Body Fluid Compartments and Transport Across Cell Membranes

This set of vocabulary flashcards covers body fluid distribution, ion concentrations, cell membrane components, and the various mechanisms of solute transport.

Total Body Water (TBW)

The total amount of water in the body, which typically accounts for approximately $60\%$ of body weight.

Intracellular Fluid (ICF)

The fluid compartment located inside the cells, making up approximately $40\%$ of the total body weight; it is characterized by high $K^+$ concentration and low ionized $Ca^{2+}$.

Extracellular Fluid (ECF)

Fluid located outside the cells, comprising approximately $20\%$ of body weight; it is subdivided into plasma ($4\%$) and interstitial fluid ($16\%$).

Average Blood Volume

A fluid compartment that varies by species but is generally estimated at $6-8\%$ of body weight or $60-80\,mL/kg$ of body weight.

Electroneutrality

The principle stating that in each body fluid compartment, the total number of positive charges must equal the total number of negative charges.

Major Ions of ECF

The primary cation is $Na^+$ and the major anions are $Cl^-$ and $HCO_3^-$.

Major Ions of ICF

The primary cation is $K^+$ and the major anions are proteins and organic phosphates.

Amphipathic

A characteristic of phospholipid molecules in the cell membrane that possess both a hydrophilic (glycerol) head and hydrophobic (fatty acid) tails.

Integral Proteins

Proteins that span the cell membrane, including transmembrane proteins that function as receptors, pores, or ion channels.

Simple Diffusion

A transport mechanism that does not require energy or a carrier protein, where solutes move down an electrochemical gradient.

Fick's Law Variables

The five factors determining diffusion rate: concentration gradient, partition coefficient (lipid solubility), diffusion coefficient (size and viscosity), membrane thickness, and surface area.

Facilitated Diffusion

A form of transport that uses a carrier protein but requires no energy input; an example is the D-glucose transporter GLUT4.

Saturation

A feature of carrier-mediated transport where the rate of transport reaches a maximum ($T_{max}$ or $T_m$) because all binding sites are occupied.

Stereospecificity

The characteristic of carrier proteins having binding sites specific to certain isomers, such as recognizing $D$-glucose but not $L$-glucose.

Primary Active Transport

Transport that moves solutes against a concentration gradient by directly using energy in the form of ATP; examples include the $Na^+/K^+$ ATPase, $Ca^{2+}$ ATPase, and $H^+/K^+$ ATPase.

Na+/K+ ATPase Pump

A primary active transporter that pumps $3\,Na^+$ out of the cell and $2\,K^+$ into the cell, consuming approximately $20-40\%$ of cellular ATP.

Cardiac Glycosides

A class of drugs including digoxin, ouabain, and oleandrin that inhibit the $Na^+/K^+$ ATPase pump.

SERCA (Sarcoplasmic and endoplasmic reticulum Ca2+ ATPase)

A primary active transporter responsible for sequestering $Ca^{2+}$ into the sarcoplasmic or endoplasmic reticulum from the ICF.

Co-transport (Symport)

A type of secondary active transport where all solutes are moved in the same direction using the $Na^+$ gradient; examples include SGLT 1 and $Na^+$/amino acid transporters.

Counter-transport (Antiport)

A type of secondary active transport where solutes move in opposite directions, such as the $Ca^{2+}/Na^+$ exchange or $Na^+/H^+$ exchange.