The Endocrine System Lecture Flashcards

Introduction to the Endocrine System and Homeostasis

  • Homeostasis is primarily maintained by two systems: the nervous system and the endocrine system.
  • The Nervous System:     * Utilizes two types of intercellular communication: electrical (direct action of an electrical potential) and chemical signaling (action of chemical neurotransmitters).     * Acts locally and rapidly.
  • The Endocrine System:     * Utilizes one method of intercellular communication: chemical signaling.     * Acts over greater distances and generally not as fast as the nervous system.     * Organs secrete hormones directly into the extracellular fluid.
  • Hormones: The chemical messengers of the endocrine system. They are transported primarily via the bloodstream throughout the body.
  • Target Cells: Any cells that possess a specific receptor for a hormone, allowing them to bind and respond to the signal.

Comparative Analysis: Endocrine vs. Nervous Systems

  • Signaling Mechanism:     * Endocrine: Chemical.     * Nervous: Chemical and Electrical.
  • Primary Chemical Signal:     * Endocrine: Hormones.     * Nervous: Neurotransmitters.
  • Distance Traveled:     * Endocrine: Long or short.     * Nervous: Always short.
  • Response Time:     * Endocrine: Fast or slow.     * Nervous: Always fast.
  • Environment Targeted:     * Endocrine: Internal.     * Nervous: Internal and external.

Physiological Processes and Target Cell Responses

  • Major Processes Controlled by Hormones:     1. Growth and Development.     2. Reproduction.     3. Metabolism.     4. Fluid and Electrolyte Balance.     5. Immunity.     6. Sleep.
  • Specific Responses in Target Cells:     * Stimulation of protein synthesis (e.g., enzymes, antibodies, hemoglobin, melanin, keratin, cartilage, and protein hormones).     * Activation or deactivation of enzymes.     * Alteration of cell membrane permeability.     * Altered rates of mitosis and cell growth (e.g., Puberty: growth hormone increasing muscle and bone growth).     * Stimulation of secretory activity (e.g., Thyroid Stimulating Hormone causing the thyroid gland to secrete thyroid hormone).

Chemical Classifications of Hormones

  • A hormone's chemical type dictates its distribution, receptor binding, and function.
  • 1. Hormones derived from Amino Acids (AAAA):     * Includes amines, peptides, and proteins.     * Examples: Melatonin, epinephrine, and growth hormone.     * Solubility Issue: These cannot pass through the cellular plasma membrane.     * Receptor Location: Outside of the cell (on the plasma membrane).
  • 2. Hormones derived from Lipids:     * Includes steroids.     * Examples: Testosterone, estrogen, and cortisol.     * Solubility: Since the plasma membrane is lipid-based, these are soluble and can pass through it.     * Receptor Location: Inside the cell (cytoplasm or nucleus).
  • Important Exception: Thyroid hormone is a protein-type hormone, yet it can pass through the plasma membrane.

Pathways of Hormone Action

  • The Pathway of Hormone Action is determined by the location of the receptor.
  • Intracellular Hormone Receptor Pathway:     * Utilized by steroid (lipid) and thyroid hormones.     * Process:         1. Lipid-soluble hormone diffuses through the plasma membrane.         2. Hormone binds with a receptor in the cytoplasm, forming a receptor-hormone complex.         3. The complex enters the nucleus and triggers gene transcription.         4. Transcribed mRNA is translated into proteins that alter cell activity.
  • Cell Membrane receptor Pathway:     * Utilized by amines, AAAA-derived, protein, and peptide hormones.     * Mechanism: Uses a second messenger.     * Primary second messenger: Cyclic Adenosine Monophosphate (cAMPcAMP).

Feedback Loops and Hormone Regulation

  • Positive Feedback Loops:     * Characterized by the release of additional hormone in response to an original hormone release.     * Only two occur in the human body: childbirth and blood clotting.     * Childbirth Example: Oxytocin signals uterine contractions, pushing the fetus toward the cervix. The stretching of the cervix signals the pituitary gland to release more oxytocin, intensifying labor. Levels decrease after birth.
  • Negative Feedback Loops:     * The most common regulation method in the body.     * Characterized by the inhibition of further secretion in response to adequate hormone levels.     * Example: Glucocorticoids from the adrenal glands. As concentrations rise, the hypothalamus and pituitary reduce signaling to the adrenal glands to stop secretion.

Mechanisms of Glandular Stimulation

  • Humoral Stimuli: Secretion in response to changes in blood levels of ions or nutrients.     * Example: Hypercalcemia (high blood calcium) stimulates the thyroid to release calcitonin, which moves Ca+2Ca^{+2} into the bones.
  • Hormonal Stimuli: Secretion in response to another hormone produced by a different gland.     * Example: Pituitary gland produces TSH (Thyroid Stimulating Hormone), which stimulates the thyroid to release thyroid hormone.
  • Neural Stimuli: Secretion in response to neural signals.     * Example: Perceived danger triggers sympathetic neurons to signal the adrenal glands to secrete norepinephrine and epinephrine.

The Hypothalamus-Pituitary Complex

  • Overview: Considered the "command center" of the endocrine system.
  • Hypothalamus: A structure of the diencephalon; produces hormones and stores some in the pituitary until signaling their release.
  • Pituitary Gland (Hypophysis): Historically called the "Master Gland."     * Infundibulum: The stem suspending the pituitary from the hypothalamus.     * Neurohypophysis (Posterior Pituitary): Composed of neural tissue.     * Adenohypophysis (Anterior Pituitary): Composed of glandular tissue.     * Sella Turcica: The bony structure of the sphenoid bone that protects the pituitary.

The Posterior Pituitary (Neurohypophysis)

  • Does not produce its own hormones; stores and secretes hormones produced by the hypothalamus.
  • Hormones:     1. ADH (Anti-diuretic Hormone): Stimulates water reabsorption by the kidneys.         * Alcohol Effect: Alcohol inhibits ADH release, leading to increased urine production, dehydration, and hangovers.         * Diabetes Insipidus: Chronic underproduction of ADH causing chronic dehydration and electrolyte imbalances. Solute concentration remains high despite fluid consumption because water reabsorption is not triggered.     2. Oxytocin: Stimulates uterine contractions during childbirth and milk let-down in breast tissue.

The Anterior Pituitary (Adenohypophysis)

  • Manufactures its own hormones, following hypothalamic signals.
  • Hormones and Targets:     1. Growth Hormone (GH): Targets long bones and skeletal muscles to undergo mitosis/cell division.     2. Thyroid Stimulating Hormone (TSH / Thyrotropin): Targets the thyroid gland to secrete thyroid hormone.     3. Adrenocorticotropic Hormone (ACTH): Targets the adrenal cortex to secrete corticosteroids like cortisol.     4. Follicle-Stimulating Hormone (FSH): Targets ovaries (maturation of egg) and testes (sperm production).     5. Luteinizing Hormone (LH): Triggers ovulation in females and testosterone production in males.     6. Prolactin: Stimulates lactation (milk production) in breast tissue.

Growth Hormone Disorders

  • Gigantism: Excess GH in children (open growth plates). Results in extreme height with normal proportions.
  • Acromegaly: Excess GH in adults (closed growth plates). Results in enlarged bones in hands, face, and feet.     * Named Examples: Andre Rene Roussimoff (Andre the Giant) in The Princess Bride; Richard Kiel (Jaws in 007 and a fan in Happy Gilmore).
  • Pituitary Dwarfism: Abnormally low GH in children. Results in small stature with normal proportions.     * Named Example: Verne Troyer (Mini-Me in Austin Powers).

Tropic Hormones

  • Definition: A hormone that has another endocrine gland as its target tissue.
  • Examples:     * ACTH (Adrenocorticotropic hormone): Targets the adrenal gland.     * Thyrotropic hormone: Targets the thyroid gland.     * Gonadotropins: Hormones regulating gonad function. Levels are regulated by GnRH through negative feedback.

The Thyroid Gland

  • Location: Anterior to the trachea, inferior to the larynx.
  • Hormones:     1. Thyroid Hormone: Includes T3T_3 (triiodothyronine) and T4T_4 (thyroxine). Regulates metabolism, heat production, and protein synthesis. These are produced in the colloid center of thyroid follicles.     2. Calcitonin: Lowers blood calcium levels.
  • Essential Element: Iodine is required for thyroid hormone production.
  • Pathologies:     * Goiter: Enlarged thyroid caused by iodine deficiency. Low T3T_3/T4T_4 causes the pituitary to overproduce TSH, overstimulating the gland.     * Cretinism (Neonatal Hypothyroidism): Thyroid deficiency in children. Causes cognitive deficits, short stature, and potential deafness/muteness. Iodine deficiency is the top cause of preventable mental retardation.     * Myxedema: Hypothyroidism in adults. Symptoms include low metabolic rate, weight gain, constipation, cold extremities, and reduced mental activity.     * Hyperthyroidism: Often caused by tumors. Graves disease is an autoimmune form.         * Symptoms: Exophthalmos (protruding eyes), tremors, weight loss, increased heart rate, and excessive sweating.

The Parathyroid Gland

  • Location: Posterior aspect of the thyroid gland.
  • Hormone: Parathyroid Hormone (PTH).
  • Function: Increases blood calcium levels by stimulating osteoclasts.
  • Disorders:     * Hyperparathyroidism: High PTH leads to excessive osteoclast activity, weakening bones and causing spontaneous fractures.     * Hypoparathyroidism: Low blood calcium levels. Increases sodium permeability, leading to muscle twitches, spasms, or fatal respiratory muscle paralysis.

The Adrenal Gland

  • Location: Superior surface of the kidneys.
  • Main Regions:     1. Adrenal Cortex: Governs three zones:         * Zona Glomerulosa: Releases mineralocorticoids (e.g., Aldosterone). Regulates salt/mineral balance, reabsorbing sodium to increase blood volume and blood pressure.         * Zona Fasciculata: Releases glucocorticoids (e.g., Cortisol and Cortisone). Regulates glucose metabolism, acts as anti-inflammatory, and resists long-term stress.         * Zona Reticularis: Produces androgens (male sex hormones).     2. Adrenal Medulla: Releases catecholamines (Epinephrine and Norepinephrine) stimulated by the sympathetic nervous system.         * Effects: Increased BP, HR, blood glucose, RR, and dilated pupils; decreased GI peristalsis.
  • Diseases:     * Cushing’s Disease: Overproduction of cortisol. Symptoms: "moon facies," "buffalo hump," weight gain, and immunosuppression.     * Addison’s Disease: Hyposecretion of corticosteroids. Symptoms: skin bronzing, low blood glucose/sodium, salt cravings, and general weakness.

The Pancreas and Pineal Glands

  • Pineal Gland: Secretes melatonin to regulate biological rhythms/sleep-wake cycles.
  • Pancreas Location: Retroperitoneal (behind the stomach, between the duodenum and spleen).
  • Islets of Langerhans: Hormone-producing cell clusters.     * Insulin: Produced by Beta cells. Decreases blood glucose levels. Antagonist to glucagon.     * Glucagon: Produced by Alpha cells. Increases blood glucose levels. Antagonist to insulin.
  • Diabetes Mellitus:     * Type 1 (5%5\% of cases): Autoimmune destruction of beta cells; requires insulin injections.     * Type 2 (95%95\% of cases): Insulin resistance. Treated with weight loss, pills, or insulin injections.     * Hyperglycemia: Elevated blood glucose.

Reproductive and Thymus Hormones

  • Testosterone: Produced by testes; governs reproductive development and secondary male characteristics.
  • Estrogen: Produced by ovaries; governs the menstrual cycle, secondary female characteristics, and pregnancy maintenance.
  • Progesterone: Produced by the corpus luteum; maintains pregnancy and regulates the menstrual cycle.
  • Human Chorionic Gonadotropin (hCG): Produced by the placenta. Promotes progesterone synthesis and protects fetus from immune rejection. Detected in urine for pregnancy tests.
  • Thymus Gland: Located behind the sternum in the mediastinum. Secretes thymosin, which aids the maturation of T lymphocytes (TT cells).

Secondary Endocrine Functions

  • Heart: Produces Atrial Natriuretic Peptide (ANP), which reduces sodium/water reabsorption to lower blood pressure.
  • Small Intestine:     * Secretin: Stimulates pancreatic bicarbonate release.     * Cholecystokinin (CCK): Promotes pancreatic enzyme secretion and bile release.
  • Kidneys:     * Renin: Released during low blood pressure; triggers the Renin-Angiotensin-Aldosterone System (RAAS) to increase blood pressure.     * Erythropoietin (EPO): Released during low blood oxygen; triggers Red Blood Cell (RBC) production in red bone marrow.