Endocrine System

Overview of the Endocrine System

The endocrine system is a network of glands that secrete hormones directly into the bloodstream to regulate various physiological processes. It plays a critical role in maintaining homeostasis, growth, metabolism, and reproduction.

• Endocrine glands: Ductless glands that release hormones into the blood (e.g., pituitary, thyroid, adrenal glands).

• Endocrinology: The scientific study of hormones and the endocrine system.

• Target: Cells or organs that respond to specific hormones due to the presence of appropriate receptors.

Hormone Types and Mechanisms

• Amino acid-based/water-soluble/nonsteroid hormones: Hormones derived from amino acids (e.g., insulin, epinephrine) that usually act via second messenger systems.

• Steroid/lipid-soluble hormones: Hormones derived from cholesterol (e.g., cortisol, estrogen) that can cross cell membranes and bind to intracellular receptors.

• Second messenger mechanism (cyclic AMP = cAMP): Many water-soluble hormones use cAMP as a second messenger to relay signals inside the cell.

• Half-life: The time required for half the amount of a hormone to be removed from the bloodstream.

• Down-regulation vs. up-regulation: Decrease or increase in the number of hormone receptors on target cells in response to hormone levels.

• Autocrines and paracrines: Local chemical messengers; autocrines act on the same cell that secretes them, paracrines act on nearby cells.

Major Endocrine Glands and Hormones

• Hypothalamus: Brain region controlling the pituitary gland; produces releasing and inhibiting hormones (e.g., CRH, GHRH, TRH, GnRH, GHIH).

• Pituitary gland: Master gland with two lobes:

  • Anterior pituitary lobe/adenohypophysis: Secretes tropic hormones (e.g., ACTH, FSH, LH, GH, PRL, TSH).

  • Posterior pituitary lobe/neurohypophysis: Stores and releases ADH and oxytocin produced by the hypothalamus.

• Thyroid gland: Produces thyroid hormones (T3, T4) and calcitonin (CT).

• Parathyroid glands: Secrete parathyroid hormone (PTH) to regulate calcium levels.

• Adrenal glands:

  • Adrenal cortex: Produces corticosteroids (mineralocorticoids, glucocorticoids, gonadocorticoids).

  • Adrenal medulla: Produces catecholamines (epinephrine and norepinephrine).

• Pancreas: Secretes insulin and glucagon to regulate blood glucose.

• Pineal gland: Produces melatonin, involved in circadian rhythms.

• Thymus: Secretes thymulin, thymopoietins, and thymosins; important for T cell maturation.

• Gonads (ovaries and testes): Produce sex hormones (estrogens, progestogens, androgens).

Key Hormones and Their Functions

• Adrenocorticotropic hormone (ACTH): Stimulates adrenal cortex to release corticosteroids.

• Antidiuretic hormone (ADH/vasopressin): Promotes water reabsorption in kidneys.

• Calcitonin (CT): Lowers blood calcium levels.

• Corticosteroids: Include mineralocorticoids (e.g., aldosterone), glucocorticoids (e.g., cortisol), and gonadocorticoids (e.g., androgens, estrogens).

• Follicle-stimulating hormone (FSH) & Luteinizing hormone (LH): Gonadotropins that regulate reproductive processes.

• Glucagon: Raises blood glucose by stimulating glycogen breakdown.

• Insulin: Lowers blood glucose by promoting cellular uptake.

• Growth hormone (GH/somatropin): Stimulates growth and metabolism.

• Melatonin: Regulates sleep-wake cycles.

• Oxytocin (OT): Stimulates uterine contractions and milk ejection.

• Parathyroid hormone (PTH): Increases blood calcium levels.

• Prolactin (PRL): Promotes milk production.

• Thyroid hormones (T3, T4): Regulate metabolism.

• Thyroid-stimulating hormone (TSH): Stimulates thyroid hormone release.

Hormone Regulation Mechanisms

• Stimulud (stimuli): Hormonal (other hormones), humoral (blood levels of ions/nutrients), or neural (nerve fibers) triggers for hormone release.

• Renin-angiotensin-aldosterone (RAA) mechanism: Regulates blood pressure and fluid balance via aldosterone secretion.

Examples and Applications

• Example: The hypothalamus releases corticotropin-releasing hormone (CRH), which stimulates the anterior pituitary to secrete ACTH, which in turn stimulates the adrenal cortex to produce cortisol.

Lymphatic/Immune System

Overview of the Lymphatic and Immune Systems

The lymphatic system is a network of vessels and organs that helps maintain fluid balance and houses immune cells. The immune system defends the body against pathogens and foreign substances through innate and adaptive mechanisms.

• Lymph: Fluid transported by lymphatic vessels, containing white blood cells.

• Lymphatic vessels/lymphatics: Transport lymph throughout the body.

• Lymph nodes: Filter lymph and house immune cells.

• Lymphoid tissues: Include lymph nodes, spleen, tonsils, and MALT.

Components of the Immune System

• White blood cells (leukocytes): Include basophils, eosinophils, lymphocytes, monocytes, and neutrophils.

• B cells/B lymphocytes: Produce antibodies; responsible for humoral immunity.

• T cells/T lymphocytes: Mediate cellular immunity; include cytotoxic T cells (TC), helper T cells (TH), regulatory T cells (TReg), and memory T cells.

• Natural killer (NK) cells: Destroy infected or cancerous cells nonspecifically.

• Macrophages and dendritic cells: Phagocytic cells that present antigens to lymphocytes.

• Plasma cells: Differentiated B cells that secrete antibodies.

Immune Response and Defense Mechanisms

• Innate/nonspecific defense system: First and second lines of defense; includes barriers (skin, mucosa), phagocytes, inflammation, fever, and antimicrobial proteins.

• Adaptive/acquired/specific defense system: Third line of defense; involves lymphocytes and the production of antibodies.

• Humoral/antibody-mediated immunity: B cells produce antibodies that neutralize pathogens.

• Cellular/cell-mediated immunity: T cells directly attack infected or abnormal cells.

• Immunity: The ability to resist infection and disease.

• Immunocompetence: The ability of immune cells to recognize specific antigens.

Antigens, Antibodies, and MHC

• Antigen (foreign)/heteroantigen: Any substance that elicits an immune response.

• Self-antigens/autoantigens: Body's own molecules; usually not attacked by the immune system.

• Antibody/immunoglobulin (Ig): Proteins produced by B cells that bind to specific antigens.

• Major histocompatibility complex (MHC) proteins: Cell surface proteins essential for antigen presentation and immune recognition.

Lymphatic Organs and Tissues

• Spleen: Filters blood, recycles red blood cells, and houses immune cells.

• Tonsils: Protect against pathogens entering through the mouth and nose.

• Thymus: Site of T cell maturation.

• Appendix/vermiform process: Contains lymphoid tissue; may play a role in immunity.

• Peyer’s patches/aggregated lymphoid nodules: Lymphoid tissue in the small intestine.

• Mucosa-associated lymphoid tissue (MALT): Lymphoid tissue in mucous membranes throughout the body.

Immune Response Lines of Defense

• 1st line: Physical and chemical barriers (skin, mucous membranes).

• 2nd line: Innate immune responses (phagocytes, inflammation, fever, antimicrobial proteins).

• 3rd line: Adaptive immune responses (B and T lymphocytes, antibodies).

Other Key Terms and Concepts

• Pathogens: Disease-causing organisms (bacteria, viruses, fungi, parasites).

• Phagocytes: Cells that engulf and digest pathogens (e.g., macrophages, neutrophils).

• Complement proteins: Plasma proteins that enhance immune responses.

• Antimicrobial proteins: Proteins that inhibit microbial growth (e.g., interferons, complement).

• Inflammation: Localized response to injury or infection, characterized by redness, heat, swelling, and pain.

• Fever: Elevated body temperature that enhances immune function.

• Autoimmune diseases: Disorders in which the immune system attacks self-antigens.

• Heparin and histamine: Chemicals released by basophils and mast cells during inflammation; heparin is an anticoagulant, histamine increases vascular permeability.

• Reticular dense connective tissue: Structural framework for lymphoid organs.

Example Table: Comparison of Innate and Adaptive Immunity

Examples and Applications

• Example: Upon infection, dendritic cells present antigens to helper T cells, which activate B cells to produce antibodies and cytotoxic T cells to destroy infected cells.

Additional info: This guide expands on the provided list by grouping terms into logical topics and providing definitions, examples, and a comparison table for clarity.