Endocrine System - Anatomy & Physiology
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The endocrine system regulates body functions by secreting hormones that control metabolism, growth, reproduction, and homeostasis.
A hormone is a chemical messenger secreted by endocrine glands that travels through the blood to target cells to regulate physiology and behavior.
An endocrine gland is a ductless gland that secretes hormones directly into the bloodstream.
Endocrine tissue or organ contains cells that produce hormones but may have other functions as well, e.g., pancreas or kidney.
A target cell has specific receptors for a hormone and responds to that hormone's signal.
The nervous system uses electrical impulses for rapid, short-term control; the endocrine system uses hormones for slower, long-lasting regulation.
Hormones are classified as amino acid derivatives, peptides/proteins, and steroids.
Amino acid and peptide hormones are water-soluble and travel freely; steroid hormones are lipid-soluble and travel bound to carrier proteins.
Water-soluble hormones bind to cell membrane receptors; lipid-soluble hormones bind to intracellular receptors in the cytoplasm or nucleus.
Water-soluble hormones activate second messenger systems; lipid-soluble hormones directly affect gene transcription.
Hormone release is controlled by humoral (blood chemistry), neural, and hormonal stimuli.
Negative feedback reduces hormone secretion when levels reach a set point to maintain homeostasis.
Positive feedback amplifies hormone secretion temporarily, such as during childbirth.
The hypothalamus connects to the anterior and posterior pituitary glands, regulating their hormone secretion.
A releasing hormone from the hypothalamus stimulates the anterior pituitary to secrete specific hormones.
An inhibiting hormone from the hypothalamus suppresses anterior pituitary hormone secretion.
A tropic hormone is secreted by the anterior pituitary and stimulates another endocrine gland to release hormones.
The hypothalamus secretes releasing and inhibiting hormones into the hypophyseal portal system to regulate anterior pituitary secretion.
The hypothalamus produces hormones stored in the posterior pituitary, which releases them into the blood upon neural signals.
Key glands include the pituitary, thyroid, parathyroid, adrenal, testes, ovaries, pancreas, kidney, thymus, heart, and placenta.
Each hormone has a specific source gland, a stimulus for release, and a defined effect on target tissues.