Endocrine Physiology Notes

Overview

• The endocrine system encompasses structures that release hormones to influence various physiological processes.

• Primary endocrine organs: Produce and secrete hormones as their main function (e.g., thyroid gland, adrenal glands).

• Secondary endocrine organs: Produce and secrete hormones as a secondary function (e.g., liver, kidneys).

Hormones

• Hormones are chemical messengers secreted into extracellular fluids, affecting metabolic activity.

• They travel through blood to regulate other cells' metabolic functions.

• Key characteristics of hormones:

  • Binding to receptors initiates responses after a lag time.

  • Solubility in water affects transport, lifespan, and receptor interaction.

Types of Hormones

1. Amino Acid-based Hormones:

   • Derived from amino acids (e.g., epinephrine, growth hormone).

   • Typically water-soluble; cannot cross the plasma membrane.

2. Steroid Hormones:

   • Synthesized from cholesterol (e.g., cortisol, testosterone).

   • Lipid-soluble; can cross the plasma membrane.

Hypothalamus & Pituitary Interaction

• The hypothalamus governs hormone release from the pituitary gland through:

  • Anterior Pituitary: Controlled by releasing hormones through the hypophyseal portal system.

  • Posterior Pituitary: Hormones released via action potentials from hypothalamic neurons.

Anterior Pituitary Hormones

• Corticotropin-releasing hormone (CRH): Stimulates ACTH production.

• Gonadotropin-releasing hormone (GnRH): Stimulates FSH and LH production.

• Prolactin-inhibiting hormone (PIH): Inhibits prolactin secretion.

• Somatostatin (SOM): Inhibits GH secretion.

• Thyrotropin-releasing hormone (TRH): Stimulates TSH production.

• Growth hormone-releasing hormone (GHRH): Stimulates GH production.

Anterior Pituitary Hormones and Their Effects

Posterior Pituitary Hormones

• Oxytocin:

  • Target: Uterine smooth muscle, mammary glands.

  • Effect: Stimulates contractions during labor; initiates milk ejection.

• Anti-diuretic hormone (ADH):

  • Target: Kidneys.

  • Effect: Stimulates reabsorption of water; inhibits diuresis.

Pineal Gland

• Regulates circadian rhythms via melatonin secretion.

• Melatonin production is stimulated by darkness and inhibited by light.

• Important precursors: Tryptophan and serotonin.

Thyroid Gland

• Thyroid hormones (T3 and T4): Affect almost all body cells.

• Increase basal metabolic rate, regulate growth, and development.

• Calcitonin:

  • Released in response to elevated blood calcium levels.

  • Inhibits osteoclast activity to decrease blood calcium.

Parathyroid Gland

• Secretes parathyroid hormone (PTH), which controls blood calcium levels.

• Low calcium levels trigger PTH release.

• PTH increases blood calcium by acting on:

  • Skeleton: Stimulates osteoclasts to release calcium.

  • Kidneys: Enhances calcium reabsorption.

  • Intestines: Promotes vitamin D activation for increased absorption.

Adrenal Glands

• Cortex: Contains three zones:

  • Zona glomerulosa: Releases mineralocorticoids (e.g., aldosterone).

  • Zona fasciculata: Produces glucocorticoids (e.g., cortisol).

  • Zona reticularis: Secretes sex steroids.

• Medulla: Secretes catecholamines (epinephrine and norepinephrine) for fight or flight response.

Pancreas

• Functions as both an endocrine and exocrine organ.

• Endocrine cells (Islets of Langerhans):

  • Alpha cells: Secrete glucagon in response to hypoglycemia.

  • Beta cells: Secrete insulin in response to hyperglycemia.

Clinical Applications of Diabetes Mellitus (DM)

• Type I DM:

  • Autoimmune destruction of beta cells.

  • Insulin-dependent.

  • Occurs at a younger age (adolescent-onset).

• Type II DM:

  • Insulin resistance; target cells do not respond properly.

  • Typically occurs in adults (maturity-onset).

  • Strong correlation with body weight; often associated with obesity.

• Both Types: Characterized by persistent hyperglycemia and various health risks (e.g., cognitive dysfunction, nerve degeneration).