6a

The Endocrine System: Endocrine Glands and Hormone Actions

Chapter Outline

  • 6.1 Primary Endocrine Organs

  • 6.2 Secondary Endocrine Organs

Learning Outcomes

  • Understanding: Named Primary and Secondary Endocrine Glands

    • Identify primary and secondary endocrine glands alongside the hormones they produce.

  • Mechanistic Descriptions:

    • Describe the relationship between hypothalamus and anterior/posterior pituitary lobes.

Endocrine Glands

  • Characteristics of Endocrine Glands:

    • Derived from epithelial tissue.

    • Formed as glands or single cells.

    • Primary Endocrine Organs:

    • Primary function: Hormone production and secretion.

    • Examples include: Hypothalamus, Pituitary, Pineal, Thyroid, Thymus, Adrenal, Pancreas, Gonads.

    • Secondary Endocrine Organs:

    • Secondary function in hormone production and secretion.

    • Examples include heart, kidneys, GI tract, liver, fat, skin.

6.1 Primary Endocrine Organs

  • List of Primary Endocrine Organs: happy penguins play tag together to avoid polar giants

    • Hypothalamus & Pituitary Gland

    • Pineal Gland

    • Thyroid Gland & Parathyroid Glands

    • Thymus

    • Adrenal Glands

    • Pancreas

    • Gonads

Hypothalamus and Pituitary Gland

  • Components:

    • Hypothalamus: part of the central nervous system (CNS).

    • Pituitary Gland:

    • Anterior Lobe: referred to as adenohypophysis.

    • Posterior Lobe: referred to as neurohypophysis.

    • Infundibulum: the stalk connecting the hypothalamus to the pituitary gland.

Hormones Released Through Posterior Pituitary

  • Hypothalamus Neurohormones:

    • Antidiuretic Hormone (ADH or Vasopressin):

    • Function: Regulation of water balance and osmolarity.

    • Released from the Paraventricular Nucleus of the hypothalamus.

    • Oxytocin:

    • Functions:

      • Contraction during labor.

      • Milk letdown and ejection, produced in the Supraoptic Nucleus.

Hypothalamic-Pituitary Portal System

  • Definition:

    • A portal system that connects two capillary beds, allowing for hormone exchange.

  • Components:

    • Hypothalamus: secretes hypothalamic tropic hormones.

    • Anterior Pituitary: responds with anterior pituitary tropic hormones to target distant endocrine glands that in turn release their final hormones.

Hypothalamic & Anterior Pituitary Tropic Hormones

  • Hormonal Pathways:

    • Prolactin:

    • Triggered by prolactin-releasing hormone; inhibited by prolactin-inhibiting hormone (dopamine).

    • Thyroid Hormones:

    • Triggered by thyroid-releasing hormone (TRH) → stimulates thyroid-stimulating hormone (TSH) → → thyroid hormones (TH).

    • Glucocorticoids & Cortisol:

    • Triggered by corticotropin-releasing hormone (CRH) → stimulates adrenocorticotropic hormone (ACTH).

    • Growth Hormones:

    • Triggered by growth hormone-releasing hormone (GHRH) → stimulates growth hormone (GH) → insulin-like growth factor.

    • Includes growth hormone inhibiting hormone.

    • Gonadotropins:

    • Triggered by gonadotropin-releasing hormone (GnRH) → stimulate luteinizing hormone (LH) & follicle-stimulating hormone (FSH) → sex hormones.

Control of Hypothalamic Tropic Hormone Release

  • Influencing Factors:

    • Neural Input: Sensory information affecting release.

    • Hormonal Input: Feedback mechanisms regulate hormone release.

    • Negative Feedback Mechanisms: Essential for regulating hormone levels and maintaining homeostasis.

  • Circadian Rhythms:

    • Governed by the suprachiasmatic nucleus of the hypothalamus.

Pineal Gland

  • Location:

    • Glandular tissue located in the brain.

  • Functions:

    • Influenced by the suprachiasmatic nucleus.

    • Melatonin Secretion:

    • Inhibited during the day and secreted at night.

    • Plays a role in circadian rhythms and seasonal processes in mammals.

    • Enhances immune function.

Hormones of the Thyroid Gland

  • Primary Hormones:

    • T4 (Tetraiodothyronine or Thyroxin):

    • Regulates metabolism; necessary for growth and nervous system development.

    • Deficiency leads to Cretinism.

    • T3 (Triiodothyronine):

    • Similar functions as T4.

  • Calcitonin:

    • Regulates blood calcium levels by promoting calcium deposition in the bone.

Parathyroid Hormone (PTH)

  • Function:

    • Regulates calcium levels in the blood, specifically preventing hypocalcemia.

  • Influence on:

    • Bone: Increases calcium release.

    • Kidneys: Enhances calcium reabsorption.

    • Intestines: Increases calcium absorption.

Thymus

  • Secretes Thymosin:

    • Hormone that regulates T-lymphocyte function, crucial for immune response.

Adrenal Gland Structure

  • Divisions:

    • Cortex: Divided into three regions; specific hormones secreted from each.

    • Zona Glomerulosa: Mineralocorticoids (e.g., aldosterone).

    • Zona Fasciculata: Glucocorticoids (e.g., cortisol).

    • Zona Reticularis: Sex hormones (e.g., androgens).

    • Medulla:

    • Composed of chromaffin cells; approximately 80% epinephrine and 20% norepinephrine, <1% dopamine.

    • Under neural control.

Hormones of the Adrenal Gland: Adrenocorticoids

  • Mineralocorticoids (Aldosterone):

    • Secreted from the zona glomerulosa; regulates sodium and potassium levels in blood.

  • Glucocorticoids (Cortisol):

    • Secreted from zona fasciculata and zona reticularis; regulates the body's stress response and metabolism.

  • Sex Hormones (Androgens):

    • Regulates reproductive functions; secreted from zona fasciculata and zona reticularis.

Pancreas Structure

  • Exocrine Function:

    • Secretion of digestive fluids and enzymes via acinar and duct cells; fluid enters digestive tract through the pancreatic duct.

  • Endocrine Function:

    • Islets of Langerhans; contains multiple cell types:

    • Alpha Cells: Secretes glucagon; raises blood glucose levels.

    • Beta Cells: Secretes insulin; lowers blood glucose levels and promotes energy storage.

    • Delta Cells: Secretes somatostatin; regulates the secretion of other pancreatic hormones.

    • F Cells: Produces pancreatic polypeptide; regulates digestion and nutrient absorption.

Tables of Endocrine Organs and Their Hormones

  • Primary Endocrine Organs and Their Secreted Hormones:

    • Hypothalamus:

    • Releases releasing hormones and inhibiting hormones (e.g., GH, ACTH, TSH, etc.).

    • Anterior Pituitary Gland:

    • Functions in growth, metabolism regulation, and reproductive functions.

    • Posterior Pituitary:

    • Releases oxytocin and ADH.

    • Pineal Gland:

    • Produces melatonin, enhancing biological rhythms.

    • Thyroid Gland:

    • Produces T3, T4 (thyroid hormones), and calcitonin.

    • Parathyroid Glands:

    • Secretes PTH.

  • Secondary Endocrine Organs and Their Hormones:

    • Heart: Atrial natriuretic peptide (ANP).

    • Kidneys: Erythropoietin (EPO).

    • GI Tract: Multiple hormones such as cholecystokinin, secretin, gastrin.

    • Liver: Insulin-like growth factors (IGFs).

    • Fat: Leptin.

6.2 Secondary Endocrine Organs

  • Heart:

    • Hormone: Atrial natriuretic peptide (ANP).

  • Kidneys:

    • Hormone: Erythropoietin (EPO), a cytokine involved in red blood cell production.

  • GI Tract:

    • Multiple hormones include:

    • Cholecystokinin:

    • Secretin:

    • Gastrin:

  • Liver:

    • Hormone: Insulin-like growth factors (IGFs).

  • Skin, liver, kidneys:

    • Produce **1,25-Dihydroxy vitamin D3.

  • Fat:

    • Hormone: Leptin, which regulates energy balance and fat storage.