Functional Organization of the Endocrine System

Vocabulary flashcards covering key definitions, classes of chemical messengers, hormone transport, patterns of secretion, feedback mechanisms, and receptor pathways from Chapter 17.

Endocrine system

The body system composed of endocrine glands and specialized endocrine cells located throughout the body that secrete hormones into the interstitial fluid and blood to regulate homeostasis and cellular functions.

Hormone

A chemical messenger secreted in very small amounts into the interstitial fluid, which diffuses into the blood to travel to distant target tissues and stimulate a specific cellular response.

Target tissue

Specific cells or tissues containing specific receptor proteins to which a circulating hormone binds to elicit a targeted biological response; also called an effector.

Exocrine gland

A gland that carries its secretions through ducts to the outside of the body or into hollow internal organs, producing substances such as saliva, sweat, breast milk, or digestive enzymes.

Endocrinology

The scientific study of the endocrine system, including its glands, hormones, and physiological functions.

Neuropeptide

A chemical messenger produced and secreted by a neuron directly into the bloodstream to act as a hormone; also referred to as a neurohormone.

Amplitude-modulated system

A communication pattern typical of the endocrine system in which the concentration of circulating hormones determines the strength of the signal and the magnitude of the response over minutes to hours.

Frequency-modulated system

A communication pattern typical of the nervous system in which the strength of the signal depends on how often action potentials are sent per unit of time rather than their magnitude.

Autocrine chemical messenger

A chemical messenger secreted by a cell into local extracellular fluid that directly stimulates or influences the activity of the exact same cell that secreted it.

Paracrine chemical messenger

A chemical messenger produced by a wide variety of tissues and secreted into extracellular fluid to exert localized effects on nearby surrounding tissues.

Neurotransmitter

A chemical messenger produced by neurons and secreted into a synaptic cleft by presynaptic nerve terminals to influence postsynaptic target cells across short distances.

Endocrine chemical messenger

A chemical messenger secreted into the blood by specialized cells that travels through general circulation to regulate distant target tissue functions.

Half-life

The amount of time required for $50\%$ of a circulating hormone to be removed from the blood circulation and excreted.

Binding protein

A selective plasma protein that attaches to hormones, protecting them from degradation or renal filtration and increasing the solubility of lipid-soluble hormones in plasma.

Bound hormone

A hormone molecule attached to a binding protein in the bloodstream, serving as a reservoir to maintain stable free hormone levels and extend half-life.

Free hormone

An unbound hormone circulating in blood plasma that is capable of diffusing through capillary walls to bind to target tissue receptors.

Conjugation

A liver process in which specific enzymes attach water-soluble molecules (such as sulfate or glucuronic acid) to lipid-soluble hormones to facilitate rapid excretion in urine or bile.

Water-soluble hormone

A category of polar hormones—including proteins, peptides, and most amino acid derivatives—that dissolve directly in plasma and typically bind to membrane-bound receptors.

Lipid-soluble hormone

A category of nonpolar hormones—including steroids, thyroid hormones, and fatty acid derivatives—that circulate bound to binding proteins and cross cell membranes to bind nuclear receptors.

Chronic hormone secretion

A pattern of hormone secretion that results in relatively constant blood concentrations of a hormone over long periods of time up to several weeks.

Acute hormone secretion

A pattern of hormone secretion where hormone concentration increases rapidly for a short duration in response to a specific stimulus before falling.

Episodic hormone secretion

A pattern of hormone secretion occurring in regular, predictable bursts at consistent intervals and concentrations, such as reproductive steroid hormones during menstruation.

Humoral stimulus

A mechanism of hormone regulation in which changing blood levels of specific metabolites or chemicals (such as glucose, $\text{Ca}^{2+}$, or $\text{Na}^+$) directly trigger hormone release.

Neural stimulus

A mechanism of hormone regulation in which action potentials in neurons stimulate an endocrine gland to release neurotransmitters that induce hormone secretion.

Hormonal stimulus

A mechanism of hormone regulation in which one hormone stimulates the secretion of another hormone.

Tropic hormone

A hormone secreted by the anterior pituitary gland that travels through the blood to stimulate another target endocrine gland to secrete its hormones.

Negative feedback

A self-limiting control system where a hormone or its biological response inhibits the further secretion of that hormone or its precursor releasing/tropic hormones once homeostatic levels are reached.

Positive feedback

A self-propagating control system where a hormone stimulates target cells or tropic hormones to promote even further synthesis and secretion of the original hormone.

Specificity

The structural and chemical tendency of a receptor site to bind only a specific type of hormone with complementary features.

Agonist

A drug or chemical that binds to a specific hormone receptor and activates it, mimicking the effect of the natural hormone.

Antagonist

A drug or chemical that binds to a specific hormone receptor and inhibits its action by blocking the natural hormone from binding.

Down-regulation

A desensitization process in which a target cell rapidly decreases its total count of active hormone receptors—often through receptor internalization and degradation—following hormone exposure.

Up-regulation

A process in which a target cell increases its sensitivity to a hormone by elevating its rate of receptor synthesis and total receptor count.

Nuclear receptor

A protein receptor located in the cytoplasm or nucleus that binds lipid-soluble hormones to form a transcription factor complex that directly regulates DNA gene transcription.

Hormone-response element

A specific sequence of nucleotides on DNA recognized and bound by a lipid-soluble hormone-receptor complex to stimulate mRNA transcription.

Membrane-bound receptor

An integral membrane protein spanning the plasma membrane with an exposed outer binding site that binds water-soluble hormones to initiate intracellular responses.

Second messenger

An intracellular mediator molecule (such as cAMP, cGMP, $\text{IP}_3$, or DAG) produced inside a cell when a hormone binds to a membrane-bound receptor, which activates cellular enzymes.

G protein

A membrane-associated protein complex composed of $\alpha$, $\beta$, and $\gamma$ subunits that binds GDP or GTP to link membrane-bound receptors with intracellular enzymes or ion channels.

Adenylate cyclase

An enzyme located on the inner surface of the plasma membrane that, when activated by an $\alpha$ subunit of a G protein, converts ATP into the second messenger cAMP.

Phosphodiesterase

An intracellular enzyme that degrades active cyclic nucleotides like cAMP or cGMP into inactive AMP or GMP, terminating second-messenger activity.

Signal amplification

A cascade mechanism in second-messenger systems where a single hormone molecule binding to a receptor activates numerous G proteins and enzymes, generating vast amounts of final products rapidly.

Insulin resistance

A metabolic state where target cells demonstrate reduced responsiveness to insulin, leading to decreased glucose transport insertion and elevated blood glucose levels.

Major Endocrine Glands and Tissues

Anatomical distribution of key hormone-secreting organs in the human body, including the hypothalamus, pituitary, thyroid, parathyroids, adrenals, pancreas, pineal gland, thymus, ovaries, and testes.

Effect of Binding Proteins

Diagram showing how water-soluble free hormones interact directly with membrane-bound receptors, while lipid-soluble hormones circulate bound to transport proteins to maintain a stable pool of free hormone.

Humoral Regulation of Hormone Secretion

Diagram demonstrating direct control of hormone secretion by circulating blood solute levels, such as low blood $\text{Ca}^{2+}$ stimulating parathyroid hormone (PTH) release.

Hormonal Regulation of Hormone Secretion

Multi-step regulatory cascade where hypothalamic releasing hormones stimulate anterior pituitary tropic hormones, which induce target gland hormone secretion and exert negative feedback control.

Nuclear Receptor Model

Detailed mechanism of action for lipid-soluble hormones binding intracellular receptors to act as transcription factors on DNA hormone-response elements for protein synthesis.