Endocrine System

Endocrine System Overview

Presented by: Dr. Nehal Sayed
1924-2024
Mandl School of Allied Health

Objectives

  1. Distinction between Endocrine and Exocrine Glands

    • Endocrine glands: Ductless glands that secrete hormones directly into the bloodstream.
    • Exocrine glands: Glands that secrete their products into ducts leading to a surface or cavity.
    • Definitions:
      • Hormone: Chemical messenger secreted into the blood.
      • Hypersecretion: Increased secretion of hormones.
      • Hyposecretion: Decreased secretion of hormones.

  2. Identification of Primary Endocrine Glands

    • Glands and Locations:
      • Pineal gland
      • Hypothalamus
      • Pituitary gland
      • Thyroid gland
      • Parathyroid glands
      • Thymus
      • Adrenal glands
      • Pancreas
      • Gonads (Ovary and Testis)

  3. Mechanisms of Hormone Action

    • Non-steroid hormones: Made of amino acids, cannot cross the plasma membrane; bind to membrane receptors.
    • Steroid hormones: Lipid soluble hormones that can diffuse through the plasma membrane into cells.

  4. Feedback Mechanisms in Hormone Regulation

    • Negative Feedback: Response that decreases the function or activity, maintaining homeostasis.
    • Positive Feedback: Output enhances the original stimulus.

  5. Prostaglandins (PGs)

    • Tissue hormones, lipid substances affecting local cells rather than distant organs.
    • Influence respiration, blood pressure, inflammation, and reproductive processes.

  6. Principal Functions of Major Endocrine Glands and Hormones Released

    • Table summarizing effects of various hormones produced by anterior and posterior pituitary, thyroid, parathyroid, adrenal glands, pancreas, ovaries, and testes.

  7. Conditions from Hormone Imbalances

    • Conditions associated with hyposecretion:
      • Gigantism: Excess growth hormone in children.
      • Diabetes Insipidus: Lack of ADH.
      • Goiter and Cretinism: Iodine deficiency impacting thyroid function.
      • Diabetes Mellitus: Insulin deficiency or action impairment affecting glucose metabolism.
      • Glycosuria: Presence of glucose in urine.

Endocrine vs Exocrine Glands

  • Endocrine Glands:
    • Secrete hormones into the bloodstream.
    • Ductless structure allows hormones to diffuse into intercellular spaces.
    • Target Cells: Specific cells activated by hormones.
  • Exocrine Glands:
    • Have ducts; their secretions are released onto surfaces (e.g., sweat and salivary glands).

Major Hormones and Their Functions

Anterior Pituitary Hormones

  1. Thyroid-stimulating Hormone (TSH): Stimulates thyroid hormones.
  2. Adrenocorticotropic Hormone (ACTH): Stimulates adrenal cortex.
  3. Follicle-Stimulating Hormone (FSH):
    • Females: Stimulates ovarian follicle development and estrogen secretion.
    • Males: Stimulates sperm production.
  4. Luteinizing Hormone (LH):
    • Females: Triggers ovulation and corpus luteum development.
    • Males: Stimulates testosterone secretion.
  5. Growth Hormone (GH): Promotes growth and increases blood glucose.
  6. Prolactin (PRL): Promotes breast development and milk production.

Posterior Pituitary Hormones

  1. Antidiuretic Hormone (ADH): Promotes water retention in kidneys.
  2. Oxytocin (OT): Stimulates uterine contractions and milk release.

Other Glands and Hormones

  • Parathyroid Hormone (PTH): Regulates blood calcium levels.
  • Adrenal Glands:
    • Mineralocorticoids (Aldosterone): Regulate electrolyte balance.
    • Glucocorticoids (Cortisol): Affect metabolism and stress response.
    • Adrenal Medulla Hormones (Epinephrine and Norepinephrine): Enhance fight-or-flight response.
  • Pancreas Hormones (Insulin and Glucagon): Regulate blood glucose levels.
  • Gonadal Hormones:
    • Estrogens: Female sexual characteristics and menstrual cycle regulation.
    • Testosterone: Male sexual characteristics.

Essential Definitions

  • Hypersecretion: Increased secretion from glands.
  • Hyposecretion: Decreased secretion from glands.
  • Non-steroid Hormones: Made of amino acids; bind to membrane receptors.
  • Steroid Hormones: Derived from cholesterol; diffuse into target cells to affect DNA directly.
  • Negative Feedback: A response mechanism that reverses a change.
  • Positive Feedback: Amplifies a change to promote a process.
  • Paracrine Signaling: Hormones affecting neighboring cells.

Mechanisms of Hormone Action

Nonsteroidal Hormone Action

  1. Bind to cell membrane receptors (first messenger).
  2. Activate G-proteins, leading to a second messenger (e.g., cAMP).
  3. This ultimately changes cellular functions through activating enzymes and regulating cellular activity.

Steroid Hormone Action

  1. Diffuse through the cell membrane.
  2. Bind to intracellular receptors.
  3. Hormone-receptor complex interacts with DNA, leading to protein synthesis.
  4. Example: Estrogen influencing breast development.

Regulation of Hormone Secretion

  • Hypothalamus: Signals pituitary to release hormones (negative feedback mechanism).
    • Example:
      • CRH stimulates ACTH release, which promotes glucocorticoid release from adrenal cortex that in turn inhibits further ACTH production feedback to hypothalamus.

Prostaglandins (PGs) and Their Functions

  1. Tissue hormones; lipid-based, modulate localized tissue behavior.
  2. Classes include PGA, PGE, PGF.
  3. Influence respiration, blood pressure, gastrointestinal secretions, and inflammatory responses.
  4. Research implicated in treatments for conditions like asthma and hypertension due to their regulatory roles.

Pituitary Gland Structure

  • Composition:
    • Anterior pituitary (adenohypophysis) — glandular.
    • Posterior pituitary (neurohypophysis) — nervous tissue.
  • Location: Lies in the sella turcica, attached by infundibulum to hypothalamus.
  • Vascular and Neural Pathways: Comprised of hypophyseal portal system connecting hypothalamus and anterior pituitary with arterial supply.

Endocrine Hormones and Homeostasis Abnormalities

Effects of Hyposecretion

  • Growth Hormone: Pituitary dwarfism in children.
  • ADH: Diabetes insipidus.
  • Thyroid Hormones: Cretinism in children; myxedema in adults.
  • Insulin: Diabetes mellitus.

Effects of Hypersecretion

  • Growth Hormone: Gigantism in children; acromegaly in adults (overgrowth of certain body parts).
  • Thyroid Hormones: Graves' disease (hyperthyroidism).
  • Insulin: Hypoglycemia (low blood sugar).

Conclusion

This comprehensive overview of the endocrine system highlights the critical elements such as hormone functions, regulation, classifications, and the implications of imbalances for health.