Endocrine System – Hormones, Signaling & Pituitary Gland

Vocabulary flashcards covering foundational terms, hormone classes, signaling pathways, and pituitary hormones from the endocrine system lecture.

Endocrine System

A collection of glands that secrete hormones (chemical messengers) into the bloodstream to regulate distant target organs and maintain homeostasis.

Hormone

A chemical messenger released by an endocrine gland that travels through blood or lymph to act on specific target cells.

Receptor (Hormone)

A lock-like protein on or in a cell that specifically binds a hormone and initiates the cell’s response.

Specificity (Lock-and-Key)

The concept that a hormone binds only to receptors whose shape complements its own, ensuring targeted action.

Dynamic Equilibrium

Rapid binding and unbinding of a hormone to its receptor, influenced by hormone concentration in the blood.

Affinity

The binding strength between a hormone and its receptor; higher affinity requires less hormone to elicit a response.

Hydrophilic Hormone

Water-loving hormone (protein, peptide, or amino-acid based) that dissolves easily in blood but cannot cross the lipid cell membrane.

Hydrophobic Hormone

Water-fearing, lipid-loving steroid hormone that needs a carrier in blood but can cross the cell’s phospholipid bilayer.

Peptide Hormone

Short chain of amino acids; hydrophilic, travels unassisted in blood, binds extracellular receptors (e.g., oxytocin).

Protein Hormone

Longer, folded amino-acid chain; hydrophilic, uses extracellular receptors and second-messenger systems (e.g., prolactin).

Amino-Acid Based Hormone

Single modified amino acid (e.g., epinephrine); hydrophilic, extracellular receptor, second messenger.

Steroid Hormone

Cholesterol-derived lipid hormone; hydrophobic, bound to carrier proteins, binds intracellular receptors (e.g., cortisol).

Cholesterol

Lipid backbone molecule used to synthesize all steroid hormones.

Carrier Protein / Chaperone

Amphipathic plasma protein that transports hydrophobic hormones through watery blood.

Amphipathic Molecule

Chemical with both hydrophilic and hydrophobic regions, able to interact with lipids and water (e.g., carrier proteins).

Phospholipid Bilayer

Double-layered lipid membrane surrounding cells that blocks hydrophilic substances.

Extracellular Receptor

Hormone receptor located on the outer surface of the cell membrane; used by hydrophilic hormones.

Intracellular Receptor

Receptor inside the cytoplasm or nucleus; used by hydrophobic steroid hormones.

Second-Messenger System

Signal pathway in which an extracellular hormone triggers internal molecules (second messengers) to produce a cellular response.

G Protein

Membrane-associated relay protein activated by a hormone-bound receptor to start second-messenger cascades.

Adenylyl Cyclase

Enzyme activated by G protein that converts ATP to cyclic AMP (cAMP).

cAMP (Cyclic AMP)

Common second messenger that activates protein kinases to produce cellular responses.

Protein Kinase

Enzyme that phosphorylates target proteins, generating the cell’s final response in a cAMP pathway.

Phospholipase C (PLC)

Enzyme in the calcium second-messenger pathway that splits PIP₂ into DAG and IP₃.

IP₃ (Inositol Triphosphate)

Second messenger that opens calcium channels in the endoplasmic reticulum.

DAG (Diacylglycerol)

Lipid second messenger produced with IP₃; helps activate protein kinase C.

Calcium Second Messenger

Signal pathway where IP₃ releases Ca²⁺ ions that activate proteins and trigger cellular responses.

Anterior Pituitary (Adenohypophysis)

Glandular lobe of the pituitary that produces and releases its own hormones under hypothalamic hormonal control.

Posterior Pituitary (Neurohypophysis)

Neural extension of the hypothalamus that stores and releases hypothalamic hormones (oxytocin, ADH).

Hypothalamus

Brain region that maintains homeostasis and regulates the pituitary via nerves (posterior) or hormones (anterior).

Hypothalamic-Pituitary Axis

Functional link where the hypothalamus controls pituitary hormone release to regulate peripheral endocrine glands.

Hypophyseal Portal System

Special blood vessel network carrying hypothalamic hormones directly to the anterior pituitary.

Follicle-Stimulating Hormone (FSH)

Anterior pituitary hormone that stimulates sperm production in testes and follicle (egg) development in ovaries.

Luteinizing Hormone (LH)

Anterior pituitary hormone that triggers ovulation and estrogen production in ovaries and testosterone production in testes.

Adrenocorticotropic Hormone (ACTH)

Anterior pituitary hormone that stimulates cortisol release from the adrenal cortex.

Thyroid-Stimulating Hormone (TSH)

Anterior pituitary hormone that stimulates the thyroid gland to secrete T₃ and T₄.

Prolactin

Anterior pituitary hormone that promotes milk production in the mammary glands.

Growth Hormone (GH)

Anterior pituitary hormone that stimulates growth of bones and soft tissues.

Oxytocin

Posterior pituitary hormone that triggers uterine contractions during labor and milk ejection from the breast.

Vasopressin (Antidiuretic Hormone, ADH)

Posterior pituitary hormone that increases water reabsorption by the kidneys to conserve body water.

Gonads

Collective term for testes and ovaries, the reproductive endocrine glands targeted by FSH and LH.

Adrenal Cortex

Outer region of the adrenal gland that produces steroid hormones such as cortisol and aldosterone.

Thyroid Hormones (T₃ & T₄)

Iodine-containing hormones produced by the thyroid that regulate metabolism; release stimulated by TSH.

Tropic Hormone

Hormone that regulates the activity or hormone release of another endocrine gland (e.g., TSH, ACTH).

Homeostasis

The body’s maintenance of a stable internal environment, largely managed by hypothalamus and endocrine signals.