Endocrine System

Overview of the Endocrine System

The endocrine system is a network of glands that secrete hormones to regulate bodily functions such as growth, metabolism, and reproduction. Hormones are chemical messengers that travel through the bloodstream to target organs.

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

• Endocrinology: The study of hormones, endocrine glands, and their functions.

• Target: The specific cell, tissue, or organ affected by a hormone.

Hormone Types and Mechanisms

Hormones are classified based on their chemical structure and solubility, which affects their mechanism of action.

• Amino acid-based/water-soluble/nonsteroid hormones: Include most hormones except those derived from cholesterol; act via second messenger systems (e.g., cAMP).

• Steroid/lipid-soluble hormones: Derived from cholesterol; include corticosteroids and sex hormones; act directly on DNA in the nucleus.

• Second messenger mechanism (cyclic AMP = cAMP): Water-soluble hormones bind to cell surface receptors, activating intracellular signaling cascades. (via adenylyl cyclase)

• 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 receptor number in response to hormone levels.

Major Endocrine Glands and Hormones

Each gland produces specific hormones with distinct functions.

• Hypothalamus: Regulates pituitary gland via releasing and inhibiting hormones (e.g., CRH, GnRH, GHRH, GHIH, TRH).

• Pituitary gland: Divided into anterior (adenohypophysis) and posterior (neurohypophysis) lobes.

• Anterior pituitary hormones: ACTH, FSH, LH, GH, PRL, TSH.

• Posterior pituitary hormones: ADH (vasopressin), OT (oxytocin).

• Thyroid gland: Produces TH (T3, T4), calcitonin.

• Parathyroid glands: Produce PTH (parathormone).

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

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

• Pancreas: Produces insulin and glucagon.

• Pineal gland: Produces melatonin.

• Thymus: Produces thymulin, thymopoietins, thymosins.

Hormone Regulation and Feedback

Hormone release is controlled by various stimuli and feedback mechanisms.

• Stimulud: Hormonal (other hormones), humeral (blood levels of ions/nutrients), neural (nerve impulses).

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

• Tropic hormones/tropins: Hormones that stimulate other endocrine glands (e.g., TSH, ACTH).

Key Hormones and Their Functions

Other Hormonal Concepts

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

• Human chorionic gonadotropin (hCG): Produced by placenta during pregnancy.

• Inhibin: Inhibits FSH production.

Lymphatic/Immune System

Overview of the Lymphatic and Immune Systems

The lymphatic system is a network of vessels and tissues that helps maintain fluid balance and provides defense against pathogens. The immune system protects the body from disease through innate and adaptive mechanisms.

• Lymph: Fluid collected from tissues, returned to the bloodstream.

• Lymphatic vessels: Transport lymph; include capillaries, trunks, ducts.

• Lymphoid tissues: Sites of immune cell development (e.g., lymph nodes, spleen, tonsils, thymus).

Lines of Immune Defense

The immune response is organized into three lines of defense.

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

• 2nd line: Innate/nonspecific defense (phagocytes, inflammation, antimicrobial proteins, fever).

• 3rd line: Adaptive/acquired/specific defense (B and T lymphocytes, antibodies).

Innate (Nonspecific) Defense System

Provides immediate, general protection against pathogens.

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

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

• Inflammation: Local response to injury; involves histamine, heparin.

• Antimicrobial proteins: Include complement proteins, interferons.

• Fever: Elevated body temperature to inhibit pathogens.

Adaptive (Specific) Defense System

Targets specific pathogens and provides long-lasting immunity.

• Humoral/antibody-mediated immunity: B cells produce antibodies (immunoglobulins, Ig).

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

• Antigen: Foreign substance triggering immune response.

• Antibody: Protein that binds to antigens.

• Immunocompetence: Ability of immune cells to recognize antigens.

• Memory B and T cells: Provide rapid response upon re-exposure.

• Major histocompatibility complex (MHC) proteins: Present antigens to T cells.

Immune Cells and Structures

Lymphatic Structures and Tissues

• Lymph nodes: Filter lymph, house immune cells.

• Spleen: Filters blood, stores immune cells.

• Tonsils: Protect against inhaled/ingested pathogens.

• Thymus: Site of T cell maturation.

• Mucosa-associated lymphoid tissue (MALT): Includes Peyer's patches, appendix.

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

Immunity and Disease

• Immunity: Resistance to disease.

• Autoimmune diseases: Immune system attacks self-antigens.

• Self-antigens/autoantigens: Normal body molecules recognized as "self".

• Pathogens: Disease-causing organisms.

Summary Table: Lymphatic and Immune System Components

Additional info:

• Some hormone and immune cell functions were expanded for clarity.

• Tables were inferred and organized for comparison and classification.