Water and Ion Balance - Cellular and Organ Physiology

Vocabulary practice flashcards generated from BIO 203 lecture slides on Water and Ion Balance.

Intracellular Fluid (ICF)

Fluid compartment located inside cells containing high $K^+$ concentration, holding approximately $25\,L$ ($23\,L$ in tissue cells plus $2\,L$ in red blood cells) of water in a $60\,kg$ human.

Extracellular Fluid (ECF)

Fluid compartment located outside cells containing high $Na^+$ concentration, consisting of interstitial fluid ($12\,L$) and blood plasma ($3\,L$), totaling $15\,L$ in a $60\,kg$ human.

Fluid Distribution in the Human Body

Compartmentalization of the $40\,L$ total body water in a $60\,kg$ person into Intracellular Fluid ($25\,L$ total) and Extracellular Fluid ($15\,L$ total).

Fick's Law of Diffusion

Passive transport equation stating that diffusion rate is proportional to the diffusion coefficient ($D_s$) multiplied by the concentration gradient $(C_2 - C_1)$.

Membrane Flux Equation

Equation describing passive solute flux across a membrane: $\frac{dQ_s}{dt} = P \times (C_2 - C_1)$, where $P$ is membrane permeability and $(C_2 - C_1)$ is the concentration driving force.

Membrane Permeability ($P$)

Property defining how easily a specific substance can cross a biological membrane.

Phospholipid Bilayer

Membrane structure composed of amphipathic phospholipids with hydrophilic polar heads exposed to aqueous environments and hydrophobic nonpolar tails forming an internal lipid barrier.

$\text{Na}^+\text{-K}^+$ ATPase

Primary active transport transmembrane enzyme pump that hydrolyzes ATP to move $3\,Na^+$ out of the cell and $2\,K^+$ into the cell against their concentration gradients.

Colligative Property

Physical property of solutions (such as osmotic pressure or freezing point depression) whose magnitude depends solely on the solute particle concentration, independent of solute chemical identity.

Osmotic Pressure

Hydrostatic pressure required to be applied to a solution to exactly counteract and oppose the osmotic movement of water across a semi-permeable membrane.

Osmolarity

Index of total solute particle concentration per liter of solution ($1\,\text{Osm} = 1\,\text{mole of solute particles / 1 L solution}$), inversely related to water concentration.

Hypotonic Solution

Solution with a lower osmolarity than intracellular fluid ($\text{Osm}_{ECF} < \text{Osm}_{ICF}$), causing net influx of water into the cell, resulting in cell swelling and potential lysis.

Hypertonic Solution

Solution with a higher osmolarity than intracellular fluid ($\text{Osm}_{ECF} > \text{Osm}_{ICF}$), causing net efflux of water out of the cell, resulting in cell shriveling (crenation).

Euryhaline

Ability of an animal to survive over a broad range of external environmental osmotic concentrations.

Stenohaline

Ability of an animal to tolerate only a narrow, highly restricted range of external environmental osmotic concentrations.

Osmoregulator

Animal that actively maintains a stable internal body fluid osmotic concentration (typically $\approx 300\,\text{mOsm}$ in vertebrates) despite external osmotic variation.

Osmoconformer

Animal whose body fluid osmotic concentration passively matches and tracks the osmotic concentration of the surrounding environment.

Freshwater Fish Osmoregulation

Physiological state where hyperosmotic fish gain water and lose ions passively across gills; they do not drink water, actively uptake ions across gills, and excrete large volumes of dilute urine.

Saltwater Fish Osmoregulation

Physiological state where hypoosmotic fish lose water and gain ions passively across gills; they drink large amounts of seawater, actively secrete excess ions across gills, and excrete small amounts of concentrated urine.

Chloride Cells (Ionocytes)

Specialized transport epithelial cells in fish gills that actively pump ions (such as exporting $Cl^-$ via basolateral $Na^+\text{-}K^+\text{-}2Cl^-$ cotransporters in marine fish) to maintain ionic balance.

Transport Epithelia

Specialized cellular layers featuring asymmetrical transporter distribution across apical and basolateral membranes, tight junctions connecting adjacent cells, and abundant mitochondria for active transport.

Transcellular Transport

Movement of solutes or water directly through epithelial cells, passing sequentially across both apical and basolateral cell membranes.

Paracellular Transport

Movement of solutes or water between adjacent epithelial cells through tight junctions and intercellular spaces.

Epithelial Glucose Transport

Transepithelial transport mechanism involving apical $Na^+\text{-glucose symport}$, basolateral GLUT facilitated diffusion, and basolateral $Na^+\text{-}K^+$ ATPase pumping to keep ICF $Na^+$ low.

Channel Proteins

Transmembrane proteins that form continuous water-filled pores through the lipid bilayer to permit passive diffusion of hydrophilic solutes and ions down concentration gradients.

Carrier Proteins

Transmembrane proteins that undergo conformational changes to transport lipophobic molecules across membranes without ever forming a continuous open channel between ECF and ICF.

Primary Active Transport

Active solute transport mechanism moving substances against concentration gradients driven directly by ATP hydrolysis via carrier proteins that function as ATPases.

Secondary Active Transport

Active solute transport moving substances against concentration gradients driven by kinetic energy from an established ion gradient (e.g., $Na^+$ gradient) without direct ATP hydrolysis by the transporter.

Kangaroo Rat Water Budget

Daily water balance model ($60\,mL/day = 100\%$) where water gain is primarily metabolic ($54\,mL$) rather than dietary ($6\,mL$), and water loss occurs mainly via evaporation ($43.9\,mL$) and concentrated urine ($13.5\,mL$).

Temporal Counter-Current Exchange

Respiratory mechanism in desert mammals that cools exhaled air in nasal passages to condense moisture and minimize respiratory water loss.