Endocrine Glands Sec 3, 4 Pt 2

Homeostasis

  • Concept Overview

    • Homeostasis refers to the body's ability to maintain a stable internal environment despite changes in external conditions.

Calcium Regulation

  • Calcitonin

    • Produced by the thyroid gland, specifically by the parafollicular cells.

    • Stimulus for secretion: Hypercalcemia, which means an increased calcium ion concentration in the blood.

    • Function: Inhibits the activity of osteoclasts, allowing osteoblasts to continue bone building.

    • Definitions:

      • Osteoclasts: Cells that break down bone.

      • Osteoblasts: Cells that build up bone (Bone Builders BBBB).

    • Secreted at higher levels: Inhibits osteoclast activity, allowing bone growth.

    • Secreted at lower levels: Appears to inhibit osteoblasts, reducing bone building activity.

Thyroid Hormones

  • Follicular Cells: Produce T3 and T4 thyroid hormones.

  • Stimulus for Thyroid Hormone Secretion: TSH (Thyroid Stimulating Hormone) from the anterior pituitary.

  • Inhibition: Increased levels of T3 and T4 hormones inhibit TRH (Thyrotropin-releasing hormone) and TSH due to negative feedback.

  • Effects of Thyroid Hormones:

    • Basal Metabolic Rate (BMR): Regulation of metabolism.

    • Thermal Regulation: Maintenance of body temperature.

    • Growth and Development: Important in physical and mental growth.

    • Synergism with Sympathetic Nervous System: Enhances the body’s response to stress.

    • Target Tissue: Every cell in the body.

Parathyroid Hormone (PTH)

  • Produced by: Chief cells of the parathyroid glands.

  • Stimulus for Secretion: Hypocalcemia, a low concentration of calcium in the blood.

  • Primary Effects of PTH:

    • Releases Calcium Ions: Increases release of calcium ions from bones by stimulating osteoclasts (bone destroyers).

    • Dietary Calcium Absorption: Increases absorption from the small intestine.

    • Kidney Function: Increases reabsorption of calcium ions in renal tubules, preventing excretion.

  • Integration with Vitamin D:

    • PTH stimulates the conversion of Vitamin D to its active form, calcitriol (Vitamin D3), enhancing calcium absorption.

  • Target Tissues:

    • Osteoclasts: Increase bone resorption.

    • Small Intestine: Enhances dietary absorption.

    • Kidneys: Increases calcium reabsorption.

Adrenal Gland Anatomy

  • Location: Pyramid-shaped glands located on top of each kidney.

  • Parts:

    • Adrenal Cortex: Outer layer with three zones:

    • Zona Glomerulosa: Produces mineralocorticoids (e.g., aldosterone).

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

    • Zona Reticularis: Produces androgens (sex hormones).

    • Adrenal Medulla: Inner layer, a neuroendocrine organ that secretes catecholamines (epinephrine and norepinephrine).

Aldosterone

  • Function: Maintains sodium and potassium ion concentrations, regulates blood pressure, and maintains acid-base homeostasis.

  • Effects:

    1. Maintains Sodium and Potassium Levels:

    • Stimulates sodium and chloride reabsorption and potassium excretion in urine.

    1. Regulates Fluid Volume:

    • Indirectly promotes water reabsorption through osmosis.

    1. Maintains Blood Pressure:

    • Part of Renin-Angiotensin-Aldosterone System (RAS).

    1. Acid-Base Homeostasis:

    • Activates hydrogen ion pumps for urine excretion.

  • Regulation:

    • Stimulated by elevated blood potassium levels, decreased blood pH, and angiotensin II.

Cortisol

  • Functions:

    1. Gluconeogenesis: Promotes glucose production in the liver.

    2. Releases Amino Acids: From muscle tissue into blood.

    3. Releases Fatty Acids: From adipose tissue for energy.

    4. Stress Response: Manages physiological response to stress (e.g., injury).

  • Anti-inflammatory Properties: Reduces inflammation and suppresses immune response; used in corticosteroid treatments.

Regulation of Cortisol

  • HPA Axis: Hypothalamic-Pituitary-Adrenal Axis.

    • Corticotropin-Releasing Hormone (CRH) from hypothalamus stimulates Adrenocorticotropic Hormone (ACTH) from anterior pituitary, leading to cortisol production in adrenal cortex.

  • Negative Feedback Loop: High cortisol levels inhibit CRH and ACTH production to maintain balance.

Disorders Related to Hormones

Thyroid Disorders

  • Hypothyroidism: Low production of thyroid hormones.

  • Hyperthyroidism: High levels of thyroid hormones.

  • Graves' Disease: Autoimmune disorder causing excessive production.

Aldosterone Disorders

  • Hyperaldosteronism: Excess secretion leads to:

    • Hypokalemia: Low potassium levels.

    • Hypernatremia: High sodium levels.

    • High Blood Pressure: Resulting from fluid retention.

Cortisol Disorders

  • Cushing's Disease: Hypercortisolism causing symptoms like moon facies and buffalo hump due to fat redistribution.

  • Addison's Disease: Hypocortisolism characterized by low secretion of cortisol and aldosterone, leading to increased risks for patients.

Adrenal Medulla Function

  • Chromaffin Cells: Within adrenal medulla, involved in catecholamine secretion (epinephrine and norepinephrine).

  • Responses: Immediate stress response including increased heart rate and blood pressure, bronchodilation, and blood vessel constriction.