endocrine system
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Here is a thorough overview of the endocrine system, drawn from Guyton and Hall Textbook of Medical Physiology, Costanzo Physiology, and Robbins & Kumar Basic Pathology.
The Endocrine System
What It Is
The endocrine system is a network of glands and tissues that secrete hormones - chemical messengers released into the bloodstream to regulate distant target organs. Its main roles are maintaining metabolic homeostasis, controlling growth and development, regulating reproduction, and coordinating the body's response to stress.
Unlike the nervous system (which communicates in milliseconds via electrical signals), the endocrine system works more slowly through chemical signaling, with effects lasting from minutes to weeks.
Major Glands, Hormones, and Functions
Hypothalamus (Control Center)
The hypothalamus is the master regulator - it links the nervous system to the endocrine system via the pituitary gland.
| Hormone | Action |
|---|---|
| Thyrotropin-releasing hormone (TRH) | Stimulates TSH and prolactin release |
| Corticotropin-releasing hormone (CRH) | Stimulates ACTH release |
| Gonadotropin-releasing hormone (GnRH) | Stimulates LH and FSH release |
| Growth hormone-releasing hormone (GHRH) | Stimulates GH release |
| Somatostatin (SRIF) | Inhibits GH release |
| Dopamine (PIF) | Inhibits prolactin release |
Anterior Pituitary ("Master Gland")
| Hormone | Action |
|---|---|
| Growth hormone (GH) | Stimulates protein synthesis and tissue growth |
| Thyroid-stimulating hormone (TSH) | Stimulates thyroid hormone synthesis |
| Adrenocorticotropic hormone (ACTH) | Stimulates cortisol, androgens, and aldosterone from adrenal cortex |
| Prolactin | Promotes breast development and milk secretion |
| Follicle-stimulating hormone (FSH) | Ovarian follicle growth; sperm maturation |
| Luteinizing hormone (LH) | Ovulation; testosterone synthesis in testes |
| Melanocyte-stimulating hormone (MSH) | Stimulates melanin synthesis |
Posterior Pituitary
Stores and releases hormones made in the hypothalamus:
| Hormone | Action |
|---|---|
| Antidiuretic hormone / Vasopressin (ADH) | Increases water reabsorption in kidneys; raises blood pressure |
| Oxytocin | Triggers milk ejection; stimulates uterine contractions |
Thyroid Gland
| Hormone | Action |
|---|---|
| T3 (triiodothyronine) and T4 (thyroxine) | Increase metabolic rate; regulate heat production, protein/fat/carbohydrate use, skeletal growth |
| Calcitonin | Promotes calcium deposition in bone; lowers serum calcium |
Parathyroid Glands (x4)
| Hormone | Action |
|---|---|
| Parathyroid hormone (PTH) | Raises serum calcium (bone resorption, renal reabsorption, activates vitamin D) |
Adrenal Cortex (three zones)
| Hormone | Zone | Action |
|---|---|---|
| Cortisol | Zona fasciculata | Regulates glucose, protein, fat metabolism; anti-inflammatory |
| Aldosterone | Zona glomerulosa | Raises Na+ reabsorption, K+ and H+ secretion (blood pressure) |
| Androgens (DHEA) | Zona reticularis | Sex hormone precursors |
Adrenal Medulla
| Hormone | Action |
|---|---|
| Epinephrine and Norepinephrine | "Fight-or-flight" - sympathetic stimulation, raise heart rate and blood glucose |
Pancreas (Islets of Langerhans)
| Cell | Hormone | Action |
|---|---|---|
| Beta cells | Insulin | Lowers blood glucose (promotes uptake and storage) |
| Alpha cells | Glucagon | Raises blood glucose (stimulates glycogenolysis, gluconeogenesis) |
| Delta cells | Somatostatin | Inhibits insulin and glucagon locally |
Gonads
| Gland | Hormones | Actions |
|---|---|---|
| Testes | Testosterone, Inhibin | Male development, spermatogenesis |
| Ovaries | Estrogen, Progesterone, Inhibin | Female development, menstrual cycle, pregnancy |
Other Endocrine Tissues
| Tissue | Hormone | Action |
|---|---|---|
| Kidneys | Renin, Erythropoietin (EPO), Calcitriol (active vit D) | Blood pressure, RBC production, calcium absorption |
| Heart (atria) | Atrial natriuretic peptide (ANP) | Lowers blood pressure, promotes Na+ excretion |
| Adipose tissue | Leptin, Adiponectin | Hunger suppression, insulin sensitivity |
| GI tract | Gastrin, Secretin, CCK, GIP, GLP-1 | Digestion, insulin secretion |
| Pineal gland | Melatonin | Circadian rhythm regulation |
| Thymus | Thymosin | T-cell maturation |
Hormone Classes and Mechanisms
By Chemical Structure
-
Peptide/Protein hormones (most hormones - insulin, GH, TSH, ADH): Water-soluble, cannot cross the cell membrane. Bind to surface receptors and act via second messengers (cAMP, IP3, etc.). Fast onset, short duration.
-
Steroid hormones (cortisol, aldosterone, estrogen, testosterone): Lipid-soluble, derived from cholesterol. Cross the cell membrane and bind to intracellular receptors, directly regulating gene transcription. Slower onset, longer duration.
-
Amine hormones (T3/T4, epinephrine, dopamine): Derived from tyrosine. T3/T4 act like steroids (nuclear receptors); catecholamines act like peptides (surface receptors).
Regulation: Feedback Loops
Most endocrine axes are regulated by negative feedback:
Hypothalamus → Releasing hormone →
Anterior Pituitary → Tropic hormone →
Target Gland → Hormone →
Back to Hypothalamus/Pituitary (INHIBITS further release)
Example - the HPT axis:
- Hypothalamus releases TRH
- Pituitary releases TSH
- Thyroid releases T3/T4
- High T3/T4 feeds back to suppress TRH and TSH
Positive feedback is rare - the classic example is the LH surge during the menstrual cycle (rising estrogen triggers a massive LH release that causes ovulation).
Key Clinical Disorders (Overview)
| Gland | Excess | Deficiency |
|---|---|---|
| Anterior pituitary | Acromegaly (GH excess), Cushing disease (ACTH excess), Prolactinoma | Hypopituitarism, Growth failure |
| Thyroid | Hyperthyroidism / Graves' disease (exophthalmos, palpitations, weight loss) | Hypothyroidism / Hashimoto's (fatigue, weight gain, cold intolerance) |
| Parathyroid | Hyperparathyroidism (hypercalcemia: "bones, stones, groans, moans") | Hypoparathyroidism (hypocalcemia: tetany, Chvostek sign) |
| Adrenal cortex | Cushing syndrome (cortisol), Conn syndrome (aldosterone), CAH | Addison's disease (adrenal insufficiency) |
| Pancreas | - | Diabetes mellitus (insulin deficiency/resistance) |
| Posterior pituitary | SIADH (ADH excess - hyponatremia) | Diabetes insipidus (ADH deficiency - polyuria) |
Summary
The endocrine system works through:
- Glands that synthesize and secrete hormones
- Hormones that travel via the bloodstream to distant targets
- Receptors on or inside target cells that transduce the signal
- Feedback loops (mostly negative) that maintain homeostasis
Sources: Guyton and Hall Textbook of Medical Physiology, Table 75.1; Costanzo Physiology 7th Ed., Table 9.2; Robbins & Kumar Basic Pathology, Chapter 20 Outline
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I now have rich content from the THIEME Atlas of Anatomy and other textbooks. Here is a focused, detailed breakdown of all the endocrine glands.
Endocrine Glands
Endocrine glands are ductless glands - they release their secretions (hormones) directly into the bloodstream, lymphatics, or intercellular spaces, rather than through ducts. The hormones then travel to distant target cells and bind to specific receptors to exert their effects.
This distinguishes them from exocrine glands (like sweat glands or salivary glands) which release secretions externally through ducts.
1. Hypothalamus
Location: Base of the brain (diencephalon)
Type: Neuroendocrine - neurons that secrete hormones
| Hormone | Effect |
|---|---|
| TRH (Thyrotropin-releasing hormone) | Stimulates TSH and prolactin from anterior pituitary |
| CRH (Corticotropin-releasing hormone) | Stimulates ACTH from anterior pituitary |
| GHRH (Growth hormone-releasing hormone) | Stimulates GH release |
| Somatostatin (SRIF) | Inhibits GH release |
| GnRH (Gonadotropin-releasing hormone) | Stimulates LH and FSH |
| Dopamine (PIF) | Inhibits prolactin |
| Vasopressin (ADH) & Oxytocin | Made here, stored and released from posterior pituitary |
The hypothalamus is the master controller - it links the CNS to the endocrine system.
2. Pituitary Gland (Hypophysis)
Location: Sella turcica of the sphenoid bone, at the base of the brain
Size: ~1 cm; two distinct lobes
Anterior Pituitary (Adenohypophysis)
| Hormone | Abbreviation | Action |
|---|---|---|
| Growth hormone | GH (STH) | Stimulates overall growth and protein synthesis |
| Thyroid-stimulating hormone | TSH | Stimulates thyroid hormone synthesis and secretion |
| Adrenocorticotropic hormone | ACTH | Stimulates cortisol, androgens, aldosterone from adrenal cortex |
| Prolactin | PRL | Promotes breast development and milk production |
| Follicle-stimulating hormone | FSH | Ovarian follicle growth; sperm maturation in Sertoli cells |
| Luteinizing hormone | LH | Ovulation; testosterone synthesis (Leydig cells) |
| Melanocyte-stimulating hormone | MSH | Stimulates melanin synthesis |
Posterior Pituitary (Neurohypophysis)
Stores and releases hormones made in the hypothalamus:
| Hormone | Action |
|---|---|
| ADH (Vasopressin) | Increases water reabsorption in kidney collecting ducts; vasoconstriction |
| Oxytocin | Milk ejection from breasts; uterine contractions during labor |
3. Pineal Gland
Location: Epithalamus of the brain (posterior, between the two cerebral hemispheres)
| Hormone | Action |
|---|---|
| Melatonin | Regulates circadian rhythm (sleep-wake cycle); secreted in darkness |
4. Thyroid Gland
Location: Anterior neck, wrapping around the trachea below the larynx
Structure: Two lobes connected by an isthmus; composed of follicles filled with thyroglobulin colloid
| Hormone | Cells | Action |
|---|---|---|
| T4 (Thyroxine) and T3 (Triiodothyronine) | Follicular cells | Increase metabolic rate; regulate heat production, growth, and protein/fat/carbohydrate use |
| Calcitonin | Parafollicular (C cells) | Lowers serum calcium by promoting bone deposition |
T3 is the active form; T4 is converted to T3 in peripheral tissues.
5. Parathyroid Glands
Location: Four tiny glands (~6 mm each) embedded in the posterior surface of the thyroid
Number: Usually 4 (can vary from 2 to 6)
| Hormone | Action |
|---|---|
| Parathyroid hormone (PTH) | Raises serum calcium - stimulates bone resorption, renal Ca²+ reabsorption, and activates Vitamin D (calcitriol) |
PTH and calcitonin are antagonists in calcium regulation.
6. Adrenal Glands (Suprarenal Glands)
Location: Atop each kidney (bilateral)
Structure: Two distinct regions with completely different functions
Adrenal Cortex (outer, 3 zones)
| Zone | Hormone | Action |
|---|---|---|
| Zona glomerulosa (outermost) | Aldosterone (mineralocorticoid) | Raises Na+ reabsorption and K+ excretion in kidney - controls blood pressure |
| Zona fasciculata (middle) | Cortisol (glucocorticoid) | Regulates glucose, protein, fat metabolism; anti-inflammatory; stress response |
| Zona reticularis (inner) | Androgens (DHEA) | Sex hormone precursors; converted to testosterone/estrogen in periphery |
Memory aid: "GFR - Salt, Sugar, Sex" (outer to inner)
Adrenal Medulla (inner core)
| Hormone | Action |
|---|---|
| Epinephrine (Adrenaline) ~80% | "Fight-or-flight" - raises HR, BP, blood glucose; bronchodilation |
| Norepinephrine (Noradrenaline) ~20% | Primarily vasoconstriction; acts as both hormone and neurotransmitter |
The adrenal medulla is essentially a modified sympathetic ganglion - its cells are derived from neural crest cells.
7. Pancreas (Endocrine Portion)
Location: Retroperitoneal, behind the stomach
Structure: Exocrine portion makes digestive enzymes (acini); endocrine portion = Islets of Langerhans (1-2% of pancreatic mass)
| Cell Type | Hormone | Action |
|---|---|---|
| Beta (β) cells ~70% | Insulin | Lowers blood glucose - promotes uptake, glycogen/fat storage |
| Alpha (α) cells ~20% | Glucagon | Raises blood glucose - stimulates glycogenolysis and gluconeogenesis |
| Delta (δ) cells ~5% | Somatostatin | Inhibits insulin and glucagon locally |
| PP cells | Pancreatic polypeptide | Inhibits exocrine pancreatic secretion |
8. Gonads
Testes (Male)
Location: Scrotum
| Hormone | Cells | Action |
|---|---|---|
| Testosterone (androgen) | Leydig cells | Male development, spermatogenesis, libido, muscle mass |
| Inhibin | Sertoli cells | Inhibits FSH (negative feedback) |
Ovaries (Female)
Location: Pelvis, flanking the uterus
| Hormone | Source | Action |
|---|---|---|
| Estrogens (estradiol) | Follicular cells, corpus luteum | Female development, uterine lining proliferation, bone density |
| Progesterone | Corpus luteum | Maintains uterine lining for implantation; sustains pregnancy |
| Inhibin | Granulosa cells | Inhibits FSH |
Placenta (during pregnancy)
| Hormone | Action |
|---|---|
| hCG (human chorionic gonadotropin) | Maintains corpus luteum early in pregnancy |
| Progesterone | Sustains pregnancy |
| Estrogens | Pregnancy-related effects |
9. Thymus
Location: Anterior mediastinum (behind the sternum); largest in childhood, involutes after puberty
| Hormone | Action |
|---|---|
| Thymosin, Thymopoietin | Promote T-lymphocyte maturation and immune development |
10. Other Hormone-Producing Tissues (Diffuse Endocrine Organs)
| Organ | Hormone | Action |
|---|---|---|
| Kidney | Erythropoietin (EPO) | Stimulates RBC production in bone marrow |
| Kidney | Renin | Activates angiotensin system; raises BP |
| Kidney | Calcitriol (1,25-OH Vit D) | Active vitamin D; promotes Ca²+ absorption |
| Liver | IGF-1 (Insulin-like growth factor) | Mediates GH effects on growth |
| Liver | Angiotensinogen | Precursor to angiotensin II |
| Heart (atria) | ANP (Atrial natriuretic peptide) | Lowers BP; promotes Na+ and water excretion |
| GI tract | Gastrin, Secretin, CCK, GLP-1 | Regulate digestion; GLP-1 stimulates insulin secretion |
| Adipose tissue | Leptin | Suppresses appetite; signals satiety to hypothalamus |
| Adipose tissue | Adiponectin | Increases insulin sensitivity |
Key Concept: Endocrine vs. Exocrine
| Feature | Endocrine Glands | Exocrine Glands |
|---|---|---|
| Ducts | No ducts (ductless) | Have ducts |
| Secretion goes to | Bloodstream / lymph | External surfaces / body cavities |
| Products | Hormones | Enzymes, sweat, saliva, mucus |
| Examples | Thyroid, adrenal, pituitary | Salivary, sweat, pancreas (acini) |
Note: The pancreas is both - endocrine (islets of Langerhans) and exocrine (acinar cells producing digestive enzymes).
Sources: THIEME Atlas of General Anatomy and Musculoskeletal System; Guyton and Hall Textbook of Medical Physiology, Table 75.1; Costanzo Physiology 7th Ed., Table 9.2
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