels and long-term stress responses, and epinephrine and norepinephrine that stimulate the fight-or-flight stress response.
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- 15.7: Pineal Gland
- Recall that the hypothalamus, part of the diencephalon of the brain, sits inferior and somewhat anterior to the thalamus. Inferior but somewhat posterior to the thalamus is the pineal gland, a tiny endocrine gland whose functions are not entirely clear. The pinealocyte cells that make up the pineal gland are known to produce and secrete the amine hormone melatonin, which is derived from serotonin.
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- 15.8: Gonadal and Placental Hormones
- This section briefly discusses the hormonal role of the gonads—the testes and ovaries—which produce the sex cells (sperm and ova, respectively) and secrete the gonadal hormones. The roles of the gonadotropins released from the anterior pituitary (FSH and LH) were discussed earlier.
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- 15.9: Endocrine Pancreas
- The pancreas is a long, slender organ, most of which is located posterior to the bottom half of the stomach. Although it is primarily an exocrine gland, secreting a variety of digestive enzymes, the pancreas has an endocrine function. Its pancreatic islets—clusters of cells formerly known as the islets of Langerhans—secrete the hormones glucagon, insulin, somatostatin, and pancreatic polypeptide.
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- 15.10: Organs with Secondary Endocrine Functions
- In your study of anatomy, you have already encountered a few of the many organs of the body that have secondary endocrine functions. Here, you will learn about the hormone-producing activities of the heart, gastrointestinal tract, kidneys, skeleton, adipose tissue, skin, and thymus.
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- 15.11: Development and Aging of the Endocrine System
- The endocrine system originates from all three germ layers of the embryo, including the endoderm, ectoderm, and mesoderm. In general, different hormone classes arise from distinct germ layers. Aging affects the endocrine glands, potentially affecting hormone production and secretion, and can cause disease. The production of hormones, such as human growth hormone, cortisol, aldosterone, sex hormones, and the thyroid hormones, decreases with age.