16b

Brazosport College - BIOL 2301 Anatomy and Physiology II

Chapter 16B - Endocrine System

16.7 - Thyroid Gland
  • Location: Lateral upper portion of the trachea, just inferior to the larynx.
  • Weight: One of the largest endocrine glands, weighing approximately 20g.
  • Structure: Highly vascular with numerous follicles:
      - Follicles:
        - Centers filled with proteins called thyroglobulin (produced by follicular cells).
        - Follicles store large amounts of thyroid hormones as part of thyroglobulin (T3 & T4).
      - Parafollicular Cells: Produce calcitonin.
Thyroid Hormone
  • Function: Body's major metabolic hormone; found in two forms:
      - T4 (Thyroxine): Major form consisting of two tyrosine molecules with four bound iodine atoms.
      - T3 (Triiodothyronine): Form with two tyrosines and three bound iodine atoms.
      - Conversion: T4 must be converted at the tissue level.
      - Type: Iodine-containing amine hormones.
      - Target Cells: Affects almost all cells of the body, except brain, testes, uterus, and spleen.
Thyroid Hormone Functions
  • Mechanism of Action: Enters target cell, binds to intracellular receptors within the nucleus, triggers transcription of various metabolic genes.
  • Effects:
      - Increases basal metabolic rate and heat production (calorigenic effect).
      - Regulates tissue growth and development - critical for normal skeletal and nervous system development as well as reproductive capabilities.
      - Maintains blood pressure by increasing adrenergic receptors (norepinephrine α and β) in blood vessels.
Thyroid Hormone Imbalances
  • Hyposecretion (Adults): Leads to myxedema.
      - Symptoms include:
        - Low metabolic rate.
        - Thick and/or dry skin.
        - Puffy eyes.
        - Feeling chilled.
        - Constipation.
        - Edema.
        - Mental sluggishness and lethargy.
      - Goiter Development: Possible if due to a lack of iodine, characterized by colloid expansion.
  • Hyposecretion (Infants): Leads to cretinism.
      - Symptoms include:
        - Intellectual disabilities.
        - Short and disproportionately sized body.
        - Thick tongue and neck.
  • Hypersecretion: Most commonly manifests as Graves’ disease.
      - Characteristics: Autoimmune disease causing abnormal antibodies directed against thyroid follicular cells that mimic TSH, stimulating TH release.
      - Symptoms: Elevated metabolic rate, sweating, rapid and irregular heartbeats, nervousness, weight loss despite adequate food intake, possible exophthalmos (eyes protruding due to edematous and fibrous tissue behind the eyes).
      - Treatments: Surgical removal of thyroid or radioactive iodine to destroy active thyroid cells.
Calcitonin
  • Production: Secreted by parafollicular cells (C-cells).
  • Control: Regulated by Ca²⁺ levels in the blood; production increases as blood Ca²⁺ levels rise.
  • Target: Primarily bone tissue.
  • Mechanism of Action:
      - Binds to membrane-bound receptors, decreasing osteoclast activity and lengthening the lifespan of osteoblasts.
  • Resulting Effects:
      - Decreased blood Ca²⁺ and phosphate levels.
      - Increased bone deposition.
16.8 - Parathyroid Gland
  • Description: Composed of 4-8 tiny yellow-brown glands embedded in the posterior thyroid.
  • Cells: Contains oxyphil cells (function unclear) and parathyroid cells that secrete parathyroid hormone (PTH).
Parathyroid Hormone (PTH)
  • Function: Most important hormone in calcium (Ca²⁺) homeostasis.
  • Stimulus for Secretion: Secreted in response to low blood Ca²⁺ levels and inhibited by rising levels of Ca²⁺.
  • Targets:
      - Skeleton: Stimulates osteoclasts to digest bone matrix and release Ca²⁺ into blood.
      - Kidneys: Enhances Ca²⁺ reabsorption and phosphate ($ ext{PO}_4^{3-}$) secretion.
      - Intestines: Promotes activation of vitamin D, leading to increased Ca²⁺ absorption by intestinal mucosa.
16.9 - Adrenal Gland
  • Structure: Paired, pyramid-shaped organs located atop the kidneys, also known as suprarenal glands.
  • Composition: Functionally and structurally two glands in one:
      - Adrenal Cortex: Three layers of glandular tissue that synthesize and secrete various hormones.
      - Adrenal Medulla: Nervous tissue that is part of the sympathetic nervous system.
Adrenal Cortex
  • Hormone Production: Produces over 24 different hormones collectively called corticosteroids.
      - Storage: Steroid hormones are not stored; release rate depends on synthesis.
  • Layers Producing Corticosteroids:
      - Zona Glomerulosa: Produces mineralocorticoids (e.g., aldosterone).
      - Zona Fasciculata: Produces glucocorticoids (e.g., cortisol).
      - Zona Reticularis: Produces gonadocorticoids (androgens).
Zona Glomerulosa: Mineralocorticoids
  • Function: Regulate electrolyte concentrations (primarily Na⁺ and K⁺) in extracellular fluid (ECF).
      - Importance of Na⁺: Affects ECF volume, blood volume, blood pressure, and levels of other ions.
      - Importance of K⁺: Sets resting membrane potential of cells.
  • Aldosterone: Most potent mineralocorticoid.
      - Effects: Stimulates Na⁺ reabsorption by kidneys, resulting in increased blood volume and blood pressure; stimulates K⁺ elimination by kidneys.
  • Aldosteronism: Hypersecretion usually due to adrenal tumors, causing:
      1. Hypertension and edema due to excessive Na⁺.
      2. Excretion of K⁺ leads to abnormal nonresponsive neurons and muscles.
Zona Fasciculata: Glucocorticoids
  • Function: Influence metabolism of most cells, helping resist stressors.
      - Maintain Blood Glucose Levels: Keeping blood glucose levels relatively constant and maintaining blood pressure by increasing the action of vasoconstrictors.
  • Primary Glucocorticoid: Cortisol (hydrocortisone); only glucocorticoid in significant amounts in humans.
      - Other Hormones: Cortisone, corticosterone.
  • Regulation of Secretion: Cortisol release in response to ACTH, governed by eating patterns and activity.
      - Effect of Acute Stress: Interrupts cortisol rhythm.
  • Cortisol Actions:
      - Increases blood levels of glucose, fatty acids, and amino acids.
      - Promotes gluconeogenesis: formation of glucose from fats and proteins.
        - Encourages cells to utilize fatty acids for fuel to preserve glucose for brain.
      - Enhances vasoconstriction:
        - Causes rise in blood pressure for rapid nutrient distribution.
Zona Fasciculata: Glucocorticoid Imbalances
  • Hypersecretion - Cushing’s Syndrome/Disease:
      - Effects:
        - Depresses cartilage and bone formation; inhibits inflammation; disrupts neural, cardiovascular, and gastrointestinal functioning.
        - Causes include tumors or overuse of corticosteroids.
        - Cushingoid signs: “moon” face, “buffalo hump.”
      - Treatment: Removal of tumor or discontinuation of drugs.
  • Hyposecretion - Addison’s Disease:
      - Symptoms:
        - Involve deficits in mineralocorticoids leading to decreased glucose and Na⁺ levels, resulting in weight loss, severe dehydration, and hypotension.
      - Treatment: Corticosteroid replacement therapy.
Zona Reticularis: Gonadocorticoids
  • Function: Adrenal sex hormones, weak androgens converted to testosterone in tissue cells, some converted to estrogens.
  • Examples: Androstenedione, dehydroepiandrosterone (DHEA).
  • Roles in Development:
      - Contribute to onset of puberty and appearance of secondary sex characteristics.
      - Influence sex drive in women and act as a source of estrogens in postmenopausal women.
Gonadocorticoids Imbalances
  • Hypersecretion:
      - Leads to adrenogenital syndrome (masculinization).
      - Effects in Males: Not noticeable as they are already masculinized by testosterone.
      - Effects in Females and Prepubertal Males:
        - Boys: reproductive organs mature; secondary sex characteristics emerge early.
        - Females: beard growth, masculine pattern of body hair; enlarged clitoris resembling a penis.
Adrenal Medulla
  • Composition: Medullary chromaffin cells synthesize catecholamines epinephrine (80%) and norepinephrine (20%).
  • Effects of Catecholamines:
      - Vasoconstriction, increased heart rate, increased blood glucose levels, blood diverted to brain, heart, and skeletal muscle.
  • Comparison of Effects:
      - Epinephrine primarily stimulates metabolic activities (e.g., bronchial dilation).
      - Norepinephrine influences peripheral vasoconstriction and blood pressure.
  • Response Duration: Responses to stressors are brief compared to adrenal cortical hormones.
Adrenal Medulla Imbalances
  • Hyposecretion: No significant issues as epinephrine and norepinephrine are not essential to life.
  • Hypersecretion: Leads to symptoms of uncontrolled sympathetic nervous system activation, such as:
      - Hyperglycemia, increased metabolic rate, rapid heartbeat, palpitations, hypertension, intense nervousness, and sweating.
16.10 - Pineal Gland
  • Function: Pinealocytes secrete melatonin, derived from serotonin.
  • Effects of Melatonin:
      - Influences timing of sexual maturation and puberty.
      - Affects day/night cycles and physiological processes that exhibit rhythmic variations (body temperature, sleep, appetite).
      - Involved in production of antioxidant and detoxification molecules in cells.
16.11 - Pancreas & Thymus
Pancreas
  • Description: Triangular gland located partially behind the stomach; contains both exocrine and endocrine cells.
      - Exocrine Cells: Acinar cells produce enzyme-rich juice for digestion.
      - Endocrine Cells: Pancreatic islets (islets of Langerhans).
        - Alpha (α) cells: Produce glucagon (hyperglycemic hormone).
        - Beta (β) cells: Produce insulin (hypoglycemic hormone).
Pancreas - Glucagon
  • Function: Extremely potent hyperglycemic agent.
  • Triggers: Decreased blood glucose levels, rising amino acid levels, sympathetic nervous system activity.
  • Mechanism: Raises blood glucose levels by targeting the liver to:
      - Glycogenolysis: Break down glycogen into glucose.
      - Gluconeogenesis: Synthesize glucose from lactic acid and other non-carbohydrates.
      - Release: Glucose enters the bloodstream.
Pancreas - Insulin
  • Function: Secreted when blood glucose levels increase.
  • Synthesis: Produced as proinsulin and modified before release.
  • Mechanism: Lowers blood glucose in three ways:
      1. Enhances membrane transport of glucose into fat and muscle cells.
      2. Inhibits breakdown of glycogen to glucose.
      3. Inhibits conversion of amino acids or fats to glucose.
  • Additional Functions:
      - Triggers oxidation of glucose for ATP production (first priority).
      - Polymerizes glucose to form glycogen.
      - Converts glucose to fat in adipose tissue.
      - Plays a role in neuronal development, learning, and memory.
      - Important to note: Insulin is not required for glucose uptake in the liver, kidney, and brain.
Diabetes Mellitus (DM)
  • Types of DM:
      1. Type 1: Hyposecretion of insulin (autoimmune).
      2. Type 2: Hypoactivity of insulin (lifestyle factors).
  • Symptoms of High Blood Glucose Levels:
      - Nausea leading to sympathetic response (fight-or-flight response) increasing blood glucose levels.
      - Glycosuria: Excess glucose spilled into urine.
  • Three Cardinal Signs of DM:
      1. Polyuria: High urine output due to glucose acting as an osmotic diuretic.
      2. Polydipsia: Excessive thirst resulting from water loss.
      3. Polyphagia: Excessive hunger; cells cannot uptake glucose and appear “starving.”
  • Metabolic Effects of DM: When sugars cannot be used as fuel, fats are metabolized, causing lipidemia (high levels of fatty acids in blood) and ketone production leading to ketoacidosis.
      - Ketoacidosis Effects: May result in hyperpnea, disrupted heart activity, oxygen transport issues, and severe depression of the nervous system, possibly leading to coma or death.
Type 1 - Insulin Dependent Diabetes Mellitus
  • Prevalence: Accounts for 10% of DM cases.
  • Onset: Early onset (juvenile).
  • Mechanism: Autoimmune destruction of insulin-producing β islet cells in the pancreas.
  • Treatment: Involves testing and insulin injections (or pumps).
  • Long-Term Problems:
      - Blood vessel and nerve damage.
      - Lipidemia and high cholesterol can lead to atherosclerotic plaques, increasing likelihood of stroke and heart attack.
Type II - Non-Insulin Diabetes Mellitus
  • Prevalence: Accounts for 90% of DM cases.
  • Onset: Typically develops after age 40 (adult-onset).
  • Causes: Associated with obesity, diet, and sedentary lifestyle.
  • Mechanism: Body produces insulin normally but receptors reduce sensitivity due to excessive glucose levels in plasma.
  • Symptoms:
      - Blurry vision, fatigue, lethargy, irritability.
      - Fasting plasma glucose levels exceeding 126 mg/dL.
  • Consequences: Untreated DM can lead to complications affecting heart, nerves, eyes, and kidneys; it is the leading cause of blindness in the United States.
Thymus
  • Characteristics: Large in infants and children; shrinks with age.
  • Hormones: Secretes thymulin, thymopoietins, and thymosins.
  • Function: Involved in normal development of T lymphocytes in immune response; classified as hormones but act as paracrines.