Endocrine System: Organization and Glands (Chapter 17/18 A&P2)

Comprehensive practice vocabulary flashcards covering the functional organization, chemical classes, receptor mechanisms, and major endocrine glands (hypothalamus, pituitary, thyroid, parathyroids, adrenals, pancreas, pineal, and thymus) along with their associated clinical disorders.

Hormones

Chemical signals secreted in small quantities by glands and endocrine cells into blood plasma, traveling to distant target cells (effectors) to regulate homeostasis, metabolism, growth, and reproduction.

Endocrine Glands

Ductless glands and specialized cells that secrete hormones directly into the circulatory system (bloodstream) rather than through ducts.

Exocrine Glands

Glands that release their non-hormonal secretions into ducts or onto exterior/interior body surfaces rather than directly into the bloodstream.

Frequency-Modulated Signals

A signaling pattern characteristic of the nervous system where stimulus intensity is encoded by the frequency of action potentials rather than their magnitude.

Amplitude-Modulated Signals

A signaling pattern characteristic of the endocrine system where signal intensity depends directly on the concentration (amplitude) of the circulating hormone in the blood.

Autocrine Messengers

Chemical messengers that are secreted into the extracellular fluid and act directly on the same cell that released them, often participating in positive feedback loops (e.g., white blood cells releasing cytokines).

Paracrine Messengers

Chemical messengers secreted into local extracellular fluid that exert their effects on neighboring, nearby target cells of a different type without traveling through the systemic circulation (e.g., clotting factors activating adjacent platelets).

Neurotransmitters

Chemical messengers released by a neuron across a synaptic cleft that activate an immediately adjacent postsynaptic cell, such as another neuron, muscle cell, or gland cell.

Endocrine Messengers

Hormones secreted by cells into the bloodstream that circulate through the cardiovascular system to affect distant target tissues.

Humoral Stimuli

The control of hormone secretion elicited by direct changes in the concentration of specific extracellular chemicals, ions, or nutrients in the blood (e.g., low blood $Ca^{2+}$ stimulating PTH secretion).

Neural Stimuli

The control of hormone secretion triggered directly by action potentials and neurotransmitter release from neurons onto endocrine cells (e.g., sympathetic stimulation prompting epinephrine release from the adrenal medulla).

Hormonal Stimuli

The regulation of hormone secretion governed by the action of another circulating hormone, typically involving hypothalamic releasing or inhibiting hormones that trigger tropic hormone release from the pituitary.

Chronic Hormone Secretion

A secretory pattern resulting in relatively stable, constant blood concentrations of a hormone maintained over extended periods of time (e.g., thyroid hormones).

Acute Hormone Secretion

A secretory pattern where hormone concentration fluctuates abruptly and irregularly in immediate response to a specific, intermittent stimulus (e.g., epinephrine release during acute stress).

Episodic Hormone Secretion

A secretory pattern where hormones are released in regular, recurring, and predictable cyclical rhythms (e.g., female reproductive cycle hormones).

Lipid-Soluble Hormones

Nonpolar, hydrophobic, typically small hormones (including steroid hormones like testosterone, estrogen, and cortisol, as well as thyroid hormones) that readily diffuse across the plasma membrane to bind intracellular or nuclear receptors.

Water-Soluble Hormones

Polar, hydrophilic hormones (including amino acid derivatives, peptides, and proteins) that cannot cross the lipid bilayer and must bind to membrane-bound receptors on the target cell surface.

Bound Hormones

Hormones reversibly attached to specific circulating transport proteins in the blood, which extends their half-life, prevents premature enzymatic breakdown, and keeps lipid-soluble molecules dissolved in plasma.

Free Hormones

Unattached circulating hormones that are readily able to diffuse across capillary walls into interstitial fluid to interact with target cell receptors.

Half-Life

The amount of time required for $50\%$ of a circulating hormone dose to be degraded, inactivated, or eliminated from the bloodstream.

Conjugation

The enzymatic liver process of attaching polar chemical groups to lipid-soluble hormones to prevent them from re-entering the bloodstream and target them for excretion in bile.

Agonist

A drug or chemical molecule that specifically binds to a hormone receptor and activates it, mimicking the physiologic response of the endogenous hormone.

Antagonist

A drug or chemical molecule that binds to a hormone receptor without activating it, thereby blocking the binding site and inhibiting the hormone's normal cellular action.

Nuclear Receptors

Receptors located within the cytoplasm or nucleus of target cells that bind lipid-soluble hormones; the hormone-receptor complex binds to specific hormone-response elements on DNA to regulate gene transcription and protein synthesis.

Membrane-Bound Receptors

Transmembrane cell-surface receptor proteins that bind water-soluble hormones and rapidly alter intracellular enzyme activity or generate second messengers without entering the cell.

Receptor Tyrosine Kinase

An enzymatic membrane-bound receptor that undergoes autophosphorylation upon hormone binding, subsequently phosphorylating intracellular proteins to initiate target cell responses (e.g., the mechanism by which insulin facilitates glucose uptake).

Down-Regulation

A decrease in the total number of active target cell receptors following prolonged or excessive exposure to a hormone, decreasing target tissue sensitivity.

Up-Regulation

An increase in the total number of target cell receptors that enhances target tissue sensitivity to a specific hormone (e.g., FSH stimulating an increase in ovarian LH receptors to trigger ovulation).

Infundibulum

The stalk of neural and vascular tissue that connects the hypothalamus to the pituitary gland.

Posterior Pituitary (Neurohypophysis)

The nervous tissue lobe of the pituitary gland that stores and secretes neurohormones (specifically ADH and oxytocin) produced in the cell bodies of hypothalamic neurons.

Hypothalamohypophysial Tract

A bundle of CNS axons passing through the infundibulum that conveys neurosecretory action potentials and transport vesicles from hypothalamic nuclei into the posterior pituitary.

Anterior Pituitary (Adenohypophysis)

The epithelial, endocrine portion of the pituitary gland that synthesizes and secretes major regulatory hormones under the influence of hypothalamic releasing and inhibiting neurohormones.

Hypothalamohypophysial Portal System

A specialized capillary-vein-capillary vascular system that transports releasing and inhibiting neurohormones directly from the hypothalamus to the capillary beds of the anterior pituitary.

Tropic Hormones

Hormones secreted by the anterior pituitary that stimulate the secretion of other hormones from target endocrine glands and control target tissue growth.

Antidiuretic Hormone (ADH / Vasopressin)

A neurohormone synthesized in the hypothalamus and released by the posterior pituitary that promotes water reabsorption in the kidney tubules to reduce urine output and raise blood volume and pressure.

Oxytocin

A neurohormone synthesized by the hypothalamus and released from the posterior pituitary that stimulates uterine smooth muscle contraction during labor and triggers milk ejection (letdown) during lactation.

Growth Hormone (GH / Somatotropin)

An anterior pituitary hormone stimulated by GHRH that accelerates tissue growth, increases protein synthesis (anabolism), promotes lipolysis, and spares blood glucose during fasting.

Pituitary Dwarfism

A condition resulting from hyposecretion of growth hormone in children, leading to short stature with typical body proportions and typical cognitive development.

Gigantism

An endocrine disorder caused by hypersecretion of growth hormone during childhood prior to the ossification of epiphyseal plates, leading to excessive lengthening of long bones.

Acromegaly

A growth disorder resulting from growth hormone hypersecretion in adulthood after epiphyseal plate closure, causing widened digits, enlarged hands and feet, and coarse facial bone features without increased height.

Thyroid Follicles

Spherical structures in the thyroid gland lined by follicular cells and filled with thyroglobulin-rich colloid, which serves as the precursor reservoir for $T_3$ and $T_4$ synthesis.

Thyroid-Stimulating Hormone (TSH / Thyrotropin)

An anterior pituitary tropic hormone stimulated by hypothalamic TRH that triggers the synthesis and release of $T_3$ and $T_4$ from the thyroid gland.

Calcitonin

A peptide hormone produced by parafollicular cells of the thyroid gland in response to elevated blood $Ca^{2+}$ levels that inhibits osteoclast activity and increases kidney calcium excretion.

Hashimoto Disease

An autoimmune disorder characterized by autoantibody attack against thyroid tissue and enzymes, representing the most common cause of hypothyroidism in the United States.

Grave's Disease

An autoimmune disorder characterized by thyroid-stimulating autoantibodies that continuously activate the thyroid gland, leading to hyperthyroidism, elevated BMR, weight loss, and often exophthalmos.

Ocular Graves

An autoimmune manifestation associated with hyperthyroidism in Grave's disease in which connective tissue and fluid are deposited behind the orbits, resulting in protruding eyes.

Goiter

An abnormal enlargement of the thyroid gland caused by persistent overstimulation from TSH (often secondary to dietary iodine deficiency) or autoimmune thyroid diseases.

Parathyroid Glands

Four small, dense endocrine glands embedded in the posterior aspect of the thyroid gland whose chief cells secrete parathyroid hormone (PTH).

Parathyroid Hormone (PTH)

A hormone secreted by parathyroid chief cells in response to low blood calcium that elevates blood $Ca^{2+}$ by stimulating osteoclasts, enhancing renal calcium reabsorption, and increasing intestinal calcium uptake via vitamin D synthesis.

Adrenal Medulla

The inner portion of the adrenal gland acting as a modified sympathetic nervous system ganglion that secretes the catecholamines epinephrine and norepinephrine.

Adrenal Glands

Pyramid-shaped paired endocrine organs located on the superior pole of each kidney, composed of an outer steroid-producing cortex and an inner neural medulla.

Aldosterone

A mineralocorticoid steroid secreted by the adrenal cortex that acts on kidney tubules to promote sodium and water retention, increase blood pressure, and enhance potassium and hydrogen ion excretion.

Cortisol

A glucocorticoid steroid secreted by the adrenal cortex in response to ACTH that promotes gluconeogenesis, enhances lipid and protein catabolism, and exerts anti-inflammatory actions.

Addison's Disease

An endocrine disorder caused by autoimmune destruction of the adrenal cortex, resulting in hyposecretion of cortisol and aldosterone, severe fatigue, weight loss, and hypotension.

Cushing Syndrome

A condition caused by hypersecretion of ACTH or cortisol (frequently from pituitary or adrenal tumors), presenting with hyperglycemia, facial roundness, a fat pad between the shoulders, easy bruising, and hypertension.

Pancreatic Islets (Islets of Langerhans)

Endocrine cell clusters within the pancreas (up to $1{,}000{,}000$ in number) that contain alpha cells producing glucagon and beta cells producing insulin.

Insulin

A peptide hormone secreted by pancreatic beta cells immediately following meals that lowers blood glucose by promoting cellular glucose uptake, glycogen synthesis in muscle and liver, and lipid storage.

Glucagon

A peptide hormone secreted by pancreatic alpha cells during periods of hypoglycemia that raises blood glucose levels by triggering glycogenolysis and gluconeogenesis in the liver.

Type 1 Diabetes Mellitus

An early-onset endocrine disorder characterized by autoimmune destruction of pancreatic beta cells, causing insulin deficiency and requiring lifelong exogenous insulin administration.

Type 2 Diabetes Mellitus

A metabolic disorder characterized by insulin resistance in target tissues (often due to receptor down-regulation or phosphorylation failure), strongly linked to obesity, diet, and genetics.

Gestational Diabetes

A temporary condition during pregnancy characterized by reduced insulin sensitivity, which elevates the risk of pre-eclampsia in the mother and increases the child's future risk of type 2 diabetes.

Pineal Gland

An endocrine gland located in the diencephalon that secretes melatonin according to the photoperiod to regulate sleep-wake circadian rhythms.

Thymus Gland

A mediastinal lymphoid organ positioned superior to the heart that secretes thymosin, a hormone essential for the development and maturation of the immune system.