Human Physiology: Homeostasis, Transport, Tissues, Digestion, and Blood Glucose Regulation

Vocabulary practice flashcards covering homeostatic mechanisms, blood glucose regulation, plasma membrane transport, epithelial cell structure and transport, and digestive system structure and function.

Homeostasis

The state of relative constancy of the internal environment that provides the required conditions for cell and organism survival.

Negative Feedback Loop

A regulatory control pathway in which the physiological response counteracts or opposes the initial stimulus, shutting off the response loop once homeostasis is restored.

Positive Feedback Loop

A control mechanism where the physiological response reinforces or amplifies the initial stimulus, driving the variable farther from the set point until an external factor shuts off the cycle.

Integrating Center

A physiological region (frequently in the central nervous system or specific endocrine cells) that receives input signals from sensors, compares them against a set point, and sends output signals to effectors.

Effector

A cell or organ that carries out a physiological response in response to output signals from an integrating center to maintain homeostatic stability.

Antagonistic Signals

Control systems involving two opposing regulatory factors (such as insulin and glucagon) that act in opposite directions to achieve refined physiological control.

Intrinsic Regulation

Inherent regulatory mechanisms that reside entirely within a specific organ, such as the built-in capability of blood vessels to constrict or dilate.

Extrinsic Regulation

Organ control exerted from outside the organ by the nervous system via nerve fibers or by the endocrine system via circulating hormones.

Alpha Cells

Endocrine cells located within the pancreatic islets of Langerhans that synthesize and secrete glucagon in response to decreased blood glucose levels.

Beta Cells

Endocrine cells in the pancreatic islets of Langerhans that synthesize and release insulin in response to elevated blood glucose levels.

Insulin

An endocrine hormone secreted by pancreatic beta cells in response to elevated blood glucose that promotes cellular glucose uptake, stimulates glycogenesis, and lowers plasma glucose levels.

Glucagon

An endocrine hormone secreted by pancreatic alpha cells in response to low blood glucose that stimulates glycogenolysis, gluconeogenesis, and lipolysis to raise blood glucose.

C-peptide

A peptide cleaved from proinsulin inside secretory vesicles that is released into the bloodstream in equal molar amounts alongside active insulin during exocytosis.

GLUT4

An insulin-dependent glucose carrier protein found in skeletal muscle, adipose tissue, and heart that translocates from intracellular vesicles to the cell membrane upon insulin signaling.

GLUT2

An insulin-independent glucose transporter with high $K_m$ and low affinity found in the liver, pancreas, and small intestine that allows bidirectional movement of glucose across the plasma membrane.

Glycogenesis

The metabolic process of converting glucose into stored glycogen in the liver, skeletal muscles, and cardiac muscles, stimulated by insulin.

Glycogenolysis

The breakdown of stored glycogen into glucose 1-phosphate and subsequently glucose 6-phosphate, stimulated by glucagon.

Gluconeogenesis

The metabolic pathway in which the liver synthesizes new glucose from non-carbohydrate precursors, such as amino acids, during fasting.

Glucose 6-phosphatase

An enzyme present in the liver (but absent in muscle) that removes the phosphate group from glucose 6-phosphate, allowing free glucose to enter the bloodstream.

Type 1 Diabetes Mellitus

An autoimmune condition in which autoreactive T cells and autoantibodies destroy pancreatic beta cells, preventing insulin synthesis and requiring exogenous insulin injections.

Type 2 Diabetes Mellitus

A metabolic disorder accounting for $95\%$ of diabetes cases, caused by low target cell sensitivity to insulin (insulin resistance) and strongly associated with obesity.

Hemoglobin A1c

A clinical blood test measuring glycated hemoglobin to evaluate average blood glucose over several months, with normal at $5\%$, prediabetic at $5.7\%$ to $6.4\%$, and diabetic at over $6.5\%$.

Passive Transport

The movement of substances across a cell membrane down their concentration gradient without requiring metabolic energy ($\text{ATP}$).

Active Transport

The movement of molecules across a plasma membrane against their concentration gradient (from lower to higher concentration), utilizing ATP and membrane carrier pumps.

Simple Diffusion

The passive movement of small, nonpolar lipid-soluble molecules, such as $O_2$, $CO_2$, and steroid hormones, directly through the phospholipid bilayer.

Facilitated Diffusion

A passive, carrier- or channel-mediated transport process that moves polar or charged molecules down their concentration gradient without consuming ATP.

Transport Maximum ($T_m$)

The maximal rate of carrier-mediated transport across a membrane achieved when all available transport carrier proteins become fully saturated with solute molecules.

Primary Active Transport

Active transport in which the carrier protein directly hydrolyzes ATP to move solutes across the membrane against their concentration gradient.

Na+/K+ ATPase

A primary active transport membrane pump found in all body cells that hydrolyzes ATP to export $3\,Na^+$ ions and import $2\,K^+$ ions.

Secondary Active Transport

Coupled membrane transport that uses the downhill electrochemical gradient of $Na^+$ (established by primary active transport) to move another substance against its concentration gradient.

Cotransport (Symport)

A form of secondary active transport in which the coupled molecule is moved across the membrane in the same direction as $Na^+$.

Countertransport (Antiport)

A form of secondary active transport in which the coupled molecule is transported across the membrane in the opposite direction from $Na^+$.

Exocytosis

An ATP-dependent bulk transport mechanism in which intracellular secretory vesicles fuse with the plasma membrane to release large molecules outside the cell.

Endocytosis

A bulk transport process where the plasma membrane invaginates to form a vesicle, bringing large extracellular substances into the cell.

Osmosis

The net diffusion of solvent (water) across a selectively permeable membrane through aquaporin channels from an area of lower solute concentration to higher solute concentration.

Osmolality

The total concentration of osmotically active particles in a solution, expressed as the number of solute particles per kilogram of solvent ($\text{Osm/kg}$).

Fluid Mosaic Model

A structural model of the cell membrane describing a double layer of phospholipids with embedded transmembrane, peripheral, and lipid-anchored proteins, as well as cholesterol and carbohydrates.

Basal Lamina

An acellular extracellular matrix layer secreted by epithelial cells that anchors the epithelium to underlying connective tissue.

Paracrine Signaling

Local cell communication where cells within an organ secrete regulatory molecules that diffuse through extracellular space to nearby target cells.

Endocrine Signaling

Long-distance regulatory signaling in which ductless endocrine glands secrete hormones into the bloodstream to act on distant target organs.

Synaptic Signaling

Cell signaling in which neurons release chemical neurotransmitters across a synaptic gap to target cells.

Tight Junctions (Zonula Occludens)

Adhesion complexes near the apical surface of epithelial cells that seal the intercellular space to prevent or restrict paracellular transport.

Transcellular Transport

The movement of solutes or fluids across an epithelial layer by passing directly through the apical membrane, cytoplasm, and basolateral membrane of epithelial cells.

Paracellular Transport

The passage of molecules and fluid through the narrow intercellular spaces between adjacent epithelial cells.

Exocrine Gland

A gland derived from epithelial tissue that retains connecting cells forming a duct through which secretions are released onto external or internal body surfaces.

Enteric Nervous System (ENS)

An intrinsic neural network in the GI tract wall comprising the submucosal and myenteric plexuses that regulates motility and secretion independently or alongside the CNS.

Peristalsis

A pattern of gastrointestinal motility consisting of sequential wave-like muscular contractions behind a bolus and relaxations ahead of it, propelling chyme forward.

Segmentation

A mixing pattern of gastrointestinal motility involving simultaneous, localized contractions that churn chyme and increase its exposure to digestive enzymes.

Chyme

The liquid mixture of partially digested food, water, hydrochloric acid, and digestive enzymes formed in the stomach and delivered to the small intestine.

Parietal Cells

Secretory cells of the gastric glands in the stomach mucosa that produce hydrochloric acid ($\text{HCl}$) and intrinsic factor.

Chief Cells

Cells located in the gastric glands that secrete pepsinogen, the inactive precursor to the protein-digesting enzyme pepsin.

Pepsin

An active proteolytic enzyme converted from pepsinogen by $\text{HCl}$ in the stomach that operates at an optimal $\text{pH 1--2}$ to cleave peptide bonds in denatured proteins.

Villi

Finger-like folds of the intestinal mucosa containing capillary networks and central lacteals that absorb monosaccharides, amino acids, and dietary fats.

Microvilli (Brush Border)

Foldings of the apical cell membrane of enterocytes that contain membrane-bound digestive enzymes facing the lumen to execute terminal nutrient hydrolysis.

Enterohepatic Circulation

The pathway where $95\%$ of secreted bile salts are reabsorbed in the terminal ileum and returned to the liver via the hepatic portal vein to regulate cholesterol and bile acid synthesis.

Secretin

A gastrointestinal hormone released by the small intestine in response to acid that stimulates water and bicarbonate secretion from the pancreas.

Cholecystokinin (CCK)

A gastrointestinal hormone secreted by the small intestine that stimulates gallbladder contraction, pancreatic enzyme secretion, and inhibition of gastric motility.