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Comprehensive Endocrine System Study Notes

Endocrine vs Exocrine

  • Endocrine glands are vascular and have no ducts; they secrete hormones directly into the bloodstream.
  • Exocrine glands secrete chemicals through ducts to surfaces such as skin or a lumen.
  • Hormones are secreted into the blood and act as chemical messengers produced by the gland.
  • Summary: Endocrine = no ducts, hormones into blood; Exocrine = ducts, secretions to surfaces.

Terminology

  • Hormones: Organic chemical substances secreted by endocrine glands and carried in the bloodstream to target organs to regulate metabolic reactions.
  • Afrikaans: Hormone: organiese chemiese stof, deur endokrienklier afgeskei, in die bloedstroom vervoer, na die teikenorgaan waar dit metaboliese reaksies reguleer.
  • Target organ: Cells that respond to a hormone with receptor sites.
  • Afrikaans: Teikenorgaan: Selle wat op die hormoon reageer met reseptorplekke.
  • Endocrine gland: Vascular, ductless gland that secretes hormones.
  • Afrikaans: Endokrienklier: Vaskulêre, buislose klier wat hormone afskei.

Endocrine System: Organs (Overview)

  • Major components include: Pineal, Hypothalamus, Pituitary Gland, Thyroid, Parathyroid, Thymus, Adrenal Cortex, Kidney, Pancreas, Testes, Ovary, Uterus.
  • Note: This list reflects organs highlighted in the material as part of the endocrine system.

Pituitary Gland

  • Also known as the Master gland (Meesterklier).
  • Posterior lobe (Posterior lob): secretes ADH (antidiuretic hormone) and Oxytocin (Oksitosien).
  • Anterior lobe (Anterior lob): secretes GH (growth hormone), TSH (thyroid-stimulating hormone), FSH (follicle-stimulating hormone), LH (luteinising hormone), and Prolactin (Prolaktien).

Disorders and Terminology

  • AFWYKINGS means disorder or abnormality.
  • Hyper- (hiper) = over secretion / oorafskeiding.
  • Hypo- (hipo) = under secretion / onderafskeiding.

Growth Hormone (GH) Disorders

  • Gigantism (Gigantisme) when excessive GH secretion occurs before puberty.
  • Dwarfism (Dwergisme) when GH is insufficient during development.
  • Acromegaly (Akromegalie) due to excess GH after epiphyseal plates closed.

Thyroid Gland (Tiroiedkliier)

  • Disorders:
    • Hyperthyroidism (Hipertiroidisme)
    • Hypothyroidism (Hipotiroidisme)
    • Myxoedema (Miksedeem)
    • Cretinism (Kretisme)

Pancreas

  • Endocrine and exocrine gland (Endokriene en eksokriene klier).
  • Islet cell types: Alpha cells produce Glucagon (Glukagon); Beta cells produce Insulin (Insulien).
  • Disorder: Diabetes.

Adrenal Gland

  • Produces Adrenaline (Adrenalien) and Aldosterone (Aldosteroon).

Ovaries and Sex Hormones

  • Ovaries (Ovariums) produce Estrogen and Progesterone.

  • Estrogen (Oestrogen/estrogeen):

    • A steroid hormone important for female reproductive development.
    • Produced by growing Graafian follicle and placenta during pregnancy.
    • Secretion is regulated by FSH.
    • Involved in formation and maintenance of secondary sex characteristics; also important in bone resorption.

  • Progesterone (Progesterone/Progesteroon):

    • A steroid hormone that prepares the uterus for pregnancy.
    • Produced by the corpus luteum and placenta during pregnancy.
    • Secretion is regulated by LH.
    • Involved in formation and maintenance of endometrium and uterus.

  • Menstrual cycle context (simplified):

    • Estrogen and progesterone regulate the cycle.
    • Phases referenced: Follicular phase, Ovulation, Luteal phase, Start of cycle.
    • Typical cycle timing cited: Day 7 (start of follicular growth), Day 14 (ovulation), Day 21 (mid-luteal), Day 28 (end of cycle).

Testosterone and Male Reproduction

  • Testosterone is produced by the testes; Hypothalamus and Pituitary regulate its secretion via a negative feedback loop.

  • Key regulators:

    • Hypothalamus releases GnRH (gonadotropin-releasing hormone).
    • Pituitary releases LH (luteinising hormone) and FSH (follicle-stimulating hormone).
    • Testosterone and Inhibin B produced by testes provide negative feedback to the hypothalamus and pituitary to limit GnRH, LH, and FSH.

  • Testes produce testosterone and inhibin B; Inhibin B helps provide feedback to the pituitary.

  • Negative feedback: A stimulus (low testosterone) triggers GnRH release, which increases LH and FSH, raising testosterone; once normal, feedback reduces GnRH and LH/FSH.

  • Conceptual schematic (text):

    • Hypothalamus → GnRH → Anterior pituitary → LH + FSH → Testes → Testosterone + Inhibin B.
    • Testosterone and Inhibin B feed back to suppress GnRH and LH/FSH.

Homeostasis

  • All body cells are surrounded by fluids; the composition of these fluids changes constantly and influences cell function.
  • Maintaining a constant internal environment irrespective of external conditions.
  • Key variables to keep constant for optimal function:
    • Glucose levels, Water content, Salt concentration, O2 and CO2 concentrations, Body temperature, Metabolic waste, pH.

Negative Feedback Mechanism (General)

  • There are mechanisms to bring deviations back to normal; Negative Feedback Mechanism (Negatieve Terugvoer Meeganisme).
  • Basic flow:
    • Input: Information is sent along an afferent pathway to a receptor (sensor).
    • Change is detected by the receptor.
    • Control center processes the information.
    • Output: Information is sent via an efferent pathway to an effector.
    • Stimulus produces a change (imbalance) in a controlled variable.
    • The effector’s response feeds back to influence the magnitude of the stimulus and returns the variable to homeostasis.

Blood Glucose Regulation

  • Blood glucose levels (Bloedglukosevlakke) are kept near a reference value.
  • When blood glucose is too high:
    • Beta cells detect raised glucose levels and release insulin.
    • Insulin acts on liver and muscles to convert glucose to glycogen and increase glucose uptake by cells (membranes become more permeable to glucose).
    • Result: Glucose levels are reduced.
  • When blood glucose is too low:
    • Alpha cells detect decreased glucose and release glucagon.
    • Glucagon promotes glycogen breakdown in the liver to glucose.
    • Result: Glucose levels are increased.
  • Reference value noted: Blood glucose level is about 90\ \mathrm{mg/100\ mL} at rest.
  • Schematic of the response cycle:
    • Stimulus: Blood glucose level rises → Beta cells release insulin → Liver uptake and storage of glucose as glycogen → Blood glucose falls.
    • Stimulus: Blood glucose level falls → Alpha cells release glucagon → Liver glycogen breakdown → Blood glucose rises.

Reproductive Hormones – Male

  • When testosterone is too low:
    • The pituitary gland is stimulated to release more FSH and LH.
    • More testosterone is released (by the testes).
    • This leads to spermatogenesis and increased sperm production.
  • Pituitary hormone effects: LH and FSH stimulate spermatogenesis and testosterone secretion by the testes.
  • Pathway:
    • Hypothalamus → GnRH;
    • Anterior pituitary → LH + FSH;
    • Testes → Sertoli cells promote spermatogenesis; Leydig cells produce testosterone; Inhibin is produced and provides negative feedback.
  • Testosterone and Inhibin inhibit GnRH and LH/FSH in a negative feedback loop.

Reproductive Hormones – Female

  • Ovaries produce estrogen and progesterone.
  • Estrogen:
    • Steroid hormone important for reproductive development; produced by Graafian follicle and placenta during pregnancy; secretion regulated by FSH; involved in formation and maintenance of secondary sex characteristics; bone resorption; and enlargement of uterus and breasts during pregnancy.
  • Progesterone:
    • Steroid hormone that prepares the uterus for pregnancy; produced by corpus luteum and placenta; secretion regulated by LH; involved in the formation and maintenance of the endometrium and uterus; supports implantation and cycle progression.
  • Regulation involves negative feedback loops between the hypothalamus, pituitary gland, and ovaries; ovulation is part of the cycle; the uterine lining proliferates under estrogen and is maintained by progesterone during the luteal phase.
  • Key cycle references: Follicular phase, Ovulation, Proliferation of uterine wall, Start of cycle, Day 7, Day 14, Day 21, Day 28, Luteal phase.

Hormone Usage and Supplements

  • Hormone usage topics included:
    • Anabolic steroids (Anaboliese steroiede)
    • Erythropoietin (Erythropoietin, EPO)
    • Growth Hormone (GH)
  • EPO details: 3000 IU (units) mentioned; storage instructions: store at 4°C (39.2°F) to -20°C (-4°F).
  • Source noted: WWW.PEPTIDES.CO.ZA.

Connections to Foundational Principles

  • The endocrine system operates via signaling cascades that regulate metabolism, growth, reproduction, and homeostasis.
  • Negative feedback mechanisms provide stability and prevent overstimulation of hormonal pathways.
  • The pancreas illustrates a dual system (endocrine and exocrine) working in concert to regulate nutrient availability and digestion.
  • Hormone regulation often involves interplay between hypothalamus, pituitary gland, and peripheral glands (the hypothalamic-pituitary axis).

Ethical, Philosophical, and Practical Implications

  • Hormone therapy and hormone usage (anabolic steroids, EPO, GH) raise ethical considerations in sports, medicine, and fair play.
  • Negative feedback mechanisms reflect a fundamental principle of biological regulation: systems favor stability and self-correction rather than unchecked change.
  • The balance of reproductive hormones has profound implications for fertility, development, and health outcomes across the lifespan.

Key Formulas and Numerical References

  • Reference blood glucose level: ext{Blood glucose level} \, \approx \, 90\ \mathrm{mg/100\ mL}.

  • Glucose regulation pathways (summary):

    • High BG:
      ext{Beta cells release insulin} \Rightarrow \text{Glucose uptake by liver/muscle and conversion to glycogen} \Rightarrow \text{BG} \downarrow
    • Low BG:
      \text{Alpha cells release glucagon} \Rightarrow \text{Glycogen breakdown to glucose} \Rightarrow \text{BG} \uparrow

  • Negative feedback schematic (textual equation form):

    • Stimulus (V) increases -> Sensor detects -> Controller processes -> Effector acts to reduce V -> V returns to set point.

  • Hormone names and primary producers (recap):

    • ADH (posterior pituitary)
    • Oxytocin (posterior pituitary)
    • GH (anterior pituitary)
    • TSH (anterior pituitary)
    • FSH (anterior pituitary)
    • LH (anterior pituitary)
    • Prolactin (anterior pituitary)
    • Glucagon (alpha cells of pancreas)
    • Insulin (beta cells of pancreas)
    • Estrogen and Progesterone (ovaries)
    • Testosterone (testes)
    • Adrenaline (adrenal gland)
    • Aldosterone (adrenal gland)

  • Note on units and storage: EPO example indicates 3000 IU and storage range between 4°C and -20°C; this reflects typical handling of peptide hormones.