Endocrine System Quiz

Endocrine System Overview

• The endocrine system is a chemical messenger system
• It's one of the body's regulatory systems
• It comprises glands and cells that make and secrete hormones

Endocrine System Functions

• Regulates the internal body environment (e.g., blood pressure, blood glucose levels)
• Controls and integrates the reproductive system
• Maintains water and electrolyte balance
• Supports growth and development
• Regulates cellular metabolism (e.g., body's basal metabolic rate)
• Mobilizes body defenses (e.g., epinephrine, norepinephrine)

Endocrine vs Exocrine Glands

• Exocrine glands have ducts, secreting products onto the epithelial surface. Examples are sweat, sebaceous, mammary, and digestive enzyme glands.
• Endocrine glands are ductless, secreting products (hormones) directly into the internal environment (blood).

Nervous vs Endocrine Systems

Major Endocrine Glands

• Hypothalamus
• Pituitary gland
• Thyroid gland
• Parathyroid glands
• Adrenal glands
• Pancreas
• Ovaries
• Testes

Hormones & Their Functions

(Note: The provided information doesn't enumerate all hormones. A more exhaustive list is missing from this document)

Secretion of Posterior Pituitary Hormones

• Hormones are made and packaged within neurons' cell bodies.
• Vesicles transport hormones down the neuron.
• Vesicles containing hormones are stored in the posterior pituitary.
• Hormones are released into the bloodstream.

Oxytocin

• Synthesized in the cell bodies of hypothalamic neurons (paraventricular nucleus)
• Stored in the posterior pituitary
• Powerful uterine stimulant (regulates contractions)
• Stimulates milk ejection ("letdown reflex")
• Increases contraction in the vas deferens (ejaculation)

Antidiuretic Hormone (ADH or Vasopressin)

• Synthesized in the cell bodies of hypothalamic neurons (supraoptic nucleus)
• Stored in the posterior pituitary
• Regulates water balance
• In the absence of ADH, collecting tubules/ducts become impermeable to water, causing excess water loss in the urine.
• ADH binds to its receptor, activating translocation of vesicles with aquaporins to apical cell membranes, increasing water reabsorption.

Control of ADH Release

• Increased extracellular fluid osmolarity stimulates ADH secretion (osmoreceptors)
• Low blood volume/pressure stimulates ADH secretion (baroreceptors)
• Physiological stress (e.g., pain, fear, trauma) stimulates ADH release.

Regulation of ADH

• Hypothalamus receives feedback from osmoreceptors, aortic arch baroreceptors, carotid baroreceptors, and atrial stretch receptors

Effects of ADH on Blood Vessels

• ADH (via V1 receptors) causes vasoconstriction increasing blood pressure.

ADH Disorders

• Diabetes Insipidus:
  • Neurogenic (central): failure of hypothalamus/posterior pituitary to produce ADH
  • Nephrogenic: kidney's inability to respond to ADH
• Syndrome of Inappropriate Antidiuretic Hormone (SIADH): excessive ADH secretion

Diabetes Insipidus Treatment

• Desmopressin (DDAVP): synthetic analog of ADH with longer duration and higher potency (less side effects)

Treatment of Nephrogenic DI

• Correction of any underlying cause
• Provision of adequate fluids and calories
• Low sodium diet
• Diuretics
• High-dose DDAVP

Summary of Posterior Pituitary Hormones Actions

• Hypothalamus-stimuli: stress, increased plasma osmolarity
• Pituitary-actions: ADH (kidney-increased water reabsorption)
• Pituitary-actions: Oxytocin (uterus-contractions, mammary glands-milk ejection)
• Neural-stimuli: suckling, parturition

Classification of Hormones

• Steroids: Derived from cholesterol. Examples include cortisol, aldosterone, estrogen, progesterone, testosterone.
• Proteins/Peptides: Chains of amino acids. Examples include insulin, glucagon, growth hormone, thyroid-stimulating hormone (TSH), follicle-stimulating hormone (FSH), luteinizing hormone (LH).
• Catecholamines: Derived from tyrosine. Examples include epinephrine (adrenaline), norepinephrine (noradrenaline).

Hormone Synthesis (Protein & Peptides)

• Synthesized on the rough ER.
• Preprohormones are created.
• Preprohormones are cleaved into prohormones.
• Prohormones undergo further processing to become hormones.
• Hormones are stored in secretory vesicles.

Hormone Synthesis (Thyroid)

• Thyroid hormones (T3 and T4) are synthesized and stored in the follicles of the thyroid gland.
• Thyroglobulin is the protein that contains T3 and T4.

Hormone Synthesis (Steroid)

• Synthesized from cholesterol and produced in endocrine cells.
• Little to no storage before release.

Transport of Hormones

• Water-soluble hormones travel dissolved in blood plasma.
• Lipid-soluble hormones are transported bound to plasma proteins.

Clearance of Hormones

• Hormones are removed from the blood by various metabolic processes, binding with tissues, and excretion by the liver or kidneys.

General Principles of Endocrine Functions

• Hormones are specific signals and subject to feedback regulation.
• Hormones bind to receptors triggering immediate and long-term effects.
• Some hormones affect several cell types.

Mechanism of Hormone Action

• Hormones can bind to membrane receptors (peptide/catecholamines) or intracellular receptors (steroids/thyroid).
• Number of receptors can change over time (downregulation/upregulation).

Endocrine Pancreas

• Composed of exocrine acini (~99%) and endocrine Islets of Langerhans
• Major hormones produced include insulin, glucagon, amylin, somatostatin, and pancreatic polypeptide (PP).

Islet Cells

• Irregular patches of endocrine tissue within the pancreas (~1-2 million)
• Major cells: alpha (glucagon), beta (insulin, amylin), delta (somatostatin), F (PP)
• Interrelated cell-to-cell communication regulating hormone release.

Insulin Secretion

• Stimulated by: glucose, amino acids, GI hormones.
• Factors inhibiting insulin secretion: glucagon, somatostatin, and catecholamines.
• Mechanism of insulin release: glucose entering the beta cells leads to increased ATP, closing K+ channels and depolarization, stimulating Ca2+ entry, initiating exocytosis of insulin storage granules.
• Insulin is important in allowing glucose uptake and use, inhibiting glucose production, stimulating protein synthesis, and promoting lipid synthesis.

General Insulin Action at Target Receptors

• Insulin binds to receptor subunits leading to cellular responses.
• The insulin receptor is a combination of 4 subunits
• Insulin regulates carbohydrate, protein, and fat metabolism
• GLUT4 translocation to cell membrane ↑ glucose uptake
• GLUT2 is critical in beta cells and the liver.

Factors Affecting Insulin Secretion

• Elevated blood glucose, amino acids, and free fatty acids stimulate insulin secretion
• Decreased blood glucose, fasting, somatostatin, and alpha-adrenergic activity inhibit insulin secretion

Diabetes Mellitus (DM)

• A syndrome of impaired carbohydrate, fat, and protein metabolism due to either lack of insulin secretion or decreased tissue sensitivity to insulin.
• Two general types:
  • Type 1: Insulin-dependent diabetes mellitus (IDDM), caused by lack of insulin secretion
  • Type 2: Non-insulin-dependent diabetes mellitus (NIDDM), caused by reduced sensitivity of target tissues to insulin

Gland Hypersecretions and Hyposecretions