16B part one

Endocrine System Overview

General Information

  • Lecture slides prepared by Karen Dunbar Kareiva.

  • Ivy Tech Community College.

  • Chapter 16 Part B discusses the Endocrine System.

  • © 2016 Pearson Education, Inc.

Thyroid Gland

Anatomy

  • Butterfly-shaped gland located in the anterior neck, inferior to the larynx, on the trachea.

  • Isthmus: A median mass connecting the two lateral lobes of the thyroid gland.

Histology

  • Photomicrograph of thyroid gland follicles at 315x magnification:

    • Parafollicular cells: Secrete calcitonin.

    • Follicular cells: Secrete thyroid hormones (TH).

    • Colloid-filled follicles:

    • Colloid: Precursor to thyroid hormones (TH) containing thyroglobulin and iodine.

Thyroid Hormones (TH)

Types

  • T4 (Thyroxine):

    • Contains four iodine atoms and two tyrosine molecules.

    • Major form of TH.

  • T3 (Triiodothyronine):

    • Contains three iodine atoms and two tyrosine molecules.

    • Converted from T4 at the tissue level.

Functions

  • Major metabolic hormone of the body.

  • Initiates protein synthesis.

  • Increases the basal metabolic rate and body heat.

  • Regulates tissue growth and development.

  • Maintains blood pressure by increasing the number of adrenergic receptors in blood vessels.

Synthesis of Thyroid Hormone

  • Steps in synthesis:

    1. Iodide is actively transported into follicular cells and then trapped.

    2. Iodide oxidized to iodine.

    3. Iodine combines with tyrosine.

    • Results in DIT (diiodotyrosine): Two iodines + one tyrosine.

    • Results in MIT (monoiodotyrosine): One iodine + one tyrosine.

    1. MIT + DIT = T3

    2. DIT + DIT = T4

    3. Thyroglobulin is synthesized in follicular cells and discharged into the lumen of the follicle.

    4. Colloid endocytosed; enzymes cleave TH from colloid, allowing hormones to diffuse into the bloodstream.

Transport and Regulation

  • T4 and T3 are transported by thyroxine-binding globulins (TBGs).

  • Both bind to target receptors; however, T3 is 10 times more active than T4.

  • Peripheral tissues contain enzymes that convert T4 to T3 by removing one iodine.

Clinical Homeostatic Imbalance

  • Hypothyroidism (Myxedema):

    • Symptoms include decreased metabolic rate, thick/dry skin, puffy eyes, feeling cold, constipation, edema, mental sluggishness, and lethargy.

    • Caused by iodine deficiency leading to decreased TH levels.

    • Results in increased TSH secretion and potential goiter formation (enlarged thyroid).

  • Hyperthyroidism:

    • Most common type is Graves’ disease, an autoimmune disorder.

    • Body produces abnormal antibodies mimicking TSH, stimulating TH release.

    • Symptoms include elevated metabolic rate, sweating, rapid/irregular heartbeat, nervousness, and weight loss.

    • Exophthalmos: Swelling tissue behind eyes leading to protrusion.

Calcitonin

  • Secreted by parafollicular cells in response to high calcium (Ca2+) levels.

  • Acts as an antagonist to parathyroid hormone (PTH).

  • Physiological role in humans is unknown; however, at higher doses, it:

    • Inhibits osteoclast activity, preventing release of Ca2+ from the bone matrix.

    • Stimulates calcium uptake and incorporation into bone.

Parathyroid Glands

Anatomy

  • Comprise four to eight small glands embedded in the posterior aspect of the thyroid.

  • Contain two cell types:

    • Oxyphil cells: Function is currently unknown.

    • Parathyroid cells: Secrete parathyroid hormone (PTH).

Function of PTH

  • PTH is essential in calcium (Ca2+) homeostasis.

  • Secreted in response to low blood Ca2+ levels.

  • Inhibited by increasing blood Ca2+ levels.

  • Targets organs include skeleton, kidneys, and intestines to regulate Ca2+ levels.

Effects of PTH

  • Stimulus: Low blood calcium.

  • Response:

    • Increased activity of osteoclasts causing Ca2+ and phosphate ion release into blood.

    • Enhanced reabsorption of Ca2+ and secretion of phosphate in kidney tubules.

    • Activation of vitamin D in the kidneys.

    • Increased Ca2+ absorption from food in the intestines.

Adrenal Gland

Anatomy

  • Paired, pyramid-shaped organs situated atop the kidneys.

  • Each adrenal gland functions as two glands:

    • Adrenal cortex: Composed of three layers of glandular tissue, synthesizing and secreting corticosteroids.

    • Adrenal medulla: Composed of nervous tissue, affiliated with the sympathetic nervous system.

Adrenal Cortex Functions

  • Produces over 24 different hormones known collectively as corticosteroids.

  • Contains three layers:

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

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

    • Zona reticularis: Produces gonadocorticoids (sex hormones).

Mineralocorticoids

  • Regulate electrolyte concentrations in extracellular fluid (ECF) — particularly sodium (Na+) and potassium (K+).

  • Importance of Na+ in affecting ECF volume and blood pressure.

  • Aldosterone:

    • The most potent mineralocorticoid, constituting >95% of this class.

    • Stimulates Na+ reabsorption, thus increasing blood volume and blood pressure, while promoting K+ secretion to eliminate excess potassium.

Regulation of Aldosterone Release

  • Aldosterone effects are short-lived (~20 mins), allowing for precise control of electrolyte balance.

  • Regulated by:

    • Renin-angiotensin-aldosterone system (RAAS).

    • Plasma potassium concentration.

    • ACTH (Adrenocorticotropic hormone).

    • Atrial natriuretic peptide (ANP).

Major Mechanisms Controlling Aldosterone Release

  • Renin-Angiotensin-Aldosterone Mechanism:

    • Decreased blood volume/pressure activates special kidney cells releasing renin into the bloodstream.

    • Renin acts on angiotensinogen (plasma protein) to form angiotensin I, which is then converted to angiotensin II by ACE (angiotensin-converting enzyme).

    • Angiotensin II stimulates aldosterone release from the adrenal cortex, leading to increased Na+ and water reabsorption in the kidneys and effective circulating volume.

Glucocorticoids

  • Primarily involved in metabolic processes and stress resistance.

  • Keeps blood glucose levels stable despite fluctuating food intake.

  • Cortisol (hydrocortisone) is the primary glucocorticoid in significant amounts in humans.

  • Other glucocorticoids include cortisone and corticosterone.

Actions of Cortisol

  • Essential for managing crises by promoting gluconeogenesis: the formation of glucose from non-carbohydrate sources (e.g., fats and proteins).

  • Protects the nervous system and maintains blood pressure through vasoconstriction, ensuring proper nutrient distribution to cells.

Regulation of Cortisol

  • The release of cortisol is stimulated through the CRH (Corticotropin-Releasing Hormone) and ACTH (Adrenocorticotropic hormone) pathway.

  • Rising cortisol levels provide negative feedback to inhibit the hypothalamus and anterior pituitary secretion.

  • Cortisol levels peak before waking and decrease before sleep.

Gonadocorticoids

  • Weak androgens predominantly serve as precursors for sex hormones (e.g., testosterone and estrogens).

  • Role in adult physiology is minor compared to gonadal hormones.

  • May contribute to secondary sex characteristics and sex drive in females.

Adrenal Medulla

  • Activation occurs during short-term stress, causing:

    • Vasoconstriction.

    • Increased heart rate and blood pressure.

    • Elevation of blood glucose levels.

    • Release of epinephrine (80% potency) and norepinephrine (20% potency) from medullary chromaffin cells.

Short vs Long-term Stress Response.

  • Short-term stress response:

    • Triggered by nerve impulses from sympathetic fibers leading to immediate physiological changes like increased heart rate and blood pressure.

  • Long-term stress response:

    • Activation of the hypothalamus leads to the sustained release of glucocorticoids such as cortisol, which retains sodium and water, thereby enhancing blood volume and blood pressure and suppressing the immune system.

Pineal Gland

Anatomy and Function

  • Located in the epithalamus of the brain.

  • Produces melatonin which regulates sleep/wake cycles.

    • Melatonin levels peak at night and drop during the day.

  • Also implicated in regulatory functions, such as timing of sexual maturation/puberty, and may affect physiological processes like body temperature and appetite.

Pancreas

Anatomy

  • Located partially behind the stomach.

  • Contains both exocrine (digestive) and endocrine (hormonal) cells:

    • Acinar cells: Produce enzyme-rich juice for digestion.

    • Pancreatic islets (islets of Langerhans): Contain endocrine cells that secrete hormones.

    • Alpha cells: Produce glucagon.

    • Beta cells: Produce insulin.

Hormonal Functions

  • Glucagon:

    • Released in response to low blood glucose, high amino acid levels, or SNS activation.

    • Functions to raise blood glucose levels through:

    • Glycogenolysis: breaking down glycogen into glucose.

    • Gluconeogenesis: synthesizing glucose from lactic acid and other substrates.

  • Insulin:

    • Produced when blood glucose levels elevate, synthesized as proinsulin, which is cleaved into two insulin molecules.

    • Lowers blood glucose through:

    • Enhancing glucose transport into cells (especially muscle and fat).

    • Inhibiting glycogen breakdown and conversion of amino acids/fats to glucose.

    • Not required for glucose uptake in liver, kidney, or brain tissue.

    • Influences neuronal development, learning, and memory.

Insulin Release Influencers

  • Elevated blood glucose levels (primary stimulus).

  • Rising levels of amino acids and fatty acids.

  • Release of ACh by the parasympathetic nervous system.

  • Influence of glucagon, epinephrine, growth hormone, and glucocorticoids inhibit insulin release.

Regulation of Blood Glucose

  • Insulin promotes glucose uptake, reducing blood glucose levels back to normal range, while glucagon promotes glycogen breakdown to elevate glucose levels when needed.

Clinical Homeostatic Imbalance: Diabetes Mellitus

  • Type 1 Diabetes: Low/absent insulin production due to autoimmune destruction of pancreatic beta cells.

  • Type 2 Diabetes: Presence of insulin but ineffective at lowering blood glucose levels.

  • Result: Blood glucose remains elevated post-meal, causing nausea and activating sympathetic responses, disrupting normal metabolism — leading to significant complications.

Cardinal Signs of Diabetes Mellitus

  • Polyuria: Excess glucose in urine leads to high urine output due to osmotic diuresis.

  • Polydipsia: Dehydration stimulates thirst centers, leading to excessive thirst.

  • Polyphagia: Inability of cells to absorb glucose leads to starvation despite high calorie intake, resulting in breakdown of proteins and fats.

Additional Hormonal Functions

Kidneys

  • Secrete erythropoietin to promote red blood cell production.

  • Secrete renin, which initiates the renin-angiotensin-aldosterone mechanism.

Skeleton

  • Osteoblasts in bones secrete osteocalcin, promoting insulin secretion from pancreatic beta cells and enhancing sensitivity to insulin.

  • Osteocalcin's levels are low in Type 2 diabetes.

Hormones from Other Organs

  • Skin: Produces cholecalciferol, a precursor to vitamin D crucial for calcium absorption.

  • Thymus: Shrinks with age; thymic hormones play a role in T cell development.