Overview: Endocrine System
- The endocrine system consists of ductless glands, clusters of endocrine cells within organs, and isolated endocrine cells of the diffuse neuroendocrine system.
- Endocrine system means the body-wide signaling system that uses hormone-secreting cells to send chemical instructions through the blood.
- Endocrine glands are ductless glands, so they release hormones into nearby blood vessels instead of sending material through ducts.
- Ductless secretion matters because endocrine messages can travel to distant organs after entering capillaries.
- Capillaries are tiny blood vessels close enough to endocrine cells to pick up hormones quickly.
- Endocrine cells can also appear as small clusters inside organs whose main job is not endocrine secretion.
- Isolated endocrine cells can sit within respiratory and gastrointestinal epithelium and still release hormones into local capillaries.
- Diffuse neuroendocrine system means scattered hormone-producing cells that share features with nerve-related signaling cells.
- Respiratory epithelium lines air passages, and some endocrine cells in this lining help regulate local or systemic physiology.
- Gastrointestinal epithelium lines the digestive tract, where endocrine cells can release hormones involved in digestion and metabolism.
- Pituitary gland is a central endocrine gland because it releases hormones that control several other endocrine organs.
- The hypothalamus is a brain region that links the nervous system to the endocrine system by controlling pituitary hormone release.
- Thyroid gland helps set metabolic rate through thyroid hormones made by follicular cells.
- Parathyroid glands regulate blood calcium through parathyroid hormone made by chief cells.
- Adrenal glands sit above the kidneys and produce steroid hormones and catecholamine hormones.
- Pineal gland helps coordinate circadian rhythm by releasing melatonin mostly during darkness.
- Pancreatic islets are endocrine clusters inside the pancreas that release blood-glucose-regulating hormones.
- Adipose tissue is fat tissue, but it also behaves as an endocrine organ by releasing signaling molecules.
- The ovaries have endocrine roles because they produce reproductive hormones such as estrogen and progesterone.
- The testes have endocrine roles because they produce testosterone and other reproductive signals.
- Metabolic activities include processes such as energy use, heat production, growth, and nutrient storage.
- Nervous system interaction lets the endocrine system respond to stress, sleep-wake cycles, reproduction, blood volume, and blood pressure.
- Slow endocrine signaling usually lasts longer than nerve signaling because hormones remain in circulation and continue acting on target cells.
- Target cells are cells with receptors that recognize a specific hormone and respond to it.
- Receptors explain hormone specificity because a hormone affects only cells able to bind and interpret that hormone.
- Homeostasis is stable internal balance, and endocrine control is one of the main ways the body preserves it.
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