Principles of Human Physiology - Endocrine System, Metabolism, and Growth

Comprehensive vocabulary flashcards covering endocrine organs, chemical messenger classification, signal transduction, metabolic regulation, thermoregulation, growth, thyroid hormones, and glucocorticoids.

Anabolism

The metabolic process of synthesizing large biomolecules from smaller units, requiring energy input depending on body needs.

Catabolism

The metabolic process of breaking down large biomolecules into smaller units to release energy or provide substrates.

Gap Junctions

Membrane protein complexes (connexons) that directly link the cytosol of adjacent cells, allowing direct intercellular communication via movement of ions and small molecules.

Paracrine Chemical Messenger

A chemical messenger that acts on nearby cells by diffusing through the interstitial fluid.

Autocrine Chemical Messenger

A subclass of paracrine messenger that binds to receptors on and signals the same cell that secreted it.

Neurotransmitter

A chemical messenger produced by neurons and released into the extracellular fluid of a synaptic cleft to signal a target cell.

Hormone

A chemical messenger produced by endocrine cells and secreted into the bloodstream via interstitial fluid to regulate distant target cells.

Neurohormone

A specialized class of hormone produced by neurosecretory neurons and released directly into the blood.

Lipophobic Ligand

A water-soluble messenger that cannot cross the lipid bilayer of cell membranes and binds to receptors located on the plasma membrane.

Lipophilic Ligand

A lipid-soluble messenger that easily diffuses across cell membranes and typically binds to intracellular receptors in the cytosol or nucleus.

Catecholamines

A group of amine messengers derived from the amino acid tyrosine, including dopamine, norepinephrine, and epinephrine.

Eicosanoids

Lipophilic paracrine messengers derived from arachidonic acid (a plasma membrane phospholipid) via the cyclooxygenase or lipoxygenase pathways.

Messenger Half-Life

The time required for the concentration of a chemical messenger in the blood or interstitial fluid to decrease by half.

Up-regulation

An increase in the number of receptors on a target cell, often triggered by prolonged low messenger concentrations, which increases cell sensitivity.

Down-regulation

A decrease in the number of receptors on a target cell, often triggered by excess messenger concentrations, resulting in reduced sensitivity and tolerance.

Agonist

A chemical messenger or drug that binds to a receptor and mimics the normal biological response.

Antagonist

A chemical or drug that binds to a receptor without producing a response, thereby competing with and blocking the binding of the endogenous ligand.

Cyclic AMP (cAMP)

An intracellular second messenger synthesized from ATP by the amplifier enzyme adenylate cyclase following G protein activation.

Primary Endocrine Organs

Organs whose primary physiological function is the secretion of hormones, such as the hypothalamus, pituitary gland, thyroid gland, adrenal glands, and pancreas.

Secondary Endocrine Organs

Organs whose primary function is non-endocrine, but which also secrete hormones, including the heart, kidneys, liver, stomach, and skin.

Hypothalamic-Pituitary Portal System

A vascular arrangement of two capillary beds connected in series by a portal vein, delivering tropic hormones directly from the hypothalamus to the anterior pituitary.

Primary Secretion Disorder

An endocrine disease originating from a defect in the primary endocrine gland that secretes the ultimate hormone itself.

Secondary Secretion Disorder

An endocrine disease caused by an abnormality in an upstream organ (such as the hypothalamus or anterior pituitary) secreting a tropic hormone.

Synergism

A hormonal interaction where the combined effect of multiple hormones is greater than the sum of their individual effects.

Permissiveness

A hormonal interaction where one hormone must be present for another hormone to exert its full biological effect.

Basal Metabolic Rate (BMR)

The rate of energy expenditure in a person who is awake, resting, lying down, and has fasted for $12\,m$ hours, representing minimum maintenance energy.

Absorptive State

The metabolic state following a meal when energy intake exceeds output, glucose serves as the primary energy source, and nutrients are stored in macromolecules.

Postabsorptive State

The metabolic state between meals when energy intake is less than output and stored energy reserves (glycogen, triglycerides, proteins) are mobilized.

Insulin

An anabolic peptide hormone secreted by pancreatic beta cells during the absorptive state that promotes energy storage and lowers blood glucose.

Glucagon

A catabolic peptide hormone secreted by pancreatic alpha cells during the postabsorptive state that mobilizes energy stores and raises blood glucose.

Glucose Sparing

The metabolic shift during the postabsorptive state in which non-nervous tissues switch to utilizing fatty acids for energy, reserving blood glucose for the nervous system.

Diabetes Mellitus

A metabolic disorder defined by chronic hyperglycemia resulting from insulin deficiency (type 1) or target tissue resistance to insulin (type 2).

Core Body Temperature

The operating temperature of deep body structures, maintained in humans at approximately $37\,^\circ\text{C}$ ($98.6\,^\circ\text{F}$).

Thermoneutral Zone

An environmental temperature range of $25\text{--}30\,^\circ\text{C}$ in which changes in cutaneous blood flow alone are sufficient to maintain core body temperature.

Shivering Thermogenesis

An involuntary, heat-generating physiological response in cold environments ($< 25\,^\circ\text{C}$) mediated by rhythmic skeletal muscle contractions.

Hypertrophy

An increase in cell size, stimulated by growth hormone to support tissue growth.

Hyperplasia

An increase in the total number of cells through cell division, stimulated by growth hormone.

Somatomedins

Insulin-like growth factors (IGFs) released by the liver and other tissues in response to GH that directly mediate most growth-promoting actions.

Osteoblasts

Bone-forming cells that synthesize and lay down uncalcified bone matrix known as osteoid.

Osteoclasts

Bone-resorbing cells that secrete acid to dissolve calcium phosphate crystals and enzymes to degrade osteoid matrix.

Osteocytes

Mature, immobilized bone cells embedded within calcified matrix that maintain surrounding bone tissue.

Epiphyseal Plate

A layer of hyaline cartilage at the ends of long bones where chondrocytes proliferate and osteoblasts replace cartilage to enable bone lengthening.

Thyroglobulin

A precursor protein located in the colloid of thyroid follicles that contains tyrosine residues used in synthesizing thyroid hormones.

Thyroxine ($T_4$)

The most abundant thyroid hormone form secreted ($90\%$), which is less active and converted to $T_3$ inside target tissues.

Tri-iodothyronine ($T_3$)

The most biologically active form of thyroid hormone at target tissues, comprising $10\%$ of thyroid gland secretions.

Calorigenic Effect

The increase in basal metabolic rate and heat generation caused by thyroid hormones through upregulation of $\text{Na}^+/\text{K}^+$ pumps and mitochondrial enzymes.

Cortisol

A glucocorticoid steroid hormone secreted by the adrenal cortex during stress and circadian rhythms that mobilizes energy stores and suppresses immune function.

Cushing's Syndrome

A disorder caused by glucocorticoid hypersecretion, resulting in hyperglycemia, tissue wasting from protein depletion, and altered fat distribution.

Addison's Disease

A disorder caused by adrenocortical hyposecretion, characterized by hypoglycemia, poor stress tolerance, and electrolyte imbalances from lack of aldosterone.

General Adaptation Syndrome

The stereotypical, non-specific physiological response to stress, characterized by increased cortisol, epinephrine, ADH, and sympathetic activity.