Human Endocrine System Study Notes

Fundamental Concepts of the Endocrine System

• Endocrine vs. Exocrine Glands:     * Endocrine Glands: Ductless glands that secrete hormones directly into the blood or lymph to affect distant target organs.     * Exocrine Glands: Glands that secrete products into ducts that carry the secretions to a specific body surface or cavity (e.g., sweat glands, digestive enzymes from the pancreas).

• Chemical Signaling Categories:     * Hormones: Long-distance chemical signals; travel in the blood or lymph throughout the body.     * Autocrine: Chemicals that exert effects on the same cells that secrete them.     * Paracrine: Locally acting chemicals that affect cells other than those that secrete them, within the same tissue.     * Pheromones: Chemical signals secreted by an individual that affect the behavior or physiology of other individuals of the same species.

• Classification by Solubility:     * Amino Acid-Based Hormones: Most hormones are in this category. They are usually water-soluble (except thyroid hormone) and cannot cross the plasma membrane.     * Steroid-Based Hormones: Synthesized from cholesterol. These are lipid-soluble and can cross the plasma membrane.

• Mechanism of Action: Water-Soluble Hormones (Cyclic AMP Second-Messenger Mechanism):     * Molecular Relay Race Metaphor: The process follows a sequence: Hormone (1st messenger) → Receptor → G protein → Enzyme → 2nd messenger.     * Step 1: The hormone ($1^{st}$ messenger) binds to a specific receptor on the plasma membrane.     * Step 2: The receptor activates a G protein ($G_s$) by displacing $GDP$ with $GTP$.     * Step 3: The G protein activates the effector enzyme, adenylate cyclase.     * Step 4: Adenylate cyclase converts $ATP$ to cAMP ($2^{nd}$ messenger).     * Step 5: cAMP activates protein kinases, which trigger responses in the target cell (activating enzymes, stimulating secretion, opening ion channels, etc.).

• Mechanism of Action: Lipid-Soluble Hormones (Direct Gene Activation):     * Step 1: The steroid hormone diffuses through the plasma membrane and binds to an intracellular receptor.     * Step 2: The receptor-hormone complex enters the nucleus.     * Step 3: The complex binds to a specific region on the $DNA$.     * Step 4: This binding initiates transcription of a gene into $mRNA$.     * Step 5: The $mRNA$ directs protein synthesis in the cytoplasm.

Hormone Classification and Transport

• Structural Classes:     * Biogenic Amines (Monoamines): Derived from amino acids (e.g., $Epinephrine$ ($EPI$), $Thyroxine$ ($T_4$)).     * Peptides/Proteins: Chains of amino acids (e.g., $Oxytocin$, $Insulin$).     * Steroids: Lipid-derived (e.g., $Testosterone$, $Estradiol$).

• Hormone Transport:     * Hydrophilic Messengers: Transported as dissolved messengers in the blood; they are secreted via exocytosis.     * Hydrophobic Messengers: Usually bound to carrier proteins (>99\% ). Only a small fraction is "Free hormone" (<1\% ); they are secreted by diffusion through the cell membrane.

• Hormone Interactions:     * Synergistic Effects: Two or more hormones work together to produce a greater effect (e.g., $FSH$ + $Testosterone$ for sperm production).     * Permissive Effects: One hormone cannot exert its full effect without another hormone being present (e.g., $Estrogen$ is necessary for the timing of $Progesterone$ action).     * Antagonistic Effects: One hormone opposes the action of another (e.g., $Insulin$ lowers blood glucose while $Glucagon$ raises it; $Estrogen$ can block the response to $PRL$).

• Hormone Clearance: The process by which hormones are removed from the blood, primarily by the liver and kidneys.

Stimuli for Hormone Release

• Humoral Stimulus:     * Definition: Hormone release caused by altered levels of certain critical ions or nutrients in the blood.     * Example: Low concentration of $Ca^{2+}$ in capillary blood stimulates the parathyroid glands to secrete Parathyroid Hormone ($PTH$), which increases blood $Ca^{2+}$.

• Neural Stimulus:     * Definition: Hormone release caused by neural input.     * Example: Action potentials in preganglionic sympathetic fibers travel to the adrenal medulla, stimulating the release of $Epinephrine$ and $Norepinephrine$.

• Hormonal Stimulus:     * Definition: Hormone release caused by another hormone (a tropic hormone).     * Example: Hormones from the hypothalamus stimulate the anterior pituitary to secrete hormones that further stimulate other endocrine glands (e.g., thyroid, adrenal cortex, gonads).

The Pancreas

• Dual Function: The pancreas is both an endocrine and an exocrine gland.

• Exocrine Function: Cells called acini produce digestive enzymes (to break down protein, fat, starch, and nucleic acids) and bicarbonate (to neutralize stomach acid). These are delivered to the duodenum via pancreatic ducts.

• Endocrine Function: Mediated by clusters of cells called the Islets of Langerhans.     * Alpha (α) Cells: Secrete Glucagon.     * Beta (β) Cells: Secrete Insulin.     * Delta (δ) Cells: Secrete somatostatin (noted in diagrams).

• Insulin (Amino acid hormone):     * Stimulus: Increased blood glucose, increased amino or fatty acids, or acetylcholine ($Ach$).     * Inhibition: Decreased blood glucose, $GH$, or sympathetic nervous system ($SNS$) activation.     * Function: Increases glucose uptake (especially in muscle and fat), promotes glycogenesis (storing glucose as glycogen) in the liver, and plays a role in the brain (learning, memory, feeding behavior).     * Hypersecretion: Results in hypoglycemia (low blood sugar).     * Hyposecretion: Results in Diabetes Mellitus.         * Type I: Genetic/autoimmune; the body destroys the Islets of Langerhans, resulting in no insulin production.         * Type II: Lifestyle-related; cells become resistant to insulin effects despite production.         * Symptoms: Polyuria (excessive urination), polydipsia (excessive thirst), polyphagia (excessive hunger), CNS depression/coma, acetone breath, and ketoacidosis.

• Glucagon (Amino acid hormone):     * Stimulus: Decreased blood glucose, $SNS$ activation, or increased amino acids.     * Inhibition: Increased blood glucose, insulin, or $GH$.     * Function: Raises blood glucose via glycogenolysis (breaking glycogen into glucose) and gluconeogenesis (creating glucose from fats and protein).     * Hyposecretion: Results in hypoglycemia.

• Glucose Homeostasis: Normal blood glucose level is approximately $90\,mg/100\,ml$.

The Thyroid Gland

• Anatomy: Consists of follicles filled with colloid and lined by follicular cells, plus parafollicular cells (C-cells).

• Thyroid Hormone (TH): Actually an amine-iodine hormone combination of two types:     * $T_4$ (Thyroxine): The primary form released by follicles.     * $T_3$ (Triiodone-thyrinine): Converted from $T_4$ at target cells.     * Stimulus: $TSH$ from the anterior pituitary.     * Functions: Increases metabolic rate and heat production (calorigenic effect), regulates development and growth, and regulates blood pressure.     * Hypersecretion: Grave's Disease (autoimmune attack producing an antibody that mimics $TSH$). Symptoms include increased metabolism, tachycardia, nervousness, weight loss, and exophthalmos (bulging eyes). Goiter can also result from overstimulation.     * Hyposecretion:         * Adults: Myxedema (mucous swelling). Low metabolic rate, chills, mental fatigue, constipation.         * Infants: Cretinism. Mental retardation and disproportionate body growth.         * Goiter: Occurs when the pituitary produces excess $TSH$ attempting to raise low $TH$ levels, overstimulating follicles.

• Calcitonin (Peptide):     * Stimulus: Increased blood calcium levels.     * Function: Lowers blood calcium and phosphate by moving them into bone.     * Mechanism: Increases osteoblast activity (bone building) and decreases osteoclast activity (bone breakdown).

The Parathyroid Glands

• Parathyroid Hormone (PTH): A protein hormone; the most important regulator of blood calcium.

• Stimulus: Decreased blood calcium.

• Function: Increases blood calcium levels and lowers blood phosphate levels.

• Target Organs:     * Bones: Increases osteoclast activity to break down bone and release calcium and phosphate into the blood.     * Kidneys: Increases reabsorption of calcium and secretion (loss) of phosphate. It also activates Vitamin D.     * Small Intestine: Vitamin D (activated by PTH) is required for the absorption of calcium from food.

The Adrenal Glands

• Adrenal Cortex (Outer part): Produces "corticosteroids." Layers are organized from outside in (mnemonic: GFR - Glomerulosa, Fasciculata, Reticularis / Salt, Sugar, Sex):     1. Zona Glomerulosa (Salt):         * Hormone: Aldosterone (a Mineralocorticoid).         * Stimulus: $Angiotensin\text{ II}$, increased blood potassium ($K^+$), $ACTH$, or decreased blood sodium ($Na^+$).         * Inhibition: Increased blood volume, increased BP, or decreased potassium.         * Function: Sodium reabsorption and potassium secretion to increase blood volume ($BV$) and blood pressure ($BP$).         * Hypersecretion: Aldosteronism (hypertension, edema, low potassium leading to paralysis).     2. Zona Fasciculata (Sugar):         * Hormone: Cortisol (a Glucocorticoid).         * Stimulus: $ACTH$.         * Inhibition: Negative feedback by increased cortisol levels.         * Function: Gluconeogenesis, mobilizing fats, protein catabolism, resistance to stress, and decreasing inflammation/immune response.         * Hypersecretion: Cushing's Syndrome (often from pituitary tumor). Characteristics: "Moon face," "Buffalo hump," unusual fat distribution, unusual tanning, easy bruising, and "steroid diabetes."     3. Zona Reticularis (Sex):         * Hormone: Androgens (estrogen and testosterone) (Gonadocorticoids).         * Role: Minimal in males; in females, drives libido and body hair, and provides estrogen after menopause.         * Hypersecretion: Masculinization or virilization in females.

• Adrenal Medulla (Middle part): Part of the Sympathetic Nervous System ($SNS$).     * Chromaffin Cells: Secrete Epinephrine ($EPI$) and Norepinephrine ($NE$).     * Stimulus: $SNS$ activation (short-term stress).     * Function: Fight-or-flight response (increased heart rate, BP, bronchodilation, glucose release, metabolic rate increase).     * Hypersecretion: Pheochromocytoma (cancer of chromaffin cells).

• Combined Stress Response:     * Short-Term: Mediated by the medulla via catecholamines.     * Long-Term: Mediated by the cortex; Aldosterone (retaining salt/water) and Cortisol (protein/fat conversion to glucose, immune suppression).

The Pituitary Gland (Hypophysis)

• Development: Developed from the Neurohypophyseal bud (ectoderm, brain) and the Hypophyseal pouch (outpocketing of oral mucosa).

• Posterior Pituitary (Neurohypophysis):     * Mechanism: A neural connection via the hypothalamic-hypophyseal tract. Hormones are synthesized in the hypothalamus and stored in axon terminals in the posterior lobe.     * Hormones:         * ADH (Antidiuretic Hormone/Vasopressin): From the supraoptic nucleus. Stimulus: increased plasma osmolarity, low $BV$, or pain. Function: stimulates aquaporin production in kidneys to increase water reabsorption. Diabetes Insipidus is the hyposecretion (polyuria), which can be Neurogenic (production failure) or Nephrogenic (kidney non-responsive). SIADH is the hypersecretion.         * Oxytocin: From the paraventricular nucleus. Stimulus: cervical stretching, suckling, touch. Function: uterine labor contractions, milk ejection, and bonding behaviors.     * Blood Transport Path (P.P.): Aorta → Common Carotid → Internal Carotid → Inferior Hypophyseal Artery → Capillaries of Posterior Pituitary → Hypophyseal Veins → Cavernous Sinus → Internal Jugular → Brachiocephalic → Superior Vena Cava.

• Anterior Pituitary (Adenohypophysis):     * Mechanism: Connected via the hypophyseal portal system (Primary capillary plexus in infundibulum → hypophyseal portal veins → secondary capillary plexus in anterior lobe). A portal system consists of two capillary beds connected by veins.     * Blood Transport Path (A.P.): Aorta → Common Carotid → Internal Carotid → Superior Hypophyseal Artery → Primary Capillary Plexus (picks up hypothalamic tropic hormones) → Hypophyseal Portal Veins → Secondary Capillary Plexus (releases A.P. hormones) → Hypophyseal Veins → Superior Vena Cava.     * Hormones of the Anterior Pituitary:         1. Growth Hormone (GH): Protein. Stimulated by $GHRH$. Function: growth of bone (cartilage), liver, and muscle; mobilizes fats; spares glucose. Gigantism (childhood excess), Acromegaly (adult excess), Pituitary Dwarfism (childhood deficiency). Side effect note: GH may shorten lifespan based on mice studies.         2. Thyroid-Stimulating Hormone (TSH): Glycoprotein. Stimulated by $TRH$. Function: Tropic to thyroid to release $T_3/T_4$.         3. Adrenocorticotropic Hormone (ACTH): Peptide. Stimulated by $CRH$. Function: Tropic to adrenal cortex. Derived from a large protein called POMC (Proopiomelanocortin), which also yields Endorphins and Melanocyte-Stimulating Hormone ($MSH$).         4. Follicle-Stimulating Hormone (FSH): Glycoprotein. Stimulated by $GnRH$. Function: Spermatogenesis (males), follicle maturation/estrogen (females).         5. Luteinizing Hormone (LH): Glycoprotein. Stimulated by $GnRH$. Function: Testosterone production (males), ovulation/progesterone (females).         6. Prolactin (PRL): Protein. Stimulated by a decrease in $PIH$ (Dopamine). Function: Lactation. Hypersecretion leads to galactorrhea.

Other Glandular Tissues

• Pineal Gland: Produces Melatonin (increases at night); regulates circadian rhythm and mood.

• Adipose Tissue: Produces Leptin, which decreases appetite.

• Stomach: Produces Ghrelin, which increases when fasting to stimulate hunger.

• Heart (Atria): Produces Atrial Natriuretic Peptide ($ANP$) to lower blood pressure.

• Kidneys: Produce Erythropoietin ($EPO$) to stimulate red blood cell production.

• Thymus: Produces Thymosin.