Endocrine System: Communication, Glands, and Hormonal Regulation

Comprehensive practice flashcards defining core vocabulary, concepts, anatomical structures, and physiological mechanisms of the endocrine system covered in the lecture notes.

Direct communication

A form of intercellular communication that occurs directly between adjacent cells across specialized junctions, such as between adjacent cardiac muscle cells enabling electrical signals to travel through myocardial tissue.

Paracrine communication

A type of local cell-to-cell communication where cells release chemical mediators into the surrounding extracellular fluid to alter the activities of neighboring cells, as seen during inflammation and blood clotting.

Synaptic communication

A highly specific neural communication process where an action potential causes the release of neurotransmitters that diffuse across a narrow synaptic cleft to bind receptors on a target post-synaptic cell.

Endocrine communication

A communication mechanism where endocrine cells secrete chemical messengers called hormones directly into the interstitial fluid and bloodstream, allowing them to travel body-wide and bind specific receptors on distant target tissues.

Hormone

A specialized chemical messenger released into the bloodstream that binds to complementary receptors on target cells to alter metabolic and physiological activities.

Endocrine gland

A ductless organ or cluster of cells that secretes chemical messengers (hormones) directly into interstitial fluid to diffuse into the bloodstream.

Exocrine gland

A gland that secretes its non-hormonal products through a duct directly onto an epithelial surface or into a lumen.

Chemoreceptor

A specialized receptor structure on the plasma membrane or inside a cell that binds specific chemical substances, such as hormones, to elicit a cellular response.

Steroid hormones

Lipid-soluble chemical messengers derived from cholesterol that diffuse freely through the phospholipid bilayer of cell membranes to bind intracellular or nuclear receptors, altering gene transcription and protein synthesis with a slower, long-lasting onset.

Non-steroid hormones

Hormones built from amino acids, peptides, proteins, or glycoproteins that are not lipid soluble and bind to receptors on the cell surface, utilizing secondary messenger pathways to produce rapid, shorter-acting cellular changes.

Humoral stimuli

Changes in the concentrations of ions or nutrient molecules in extracellular fluids and blood (such as fluctuations in blood calcium or glucose) that directly stimulate hormone release.

Neural stimuli

Nerve impulses that prompt the release of hormones from endocrine tissue, such as sympathetic nerve fibers stimulating the adrenal medulla.

Hormonal stimuli

A mechanism of hormone regulation in which the arrival or blood level of one hormone stimulates or inhibits the release of another hormone.

Hypothalamus

The primary regulatory center of the endocrine system that maintains homeostasis by integrating nervous and endocrine activity through the secretion of releasing and inhibiting hormones and direct neural pathways.

Infundibulum

The slender, stalk-like neck of tissue that physically and vascularly connects the hypothalamus to the pituitary gland.

Posterior pituitary

The neurohypophysis lobe of the pituitary gland that contains the axon terminals of hypothalamic neurons and stores and secretes antidiuretic hormone (ADH) and oxytocin (OT).

Antidiuretic hormone (ADH)

A peptide hormone synthesized in the hypothalamus and released by the posterior pituitary that targets kidney tubules to increase water reabsorption, conserving fluid and decreasing urine output.

Oxytocin (OT)

A peptide hormone stored and released by the posterior pituitary that stimulates uterine smooth muscle contraction during labor and promotes milk ejection from mammary glands during lactation.

Tropic hormones

Hormones, primarily produced by the anterior pituitary, that specifically target other endocrine glands to stimulate their growth and regulate their hormonal secretions.

Growth hormone (GH)

An anterior pituitary hormone that stimulates overall cell growth, cell division, and protein synthesis, particularly in skeletal muscle and chondrocytes; a deficiency during childhood results in short stature.

Prolactin (PRL)

An anterior pituitary hormone that promotes mammary gland development and stimulates the production of breast milk during pregnancy and nursing.

Thyroid-stimulating hormone (TSH)

A tropic glycoprotein hormone secreted by the anterior pituitary that stimulates the thyroid gland to produce and secrete thyroid hormones ($T_3$ and $T_4$).

Adrenocorticotropic hormone (ACTH)

A tropic hormone produced by the anterior pituitary that stimulates the adrenal cortex to synthesize and release glucocorticoids, which regulate glucose metabolism.

Gonadotropins

Anterior pituitary hormones, namely follicle-stimulating hormone (FSH) and luteinizing hormone (LH), that regulate the developmental and secretory activities of the gonads (testes and ovaries).

Thyroid gland

A butterfly-shaped endocrine gland situated on the anterior surface of the trachea immediately inferior to the thyroid cartilage, consisting of two lobes connected by an isthmus.

Thyroid hormones ($T_3$ and $T_4$)

Iodine-containing metabolic hormones (triiodothyronine and thyroxine) produced by thyroid follicles that increase cellular metabolic rate, mitochondrial ATP synthesis, oxygen consumption, and body temperature.

Calcitonin

A polypeptide hormone secreted by the C cells of the thyroid gland in response to elevated blood calcium levels that acts to lower blood calcium by enhancing kidney calcium excretion and reducing intestinal absorption.

Parathyroid glands

Small, pea-sized endocrine glands embedded in the posterior aspect of the lobes of the thyroid gland that secrete parathyroid hormone (PTH).

Parathyroid hormone (PTH)

A vital peptide hormone secreted by principal cells of the parathyroid glands when blood calcium drops; it raises blood calcium levels by stimulating osteoclast activity, promoting renal calcium reabsorption, and enhancing calcitriol synthesis.

Hyperthyroidism

A clinical state resulting from excessive production of thyroid hormones, characterized by an accelerated metabolic rate, weight loss, warm clammy skin, and a fast, bounding pulse.

Adrenal cortex

The outer region of the retroperitoneal adrenal gland that synthesizes and secretes over 20 different steroid hormones known collectively as corticosteroids.

Mineralocorticoids

Steroid hormones produced by the adrenal cortex (primarily aldosterone) that promote the renal reabsorption of sodium ($Na^+$) and water while accelerating potassium ($K^+$) excretion, thereby increasing blood volume and blood pressure.

Glucocorticoids

Adrenal cortex steroid hormones (mainly cortisol) whose release is stimulated by ACTH; they mobilize stored energy reserves, exert a glucose-sparing effect, and produce anti-inflammatory actions.

Adrenal medulla

The modified sympathetic ganglion core of the adrenal gland that secretes adrenaline (ADR) and noradrenaline (NA) in response to sympathetic nerve stimulation to prolong fight-or-flight reactions.

Pancreatic islets

Microscopic clusters of endocrine cells (forming roughly $1\%$ of pancreatic tissue) scattered throughout the exocrine pancreas that synthesize and secrete glucagon and insulin.

Insulin

A peptide hormone produced and secreted by pancreatic beta cells in response to elevated blood glucose; it reduces blood glucose levels by accelerating cellular glucose uptake and stimulating glycogen synthesis in the liver and skeletal muscle.

Glucagon

A hormone secreted by pancreatic alpha cells in response to reduced blood glucose levels that elevates blood sugar by stimulating the breakdown of glycogen into glucose within liver cells.

Diabetes mellitus

A clinical metabolic condition resulting from impaired blood glucose homeostasis, characterized by chronically elevated blood glucose levels (hyperglycemia).

Melatonin

An amine hormone secreted by the pineal gland during darkness that assists in establishing and synchronizing circadian rhythms and sleep-wake cycles.

Thymosins

A group of peptide hormones produced by the thymus gland, especially active during childhood, that govern the development, maturation, and maintenance of immune system cells.

Erythropoietin (EPO)

A peptide hormone produced primarily by the kidneys that targets red bone marrow to stimulate red blood cell production, enhancing oxygen transport capacity.

Atrial natriuretic peptide (ANP)

A cardiac hormone released by atrial heart cells when stretched by increased blood volume that acts on the kidneys to promote sodium and water loss, reducing blood volume and blood pressure.