Unit 3: Endocrine Functional Organization and Glands

Comprehensive practice flashcards covering vocabulary from Chapter 17 (Endocrine Functional Organization), focusing on hormone classes, modes of regulation, secretion patterns, receptor mechanisms, and target cell sensitivity.

Hormone

A chemical messenger produced in small quantities with a long-lasting impact, secreted into the circulatory system to affect distant target tissues (effectors).

Endocrine Glands

Ductless glands and organs that synthesize and secrete hormones directly into the circulatory system to regulate distant effectors.

Exocrine Glands

Glands whose secretions are released into ducts or other external/internal spaces rather than directly into the bloodstream.

Pancreas

A dual-functioning organ that acts as both an exocrine gland (secreting into ducts) and an endocrine gland (secreting hormones into circulation).

Frequency-Modulated Signals

A characteristic signaling pattern of the nervous system where signal intensity is communicated by the frequency of action potentials (low action potentials = weak stimulus).

Amplitude-Modulated Signals

A characteristic signaling pattern of the endocrine system where signal intensity is determined by the concentration of circulating hormones in the blood.

Autocrine Messengers

Chemical messengers that act on the same cell that secretes them, frequently involved in positive feedback loops (e.g., white blood cells releasing cytokines to stimulate more white blood cells).

Paracrine Messengers

Chemical messengers secreted into extracellular fluid that act locally on adjacent, nearby cells (e.g., clotting factors acting on nearby platelets).

Neurotransmitters

Chemical messengers secreted by a neuron across a synaptic cleft to activate an adjacent cell, such as another neuron, a muscle cell, or a gland.

Endocrine Messengers

Chemical messengers (hormones) secreted directly into the bloodstream that travel through the circulatory system to distant target cells.

Humoral Stimuli

The control of hormone secretion triggered by changes in the blood concentration of non-hormone chemicals, such as low blood $Ca^{2+}$ stimulating the parathyroid gland to release parathyroid hormone (PTH).

Neural Stimuli

The control of hormone secretion prompted by the release of a neurotransmitter into a synapse with an endocrine gland, such as sympathetic action potentials stimulating the adrenal gland to secrete epinephrine.

Hormonal Stimuli

The control of hormone secretion where the release of one hormone causes the secretion of another hormone, such as hypothalamic releasing hormones prompting pituitary tropic hormone release.

Tropic Hormone

A hormone secreted by the anterior pituitary gland that travels through the bloodstream to stimulate an endocrine target gland to release another hormone.

Chronic Hormone Secretion

A pattern of hormone release maintaining relatively constant blood concentration levels over long periods of time, exemplified by thyroid hormone.

Acute Hormone Secretion

A pattern of hormone release characterized by sudden, irregular fluctuations in blood concentration in response to each distinct stimulus, exemplified by epinephrine release during stress.

Episodic Hormone Secretion

A pattern of hormone release that occurs in a predictable, cyclic manner over days or weeks, exemplified by female reproductive hormones.

Lipid-Soluble Hormones

Nonpolar, hydrophobic, small chemical messengers (such as steroid hormones and thyroid hormones) that can diffuse directly across the plasma membrane to bind intracellular receptors.

Water-Soluble Hormones

Polar, hydrophilic chemical messengers (including amino acid derivatives, peptides, and proteins) that cannot cross the plasma membrane and must bind to receptors on the target cell surface.

Bound Hormones

Hormones reversibly attached to specific plasma proteins, which prevents rapid enzymatic breakdown in the blood, allows lipid-soluble hormones to circulate freely, and maintains a stable supply.

Free Hormones

Hormones circulating unbound in the bloodstream that are readily able to diffuse across capillary walls to bind their specific target tissue receptors.

Negative Feedback (Endocrine)

The primary inhibitory regulatory mechanism in which elevated blood levels of an endocrine gland's hormone inhibit further upstream release of its releasing and tropic hormones (e.g., $T_3$ and $T_4$ inhibiting TRH and TSH).

Hypothyroidism

A disorder characterized by low thyroid hormone production ($T_3$ and $T_4$), clinically detected by an elevated value in a thyroid-stimulating hormone (TSH) laboratory test due to lack of negative feedback.

Half-Life (Hormone)

The duration of time required for $50\%$ of a circulating hormone to be cleared and excreted from the bloodstream.

Conjugation

The metabolic attachment of another molecule to a lipid-soluble hormone by liver enzymes, preventing its re-entry into the bloodstream and facilitating excretion in bile.

Agonist

A drug or chemical compound that binds to a specific hormone receptor and activates it, initiating a target cell response.

Antagonist

A drug or chemical compound that binds to a specific hormone receptor and blocks or inhibits its physiological action.

Nuclear Receptors

Intracellular receptor proteins located in the cytoplasm or nucleus that bind lipid-soluble hormones to regulate DNA transcription and stimulate synthesis of new proteins.

Hormone-Response Element

A specific sequence or region of nuclear DNA to which a lipid-soluble hormone-receptor complex binds to initiate gene transcription into mRNA.

Membrane-Bound Receptors

Cell-surface receptors that bind water-soluble hormones (first messengers) and activate rapid cellular cascades by altering G protein activity or activating intracellular enzymes directly.

Second Messengers

Intracellular mediator molecules (such as cyclic AMP or $Ca^{2+}$) produced in response to water-soluble hormone binding at the cell membrane that activate existing intracellular enzymes.

G Protein-Coupled Receptor (GPCR)

A membrane-bound receptor where hormone binding prompts GTP to replace GDP on the $\alpha$ subunit, causing subunit dissociation to alter intracellular second messengers such as cAMP or $Ca^{2+}$.

Receptor Tyrosine Kinase

A membrane-bound enzymatic receptor that undergoes autophosphorylation upon hormone binding (such as insulin) and phosphorylates intracellular proteins to trigger target cell responses.

Down-Regulation

A decrease in the total number of target cell receptors caused by decreased receptor synthesis and increased degradation following prolonged exposure to a hormone, causing desensitization.

Up-Regulation

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