Human Physiology

Comprehensive vocabulary flashcards covering cell transport, homeostasis, membrane structure, membrane potentials, and nervous system cell biology.

Pathophysiology

The study of disease states, often described as physiology gone wrong.

Extracellular Matrix (ECM)

A complex network formed by connective tissue around cells that provides structural support and anchors cells.

Intracellular Fluid (ICF)

The body fluid compartment inside cells, containing approximately $28\,L$ ($67\%$) of total body water.

Extracellular Fluid (ECF)

The fluid compartment outside cells, comprising about $14\,L$ ($33\%$) of total body water, subdivided into interstitial fluid, plasma, and transcellular fluid.

Interstitial Fluid

The portion of extracellular fluid located in the spaces surrounding cells outside blood vessels, accounting for $11\,L$ ($26\%$) of total body water.

Plasma

The fluid component of blood inside blood vessels, representing $3\,L$ ($7\%$) of total body water, which undergoes immediate volume changes upon administration of intravenous (IV) fluids.

Dynamic Constancy

A state of physiological balance where a variable may fluctuate over short intervals but remains stable over long periods.

Negative Feedback

A homeostatic mechanism where a change in a regulated variable triggers responses that move the variable in the direction opposite to the initial change.

Positive Feedback

A control mechanism in which an initial disturbance in a variable causes responses that amplify the change in the same direction, such as platelet activation during blood clotting or oxytocin release during childbirth.

Feedforward Regulation

A homeostatic process that anticipates changes in a physiological variable and initiates responses before the change occurs to minimize fluctuations.

Steady State

A condition in which a physiological variable remains constant over time, requiring continuous energy input to maintain.

Equilibrium

A state in which a system variable does not change over time and no input of energy is required to maintain stability.

Set Point

The target value or optimal range within which a homeostatic system maintains a regulated parameter.

Hormone

A chemical messenger secreted by endocrine gland cells into the bloodstream to act on target cells at distant sites.

Paracrine Substance

A chemical messenger secreted by cells into the local extracellular space to target neighboring cells in close proximity.

Autocrine Substance

A locally acting chemical messenger secreted by a cell that binds to receptors on and acts directly upon the same cell.

Neurotransmitter

A chemical signal released by neuron terminals across a synapse to communicate with an adjacent neuron or effector cell.

Phospholipid Bilayer

An amphipathic structural membrane framework consisting of nonpolar hydrophobic fatty acid tails pointing inward and polar hydrophilic heads contacting ECF and ICF.

Integral Membrane Proteins

Membrane proteins that are embedded within the lipid bilayer, most of which are transmembrane proteins spanning the entire lipid bilayer.

Peripheral Membrane Proteins

Proteins located at the intracellular or extracellular membrane surface, non-covalently bound to polar regions of integral proteins or membrane lipids.

Desmosomes

Specialized anchoring cell junctions composed of cadherins and keratin filaments that provide strong mechanical attachment between cells in tissues subjected to stretching, such as the skin.

Tight Junctions

Occluding cell junctions that form extracellular seals between adjacent epithelial cells, restricting solute movement through the paracellular pathway.

Gap Junctions

Communicating junctions composed of connexins forming $1.5\,nm$ diameter channels that connect the cytoplasm of adjacent cells to allow direct passage of ions and small molecules.

Simple Diffusion

The passive net movement of molecules or ions down a concentration gradient resulting solely from random thermal motion.

Ligand-Gated Ion Channels

Ion channels that open or close in response to the specific binding of a chemical signaling molecule.

Voltage-Gated Ion Channels

Transmembrane channels whose open or closed states are governed by changes in electrical membrane potential.

Mechanically Gated Ion Channels

Channels that open or close in response to mechanical deformation or physical forces such as vibration, pressure, or stretch.

Aquaporins

Integral membrane protein channels that specifically facilitate the rapid net diffusion of water molecules across cell membranes.

Osmolarity

The concentration of a solution expressed as the total number of osmotically active solute particles per liter ($Osm/L$ or $mOsm/L$).

Isotonic Solution

A solution containing $300\,mOsm$ of nonpenetrating solutes, matching normal extracellular fluid concentration and producing no net change in cell volume.

Hypotonic Solution

A solution containing a lower concentration of nonpenetrating solutes than normal extracellular fluid ($< 300\,mOsm$), causing water influx and cell swelling.

Hypertonic Solution

A solution containing a higher concentration of nonpenetrating solutes than normal extracellular fluid ($> 300\,mOsm$), drawing water out of the cell and causing it to shrink.

Facilitated Diffusion

Passive mediated transport of solutes across a membrane down their concentration gradient using specific transmembrane carrier proteins without energy consumption.

Primary Active Transport

Direct carrier-mediated movement of solutes against their concentration gradient powered directly by ATP hydrolysis.

Na+/K+-ATPase Pump

A primary active pump that hydrolyzes one ATP molecule to transport $3\,Na^+$ ions out of the cell and $2\,K^+$ ions into the cell per cycle.

Secondary Active Transport

Active transport of a solute against its concentration gradient coupled to the downhill movement of an ion (typically $Na^+$) down its electrochemical gradient.

Cotransport (Symport)

A form of secondary active transport in which the ion providing energy and the transported solute move across the membrane in the same direction.

Countertransport (Antiport)

A form of secondary active transport in which the driver ion and transported solute move across the membrane in opposite directions.

Pinocytosis

A type of endocytosis referred to as cellular drinking, involving nonspecific vesicular uptake of extracellular fluid and dissolved solutes.

Phagocytosis

A type of endocytosis referred to as cellular eating, where pseudopodia engulf large particles or microorganisms to form intracellular phagosomes.

Receptor-Mediated Endocytosis

Selective vesicular uptake of specific extracellular molecules triggered by ligand binding to cell-surface receptors localized in clathrin-coated pits.

Paracellular Pathway

The pathway of transepithelial transport where substances pass through the intercellular spaces between adjacent epithelial cells.

Transcellular Pathway

The pathway of transepithelial transport where substances cross through the epithelial cell, entering across one membrane surface and exiting through the opposite membrane surface.

Resting Membrane Potential

The baseline electrical potential difference across the plasma membrane of an unstimulated cell, measuring approximately $-70\,mV$ in typical neurons.

Depolarization

A change in membrane potential that makes the potential less negative (more positive) relative to the resting membrane potential.

Repolarization

The phase during which a depolarized membrane potential returns toward its baseline resting membrane potential value.

Hyperpolarization

A change in membrane potential that makes the inside of the cell more negative than its normal resting membrane potential.

Graded Potentials

Localized changes in membrane potential that vary in magnitude according to stimulus intensity and decay decrementally with distance from the site of stimulation.

Afferent Neurons

Sensory neurons that transmit electrical signals from peripheral sensory receptors toward the central nervous system.

Efferent Neurons

Motor neurons that carry electrical signals away from the central nervous system to effector organs such as muscles or glands in the PNS.

Interneurons

Neurons situated entirely within the CNS that integrate information between afferent and efferent pathways, comprising over $99\%$ of all neurons.

Nerve

A bundled collection of peripheral neuron axons wrapped in connective tissue located in the peripheral nervous system.

Astrocytes

Star-shaped CNS glial cells that maintain extracellular fluid composition, support neuron metabolism, and induce tight junction formation for the blood-brain barrier.

Oligodendrocytes

Central nervous system glial cells that form myelin sheaths around multiple surrounding central axons.

Microglia

Phagocytic glial cells that perform immune defense functions and clear debris and dead cells within the central nervous system.

Ependymal Cells

Glial cells lining the fluid-filled ventricles of the brain and central canal of the spinal cord that synthesize and circulate cerebrospinal fluid (CSF).

Schwann Cells

Peripheral nervous system glial cells responsible for forming insulating myelin sheaths around individual peripheral axons.

Growth Cone

A dynamic, actin-rich structure at the tip of a growing or regenerating axon that senses chemical cues and directs axon extension toward target cells at a rate of approximately $1\,mm/\text{day}$ in peripheral nerves.